What Size Solar System Do You Need?
If you are in the market for solar, you’re probably thrilled with the idea of zeroing out your energy bill. But you can’t just tack a few solar panels on your roof and expect to reduce your bill to nil.
That’s why many customers ask us: what size solar system do I need? The size of your solar system comes down to how much energy you use on a daily basis and how many solar panels it takes to produce that amount of energy.
Some people are surprised to learn that the size of their home is not the main factor in determining the size of their solar system. It’s all about your energy needs.
It’s a common misconception: the size of your home is not the deciding factor in the size of your solar system.
A retired couple that lives in a 3,500 square foot home is likely to use less electricity than a family of four that lives in a 2,500 square foot home. In this case, the family of four would need more solar power to cover their energy demand.
When we are approached by a customer wondering what size solar system they need, we always ask how much electricity they use. You could live in a mansion in Beverly Hills yet your average electricity usage might be less than a suburban family of four.
If you think “my house is small, so I only need a panel or two,” you could be wrong. Same goes for those with larger homes who fear they’ll need a massive, expensive solar system.
Calculating your average daily energy usage is the determining factor in how much solar you need.
To calculate daily energy usage, refer to your past 12 months of energy bills. If you don’t have them handy, log in to your PGE. SMUD. or other energy provider’s account to find your statements online.
Let’s assume that you used 10,000 kilowatt hours (kWh) of electricity in the past 12 months, or an average of 833 kWh per month. Divide 833 by 30 to calculate your average daily energy usage. 833 divided by 30 equals 27 kWh. You can even take the calculation one step further by finding the average energy you use during the summer months, the time of year in which you use the most energy. This value will give you the maximum average energy you use, so you won’t have to deal with energy bills when your usage is more than normal.
But let’s return to our assumed average daily energy usage of 27 kWh. What size solar system do you need in that case? There are a number of factors that come into play. For one, you want to make sure the system you purchase is capable of producing at least 27 kWh of electricity per day. But that’s not all: how much power is generated by your panels depends on where you live and how much shade your home or business gets, since solar panels are rated on the number of kilowatts of energy they produce given 24/7 direct sunlight. Obviously, more shade will knock down the amount of energy your panels can produce.
The chart above shows what a typical day might look like. The yellow bars represent the energy produced by your solar panels. Notice that at midday, when the sun is directly overhead, energy production peaks. As the afternoon progresses and the sun goes down, total energy production decreases. While this is just an example, it’s a good way to visualize solar energy production throughout the day.
Say you’re looking at a solar system that is rated to produce 5 kilowatts (kW) under ideal conditions. In our example, divide 27 kWh by 5 kW and get 5.4 hours, or the amount of time a 5 kW system charging at maximum capacity would need to charge up. If you get 6 hours of sunlight in one day, and your panels produce at peak efficiency, then a 5 kW solar system is large enough to cover your energy needs.
The more intense the sunlight, and the longer you receive direct sunlight, the fewer solar panels you need. But if your roof is partially shaded for much of the day, or much of the roof is at an angle where it does not receive direct sunlight for much of the day, then you will need more panels to compensate.
Compare solar models to find the exact number of solar panels that you need.
Once you figure out the system size you need, you can actually determine the exact number of solar panels that are equal to this energy output.
There are many solar manufacturers with different models. Each solar system model is rated differently. At Ilum Solar, we sell only premium solar systems from reputable manufacturers such as REC. One of our most popular solar models is the REC Alpha 370. a system with a rated power of 370 watts. Divide the system output that you need—in our example, 5,000 watts—by 370 to get 14. In other words, you will need 14 REC Alpha 370 panels in order to generate the desired energy output we previously established.
Go one step further and you can calculate the space that 14 REC Alpha 370 panels would occupy on your roof.
REC Alpha 370 panels measure 5.6 feet long and 3.3 feet wide, or roughly 18.5 square feet. Multiply 14 by 18.5 to arrive at 259 square feet. 14 REC Alpha Panels occupy 259 square feet of space.
Calculating the size of your solar system is not all that difficult. Once you establish the average daily energy that you use, you can pretty quickly work out what size solar system you need in order to generate this power.
Can the right size solar system reduce your energy bill to zero?
Energy are only increasing—rising more than 40% since 1995. The good news? You can completely avoid rising energy costs with solar!
It’s become very clear that solar-sourced energy costs less than traditional energy from the grid. But for some, significant savings is not enough. You might want a zero dollar monthly energy bill, and think solar is the way to achieve it.
Solar will reduce your energy bill. However, only in those months with particularly high energy production relative to energy demand will you see your bill reach zero. Even with the right size solar system, most months your energy bill won’t quite be zero.
That’s not to say your monthly energy costs won’t significantly decrease — they will.
As we mentioned above, peak solar production can only occur with direct sunlight. Sure, if your solar panels had direct sunlight for 8 hours a day, you should expect no monthly energy costs. The reality is that you might get an hour or two of direct sunlight per day, which means an hour or two of peak production.
Most homeowners use electricity during the evening after work. Lights, kitchen appliances, TV, laundry machine, and air conditioning might be running at the same time. But because it is now 6pm and your panels are no longer receiving direct sunlight, you might have to tap into the energy grid to meet your energy demand. That’s why your monthly bill is not zero, even though your solar system is sized correctly.
You can cover at least some of your evening time energy demand to shave off more money from your energy bill with a battery storage system. which is a great way to store and use the excess energy that your panels generate.
The bottom line: solar will decrease your monthly energy costs. How much depends upon your system’s production versus your energy demand. Installing the right size system will get you as close to zero dollar bill as possible.
Determining the size of your solar system is the first step in the solar buying process. While you’re shopping around for a solar company and looking for system quotes, you can make more informed decisions if you calculate what size solar system you need. Once you engage with a solar company, they will also run some calculations to figure out how much solar you need, to ensure you get the perfect system for your home or business.
Have more questions? Contact Ilum Solar! Our staff of engineers can help you determine the size of your solar system and answer any other questions you may have.
kwh solar system
Customer Testimonials
“Having solar panels has given me reassurance that my bills will stay manageable.”
— Kerrie Lane, Egg Harbor City, NJ
With Sunrun, you can take control of your own electricity, and not be left at the whims of your utility company with the next inevitable rate hike. Join the more than a quarter of a million American households who have gone solar with Sunrun.
Whenever you’re ready, our solar advisors are here to help. We can provide you with a free quote. Feel free to try our Cost of Solar Calculator.
A single solar panel costs between 2.67 and 3.43 to buy and install. 4 The price of the whole system is based on its capacity, measured in watts. How big a system you need will be based on how much energy you use, your roof’s sunlight exposure and panel efficiency.
The amount you save will rely on several factors, including: 18
- Region’s sunlight exposure
- Cost of solar system (including battery, where available)
- Cost of electricity
- Energy use
- Local rebates and incentives
The average payback period for a residential solar system is between six to nine years, depending on the cost of your system, incentives and savings from your energy bill. 19
Here’s a closer look at some of the things that determine your system costs.
Here’s a breakdown of installation costs, based on findings from the National Renewable Energy Laboratory: 10
| Labor | 0.30 per watt |
| Panels | 0.47/Wdc |
| Inverter | 0.12/Wdc. 0.39/Wdc |
| Permit/Inspection | 0.06/Wdc |
| Structural BOS | 0.10/Wdc |
| Electrical BOS | 0.19–0.27/Wdc (Varies by inverter option) |
| Sales tax | Sales tax varies by location; weighted national average: 6.9% |
| Electrician | 19.74–38.96 per hour (Varies by location and inverter option) |
| Laborer | 12.88–25.57 per hour (Varies by location and inverter option) |
| Burden rates | (% of direct labor) Total nationwide average: 31.8% |
Figures based on the average 6.2 kilowatt residential solar system.
These figures are based on the average 6.2 kilowatt solar system. The cost of each system will vary based on panel manufacturer, inverter option, location and equipment.
Maintenance of solar panels ranges from routine cleaning to major repairs. On average, households pay 150 for one solar panel cleaning. 11 Companies charge between 3 to 10 per panel based on roof slant, home height and system size. 11 Some firms charge a flat rate fee for a system cleaning. If you clean your system twice a year, as recommended, you can expect an annual bill of about 300.
If your system is damaged, you may incur additional expenses. Repairs to your system will include the of equipment and labor. Labor is typically priced at 100 per hour. Materials for solar repairs could be as little as 180 but may also be greater than 1,000. Your combined, total bill may range from 200 to 3,000. 11
The four most common types of damage to a home solar system and the costs of repairs area: 11
| Broken glass panel | 20 – 350, plus labor |
| Cracked panel | 100 – 400 |
| Loose connection | 100 |
| Replacement inverter | 100 – 2,500 |
Like cleaning costs, the amount you pay for repairs is affected by things like roof steepness, system type and system size. And if your panels have extensive damage, it may be cheaper to replace them. 11
When you own your system, you are responsible for the cost of maintenance and repairs. Your warranty should cover specific system failures; contact your installer before you begin repairs. A Sunrun solar lease, lets you save thousands of dollars in maintenance and repairs. You simply pay for the power, and we take care of the rest. Our comprehensive service package features the Sunrun Guarantee, which includes best-in-class monitoring, free equipment replacement, and system repairs including parts and labor, and routine maintenance.
If you’re set on buying your own home solar system, you can also purchase our full-service package, Protection Plus. While the system is yours, we’ll give you the same comprehensive support that comes with our Sunrun Guarantee.
Panel manufacturer
The price of your panels will depend on the manufacturer. Sunrun partners with LG, a world leader in solar technology and panels. Striking the right balance between quality, efficiency and affordability, LG offers some of the best solar panels on the market today. 14
For a given manufacturer, higher efficiency panels cost more. 5 The good news is that the highest efficiency panels aren’t always needed, especially if your roof gets plenty of sun.
Your state
Another factor in determining your total home solar system cost is where you live. Here are the states with the lowest and highest average solar system costs:
| Florida: 9,198 – 11,970 | Rhode Island: 13,104 – 15,792 |
| Arizona: 10,332 – 12,096 | New York: 12,264 – 16,044 |
| Maryland: 10,332 – 12,768 |
after 30% federal solar tax credit 5
Just because a residential solar system costs more in a given state doesn’t mean it’s any less cost effective. 5 Local incentives can make a huge dent in solar installation costs even in the most expensive states.
The three most common kinds of solar panels are monocrystalline, polycrystalline and thin film. 6 Here are the essential features of each:
| Monocrystalline | Monocrystalline are the most efficient on the market today, which means you need less panels to fulfill your energy needs. Monocrystalline solar cells are single-crystal silicon, giving electrons greater freedom to move and create electricity. 7 These panels are typically black, giving your roof a sleeker aesthetic. Monocrystalline panels also maintain their efficiency for longer and usually come with a 20 to 25 year guarantee. 6 |
| Polycrystalline | Polycrystalline panels are made with many fragments of silicon. 7 While this makes them less efficient, it creates less waste in production and are more affordable. 6 They also tend to have a blue tint, likely the most recognizable feature of solar panels today. |
| Thin-film | Unlike monocrystalline and polycrystalline panels, thin-film isn’t made of silicon. Instead, it’s layered with photovoltaic materials on metal or glass. 8 While the least expensive option, thin-film isn’t as efficient and likely won’t cover the average household’s energy needs without taking up lots of space. 6 |
The type of solar panel you choose will factor into the price of the system. But remember that if you decide to lease, you’ll be paying for the power rather than the panels. The cost of solar will come down to the amount of solar power you’re projected to use under your lease agreement.
Solar panel mount types
While solar panels are the main component of a solar system, how you mount the panels will also affect the cost. There are three mount types: 6
| Fix-mount | 10–15 per mount | Fixed-mount are stationary and can’t be moved to capture more sunlight, making them less efficient than other mounts. However, they’re also the least expensive. In addition, states with consistent sunlight such as Arizona and California are the least affected by this. |
| Adjustable-mount | 50 per mount | Adjustable-mounts can be tilted to maximize solar energy production. This mount type can also lay your panels flat during a storm to avoid wind damage. While they’re more expensive than fixed mounts, adjustable-mounts might make the most sense for regions with more seasonal climates and less space to maximize energy production. |
| Tracking-mount | 500–3,000 per mount | Tracking-mount follows the sun’s arch for maximum energy production. Even though they’re the most expensive, track-mounting could add 45% in energy production and might be worth considering based on your location. However, track mounting also requires more attention and will be much more costly to maintain. |
How Many kWh Does A Solar Panel Produce Per Day?
As a general rule, with an average irradiance of 4 peak-sun-hours/day, 1 watt of solar panel rated power will produce on average 4 watt-hours (Wh) of energy. This amount equates to 0.004kWh, so a 300 watt solar panel will generate 1.22kWh/day. The precise amount depends on the location irradiance.
How much kWh does a solar panel produce?
The amount of energy generated by any solar panel depends heavily on the irradiance for the panel’s location measured in kilowatt-hours per square meter per day (kWh/m2/day).
For convenience, it’s also known as the location’s Peak-Sun-Hours and can be used as a quick estimated of a solar panel arrays output per day or year measured in kWh.
It’s an important number. For example, the energy output for a solar panel in Arizona with of 7.5 peak-sun-hours/day is very different from Indiana with 3/day!
See the table below to see solar panel outputs compared across US states:
TABLE: Average solar panel output per day (kWh) by US States compared
Wh/day Produced By 50 Watt Solar Panel
Wh/day Produced By 100 Watt Solar Panel
Wh/day Produced By 200 Watt Solar Panel
Wh/day Produced By 300 Watt Solar Panel
How many solar panels to produce 30 kwh per day?
With an average irradiance of 4 peak-sun-hours 25 solar panels rated at 300 watts each would be needed to produce 30kWh per day. This equates to a 7.5kW solar power installation. The solar output will vary depending on the irradiance at any particular location.
Domestic solar panels can have power ratings anywhere from 200 watts to 350 watts.
Lower rated panels such as the very common 100 watt variety are mainly used for small projects, local battery charging, camping vans/RVs, sheds and pergola roofs.
Let’s say the panels in question are 300 watts (we can use this one for reference). I’ll also choose the location of the panels as Atlanta, Ga. Other factors, such as voltage, doesn’t really matter.
Solar panels irradiance in Atlanta, Ga
Find this value on the site Global Atlas Info – see the image below:
Irradiance at this location is 4.634 peak-sun-hours/day, so a 300 watt solar panel will generate:
4.634 x 300 = 1.39kWh/day
Now we can divide the 30kWh target by the daily energy production to0 find the number of panels needed:
30kWh/1.39 = 21.6 (22) solar panels @ 300 watts rating each
Total solar installation power required is 6.6kW.
Note: Your professional installer would advise, but it’s normal to more capacity because theoretical estimates don’t take into account the various solar losses.
How many solar panels do I need to produce 50 kwh per day?
With a typical irradiance of 4 peak-sun-hours 62 solar panels rated at 200 watts each are required to produce 50kWh per day. This is equivalent to a 7.5kW solar power system. Solar output is dependent on the irradiance at any geographic location.
Home-mounted solar panels normally have individual power ratings from 175 watts to 400 watts.
Smaller panels below 200 watts tend to be used for smaller projects, such as battery charging, emergency power supplies, RVs, or garden structures like pergola roofs.
For this example I’ll use 200 watt panels with an average irradiance value of 4 peak-sun-hours.
4 x 200 = 0.8kWh/day
If we divide 50kWh by the daily energy generation we get the number of solar panels required:
50kWh/0.8kWh = 62 solar panels @ 200 watts rating each
Total solar power needed is 12.2kW.
Note: Solar system losses of up to 23% means that extra capacity should be added to ensure the target kWh is comfortably achieved.
How many kwh per day does a 5kw system produce?
A 5kW solar power system with an average irradiance of 4 peak-sun-hours per day will theoretically generate 20kWh per day. This assumes clear skies with no shading and will vary according to location. In practice, a 5kW system may produce less than this, as solar losses reduce the power output.
5000 watts (5kW) of solar power is just about the average size of a US domestic solar system and represents 17 solar panels @ 300 watts each.
The energy output of any solar power systems depends on the sun’s energy, or irradiance, and this varies from state to state.
For example, the irradiance in peak-sun-hours in Arizona is 3 times more than in Alaska, so the difference can be very big. For most purposes, and estimate using an average irradiance value of 4 peak-sun-hours gives a good idea of solar output.
(Note: Peak-sun-hours is a convenient way of expressing irradiance, which strictly-speaking, is measured in kWh/m2/day (or year.)
How many solar panels to produce 10 kwh per day?
With a typical irradiance of 4 peak-sun-hours 13 solar panels rated at 200 watts each are required to produce 10kWh per day. This is a 2.5kW solar power system. Solar output will vary according to the irradiance for any geographic location.
I’ll stick with the 200 watt panels used in the last example and average irradiance of 4 peak-sun-hours.
How Many Solar Panels Do I Need To Power a House?
The goal for any solar project should be 100% electricity offset and maximum savings — not necessarily to cram as many panels on a roof as possible. So, the number of panels you need to power a house varies based on three main factors:
In this article, we’ll show you how to manually calculate how many panels you’ll need to power your home. Once you have an estimate for the number of panels, you’re one step close to figuring out how much solar costs for your home, and how much you can save on electricity bills
How many solar panels do you need to power a house?
While it varies from home to home, the US households typically need between 10 and 20 solar panels to entirely offset their average annual electricity consumption.
The goal of most solar projects is to offset your electric bill 100%, so your solar system is sized to fit your average electricity use. Here’s a basic equation you can use to get an estimate of how many solar panels you need to power your home:
Solar panel wattage x peak sun hours x number of panels = daily electricity use
Obviously, electricity use, peak sun hours, and panel wattage will be different for everyone. And since you didn’t come here to do algebra, we’ll go through how to figure out each variable and run through an example scenario based on national averages.
First, find how many kilowatt-hours you use to run your house
According to the US Energy Information Administration (EIA), the average US household in 2021 used 10,632 kilowatt-hours (kWh) of electricity per year. That’s equal to:
It’s important to note that this usage varies quite a bit from state to state. For example, the average daily usage was ~18 kWh in Hawaii and 40 kWh in Louisiana, which is quite a spread. But we’ll use the national average 30 kWh per day as the figure for our example.
The easiest way to find your daily electricity usage is to dig up some recent utility bills. Your bill should show your usage for 30 days (or whatever your payment period is) and you can use this to get a sense of your daily electricity consumption. Just take the total usage during the period, and divide it by the number of days.
Electricity usage varies from month to month, so the more bills you can average together, the more accurate your calculation will be.
Once you have your daily electricity usage, the next step is to figure out how many peak sun hours your system will get per day!
Next, determine how many peak sun hours your location gets
A big factor in determining how many solar panels you need to power your home is the amount of sunlight you get, known as peak sun hours.
A peak sun hour is when the intensity of sunlight (known as solar irradiance) averages 1,000 watts per square meter or 1 kW/m 2.
In the US, the average peak sun hours range from over 5.75 hours per day in the Southwest to less than 4 hours per day in the northernmost parts of the country.
Use the map above to estimate the average peak sun hours for your area, or use this peak sun hours calculator to get a precise figure for your location.
Here are some tips for using the peak sun hours calculator:
- The average roof pitch is between 14 and 27 degrees
- Use the compass on your phone to determine the Azimuth Angle of the roof face you’d put solar panels on (closest to 180 degrees is best)
Based on the map, about half the US gets less than 4.5 peak sun hours and half gets more, so we’ll use 4.5 peak sun hours as the figure for our example.
Finally, pick a solar panel power rating
The final variable is how much electricity each solar panel can produce per peak sun hour. This is called power rating and it’s measured in Watts.
Solar panel power ratings range from 250W to 450W. Based on solar.com sales data, 400W is by far the most popular power rating and provides a great balance of output and Price Per Watt (PPW).
If you have limited roof space, you may consider a higher power rating to use less panels. If you want to spend less per panel, you may consider a lower wattage. Everybody has different goals, and you should feel free to choose the panels that best suit your needs.
For the purposes of our example scenario, we’ll use 400W panels.
Calculate how many solar panels it takes to power a house
Now that we have our three variables, we can calculate how many solar panels it takes to power a house.
- Daily electricity consumption: 30 kWh (30,000 Watt-hours)
- Average peak sun hours: 4.5 hours per day
- Average panel wattage: 400W
To solve for the number of solar panels, we can rewrite the equation above like this:
Daily electricity consumption / peak sun hours / panel wattage = number of solar panels
Now let’s plug in our example figures:
30,000 Watt-hours / 4.5 peak sun hours / 400W = 16.66 panels
If we round up, it takes 17 solar panels to power the average American household and meet the goal of 100% electricity offset.
Now since we’re talking national averages, the national average electricity price in the US was 16.5 cents per kilowatt-hour in May 2023. Meanwhile, the average price of electricity from solar systems purchased on solar.com is between 6 and 8 cents per kilowatt-hour.
I’ll let you do the math there.
The easy way to find out how many solar panels you need
Now that we’ve gone through the manual calculations of finding out how many solar panels you need to power a house, we’ll show you the easy way. (I know, it’s middle school math class all over again).
Modern home solar projects are planned using satellite technology, and you can start planning your own project using our solar calculator. Simply punch in your address and set your average energy bill to calculate how big your solar system needs to be and how much you can save by switching to solar.
Under the average energy bill slider, the calculator will give you an estimated system size in kW. You can use this number to figure out how many panels you would need.
First, convert kW into Watts by multiplying by 1,000. So 5.2 kW would be 5,200 W.
Next divide the total system size in Watts by the power rating of the panels you’d prefer. If we use 400W, that would mean you need 13 solar panels.
System size (5,200 Watts) / Panel power rating (400 Watts) = 13 panels
Of course, the easiest way to know how many solar panels you need is to team up with an Energy Advisor to design a custom system.
Frequently asked questions
How many solar panels does it take to power a house?
Based on average electricity consumption and peak sun hours, it takes around 17 400-Watt solar panels to power a home. However, this number will vary between 13-19 based on how much sun the panels get and how much electricity the home uses.
Use the equation below to get an estimate of how many solar panels you need to power a house.
Daily electricity consumption / peak sun hours / panel wattage = number of solar panels
Can I run my house on solar only?
Absolutely. By pairing solar panels with battery storage, it is very possible to run a house on solar power alone. And in many areas it’s cheaper than paying for electricity through a local utility.
Without battery storage, you can still offset your grid electricity use with solar panels through net metering and eliminate your electricity bill. You will still be using grid electricity when solar generation is down, but you will only pay for your solar equipment.
Is 10 kW enough to run a house?
Yes, in many cases a 10 kW solar system is more than enough to power a house. The average US household uses around 30 kWh of electricity per day, which would require 5 kW to 8.5 kW solar system (depending on sun exposure) to offset 100%.
How Much Solar Power Do I Need?
My advice on solar power system sizing has changed over the years due to the cost of solar panels continuing to reduce over time.
This video explains the system size providing the best bang for buck for the typical Australian household:
Spoiler alert – if you don’t feel like watching, my advice on system sizing is: “if you have reasonable electricity consumption and a decent feed-in tariff, install as many solar panels as you can fit and afford.”
This article digs a bit deeper into why my advice is to ‘fill your roof’ rather than a specific size and shows how to use my nifty solar calculator to see what a solar system can do to your bills.
The size of a solar power system is described by total panel capacity, expressed in kiloWatts (kW).
A Watt is a basic measure of electrical power, and the kilo means there are 1000 of them. i.e. 1 kW = 1000 Watts
For example – a system made up of 16 x 415W solar panels = a 6.6 kW system.
When buying a solar power system, it’s common for installers to quote on ‘oversized’ systems.
The linked article goes into more detail, but in short – you get huge bang for buck by putting on 33% more panels than an inverter is rated for.
This is why 6.6kW solar systems, using 5 kW inverters, are still very popular in 2023 – they represent a sizing ‘sweet spot’ for what the typical home can fit on the roof. They’re also usually the maximum size a Distributed Network Service Provider (DNSP) will allow on a single-phase home (more on DNSPs below).
Note when you have an ‘oversized’ system, you will never produce more power than what the inverter is rated for. For example, a 6.6 kW solar system using a 5 kW inverter will never produce more than 5 kW at any given point in time.
People assume this would result in ‘lost’ energy generation due to ‘clipping’ of production at 5 kW, but averaged over a year you lose little – so don’t stress.
A word of caution: Be sure to understand what an installer is quoting you on. Make sure it is the solar PV peak Watts and NOT the inverter size they are quoting. In the past, I’ve seen advertising selling a “10 kW system” consisting of a 10 kW inverter with only 8 kW of solar panels!
To give you a feel for the amount of power a kiloWatt represents, the table below shows what you can run with 1 kW of power:
Wow, so could you power 60 globes with a 1 kW solar power system? Kind of.
A 1 kW solar panel system will only produce 1 kW of power for a few hours a day, and then only if it is a clear, sunny day. So your 60 globes will only be all on for a few hours a day.
The graph below shows what the electricity output of a 1 kW solar power system might look like over a summer’s day. You can see 1 kW is only generated at midday when the sun is at its strongest:
This is the reason most solar systems are connected to the mains electricity grid. The grid will absorb any electricity generated by the solar panels not used by your home. When you are using more electricity than can be produced by the sun (and at night, of course), the grid automatically tops up your electricity needs.
How many solar panels will you need to offset your electricity usage?
First – I want to address most people’s motivation for going solar: “I want to put on solar panels and get my electricity bill to zero!”
Getting a zero dollar bill isn’t as simple as installing enough solar panels to generate whatever your average daily usage is. As I explained previously, when you use electricity is almost as important as the amount of electricity you use. This is because a solar system only generates its maximum at midday on a ‘good’ day.
Solar power, regardless of system size, won’t do much to a massive electricity bill if it’s all caused by overnight air-conditioner usage!
Getting zero bills with only solar panels – and no battery storage – is possible. Until I added an electric car to my home, I regularly got credits on my electricity bills.
Yup – my retailer was paying me for the privilege of being connected to the electricity network!
But – for those of you with sky-high energy costs and not enough room for a huge solar power system, it’s important to FOCUS on the amount solar energy can reduce your bills by, not whether or not it can simply get your bills to 0.
Or, to put it another way – if solar power could take your 500 quarterly bill down to 50, would you be annoyed you didn’t get your fabled ‘0 bill’, or would you be jumping for joy your solar system saved you 450 in 3 months?
Now – let’s get into some hard numbers on system sizes with the help of my nifty solar calculator.
Example for a typical home in Sydney
Let’s take a home in Sydney with 500 quarterly electricity bills, paying 0.29 per kWh for grid electricity and gets paid a 0.12 per kWh feed-in tariff. They paid 5,500 for a north-facing 6.6 kW solar system and have 25% solar electricity self-consumption.
Putting these assumptions into my calculator yields the following result:
1,585 in electricity bill savings in the first year! Not too shabby.
Or, to look at it from the perspective of “What will my electricity bills be before/after solar?”:
A 9 spring electricity bill! Most homeowners would be thrilled.
So – a 6.6 kW system in Sydney, assuming 25% solar energy self-consumption, won’t quite get a 500 quarterly bill to 0 – but it gets close!
If this home could fit more than 6.6kW on their roof – closer to 10kW total – with the same consumption patterns they’d be in credit year-round.
What roadblocks are there to filling your roof with solar panels?
Besides the obvious (physical roof size/space and your budget), the other obstacle preventing you from maxing out your roof with panels are limitations set by your “Distributed Network Service Provider”, or DNSP.
But, to use an example, here in South Australia, SA Power Networks (SAPN) is our only DNSP.
They have set system size limits of:
- Single-phase homes: 10 kW inverter limit, 5 kW export limit.
- Three-phase homes: 30 kW inverter limit, 15 kW export limit.
Meaning – if you have a single-phase home in SA, you could have up to 13.3 kW of solar panels on your roof with a 10 kW inverter. Remember – you can oversize an inverter by 33%. But – you’d be export limited to 5 kW
Export limitations mean your inverter will intelligently ramp down solar power production to ensure a maximum of 5 kW is sent to the grid at any one time.
Some homeowners ask me “If my energy needs are low-to-moderate, isn’t it pointless to install a large system if I’m export limited?”
The answer, surprisingly, is no. You lose less generation than you’d expect with export limitation due to a variety of factors.
Why bigger is better, even if you believe you won’t need a larger system
Even with feed-in tariffs dropping and export limitations for single phase homes in certain states, maxing out your roof with solar panels is a Smart move for two reasons:
- Winter and summer are typically the highest energy usage seasons for households. But in summer, solar power systems pump out a ton of energy, so they can help offset big electricity bills easily. In winter, your solar energy generation can be less than half of what it is in summer, so big winter bills are harder to offset unless you have a larger solar system (10 kW or more).
- Future-proofing. I believe by 2030 many homes will have battery storage and electric cars.
Batteries and electric cars need lots of solar generation to reliably charge, and then you need more solar electricity to offset the energy needs of the rest of your home.
I recently installed more solar panels on my south-facing roof (which some installers refuse to install on!) because my home now has two electric cars and a Tesla Powerwall battery. My previous 6kW system struggled to generate enough energy for them all.
I’ll finish by saying – while I regularly hear from homeowners who wish they put on more solar panels when they had the chance (because it’s expensive and a technical headache to add panels to an existing system), I have never heard anyone complain they’ve installed too much solar power.