Solar Panel Sizing - How To Calculate Home Solar System Size
Sizing Solar Panels – How many solar panels does it take to power your home?
Solar energy is the buzz at the moment and many homeowners want to take advantage of the Federal Solar Tax Incentive while it lasts, but how do you know how many solar panels you would need to power your house?
Let’s get sizing with step-by-step solar calculations!
How Many Solar Panels Do I Need To Power My House
Whether interested in installing your own DIY solar power system or or just curious, the first step to calculate solar panel size to power your home always begins with energy consumption.
These are the things you need to know before sizing your home’s solar system:
- What is your home’s annual energy consumption in kWh (kilowatt-hours)?
- How much do you pay per kWh of electricity (use this to calculate payback period)
- What is the irradiance (sun’s energy) level in your location?
- Will your system be grid-tied or off-grid (off-grid needs batteries)
- If off-grid, what type of battery will you use (lead-acid or lithium iron phosphate)?
- What size solar panels in watts will you use?
- Inverters – will you use a central, string or micro-inverter topology (system design)
- What are the PV system losses?
What size solar system do I need for my home?
As a general rule a home solar power system needs 6 solar panels each rated 300 watts with average irradiance of 4kWh/m2/day for every 5kWh of daily energy consumption. The average US home consumes about 30kWh per day and will need 36 solar panels rated at 300 watts.
Video – How many solar panels do you need for your home?
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How to calculate home energy consumption
You could add up the wattage of all your appliances and estimate how many hours they are used each day, but this is very cumbersome and not accurate.
For example, some appliances, such as refrigerators, are difficult to estimate. This is because they have compressor motors on board and have variable instantaneous power values.
By far the easiest way is to take it from last year’s utility bill, which normally shows the full year’s energy consumption in kilowatt-hours (kWh).
Simply divide this figure by 12 to get the monthly consumption, or 365 for the average daily consumption.
The average amount in the US is about 30kW per day, but your own will depend on where you live.
How do I calculate solar panels for my home?
Once you have found your daily energy consumption, it can be used to size your solar panels.
It needs to be said that solar panel power ratings are not an accurate reflection of the actual power generation in real-life conditions.
In fact, there are many losses associated with a working home solar system. As much as 23% is normally lost and must be deducted from the solar array power rating – see infographic below:
The calculator I provide later on in the post takes these losses into account, but for this example I’ll go through the process manually step-by-step.
Download this web-page as PDF
How Many Solar Panels Do You Need To Power Your House?
Example – Calculate Solar Panel Size
How many solar panels and their power rating would be needed for a fixed solar system to cover the energy consumption of an average U.S. home?
Let’s say the house is in Burns, Oregon USA, with an average electricity consumption per day of 33 kWh.
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Checking the site Global Solar Atlas, we see that the irradiation in this location is 5.83kWh/m2/day.
Use historical irradiance data to find sun’s energy in your location
- Geographic Location: Burns, Oregon, USA
- Average daily energy consumption: 33kWh/day
- Irradiation (peak-sun-hours) for Burns, Oregon is 5.83kWh/m2 per day
PV System Losses
I previously said that system losses can be 23%, but they can also be higher. I’ll use 30% as the overall system loss, to make sure I don’t underestimate the number of solar panels to cover the home’s energy needs.
With 30% losses in mind, the energy needs are:
33kWh/day x 1.44 = 47.5 kWh/day load.
Now to take into account the efficiency of inverter, which is about 96%.
The power to be supplied to the inverter = 47.5/0.96 = 49.5kWh/day.
The average daily irradiation is 5.83kWh/m2 (also known as peak-sun-hours)
49.5kWh/day of power can be produced by:
49.5/5.83kWh/m2= 8.5kW or 9kW of solar panels working at 100% capacity rating.
To find the number of solar panels needed, divide the wattage needed by wattage of each solar panel (say, 300 watts):
Number of panels needed = 9kW/300 watts per panel = 30 solar panels.
Solar Panel Size Chart
|
Size in Watts |
Width (inches) |
Length (inches) |
Thickness (inches) |
|
50 |
21 |
26 |
1 |
|
100 |
21 |
47 |
1.5 |
|
160 |
26 |
58 |
1.5 |
|
200 |
28.5 |
66 |
1.6 |
|
250 |
39 |
65 |
1.6 |
|
350 |
39 |
77 |
1.6 |
How Many Solar Panels Do I Need Calculator
Use the calculator below to estimate the number of solar panels needed to run your home:
Solar panel sizing calculator
How many batteries are needed to power a house?
How many batteries would you need to power the average home?
If your home consumes 30kWh every 24 hours and you want to have 24 hours autonomy, then you can easily work out how many batteries you would need.
There are two choices for home energy storage batteries – lead-acid deep-cycle or lithium iron phosphate. I prefer LiFeP04 but as lead-acid are still the most common, I’ll use them in an example.
Deep-cycle batteries can be discharge up to 80% of their capacity, but 50% is recommended in order to maximize battery life.
This means that we need to double the amount of energy required from the batteries, simply beacuse half of it isn’t normally available. 80% Depth of Discharge is OK from time to time.
The calculation looks like this:
Energy for 24 hours = 30kWh
30kWh in battery amp-hours (Ah) = 2500Ah (12V batteries)
Total battery capacity needed for 24 hours = 2500Ah x 2 = 5000Ah
25 batteries rated at 200Ah will be required to power this home for 24 hours.
How many solar panels do I need for a 2000 sq ft home?
An average 2,000 square foot home consumes about 950 kWh of energy per month or about 30 kWh per day.
The average electricity bill is 0.6 cents per square foot.
About 30 solar panels each rated at 300 watts would be needed to power a 2000 sq.ft. home.
How many solar panels do I need for 5000 watts?
You light think it’s obvious, but the answer isn’t as simple as it sounds. If we propose to use 300 watt solar panels then in theory you would divide the power required by 300, giving 16.6, or 17 panels if rounded up.
Unfortunately, the real world isn’t like that!
Solar panels are rated at 300 watts, but this is the very best you can expect in the laboratory, or Standard Test Conditions (STC), which specifies an irradiance level of 1000 watts per square meter, among other things.
Apart from the difference in irradiance, all solar systems expereince losses of about 23%, which means that you need to multiply the amount of solar panel rated power to account for this loss. It’s the same as multiplying total power by 1.44:
- 5000 watts x 1.44 = 7000 watts
- Number of solar panels needed = 23 panels each rated at 300 watts
What size solar system do I need to run a fridge?
The average size refrigerator consumes about 350kWh each year, but older ones definitely consume more, so add 5% for each year of use. Modern fridges are very efficient and don’t cycle so much. This is important, as compressor motor surge current pays a big part in driving up the energy consumed.
How many solar panels does a fridge need?
A 100 watt solar panel generates 400 watts per day at an average irradiance of 4 peak-sun-hours. This is equivalent to 146kWh for the year.
- Annual energy consumption 350kWh/146kWh = 2.4 solar panels (100 watt rating)
Always round-up to the highest whole number, so 3 solar panels would be needed. I would use 4 solar panels, as they are relatively cheap nowadays – it takes care of poor power generation on cloudy days!
You’ll need an inverter, which should be as close to 300 watts as you can make it (400 watts if you go for 4 panels.) This is because inverters are most efficient when fully loaded. You need to be as efficient as possible overall, because sometimes the inerter will be very lightly loaded when the fridge compressor isn’t running.
You will also need a 100Ah deep-cycle battery, preferable Lithium Phosphate, to run the fridge through the night when the solar panels don’t provide power.
Do solar panels work on a flat roof?
Solar panels do work on a flat roof but don’t generate as much power as when mounted at the optimum angle for the location. However, the loss in power is not as great as you might imagine.
Flat roof solar panel calculator
The table below shows the monthly irradiance difference throughout the year in Phoenix, Az. Solar panels at a fixed angle of 57 degrees will generate just 10% more power than a flat installation. The amount of power difference between flat/angled panels will be different according to the location’s irradiance.
|
Irradiance for Phoenix Arizona by month (kWh/m2/day – Peak-Sun-Hours) |
|||||||||||||
|
|
Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
Year |
|
Flat |
3.2 |
4.0 |
5.4 |
6.6 |
7.3 |
7.5 |
6.7 |
5.9 |
5.4 |
4.4 |
3.5 |
2.9 |
63 |
|
57 ° |
4.9 |
5.5 |
6.4 |
6.8 |
6.7 |
6.5 |
6.0 |
5.7 |
6.0 |
5.8 |
5.2 |
4.7 |
71 |
How long does it take for solar panels to pay for themselves?
Solar panel payback period varies according to your location, as the sun’s energy is stronger in some locations. It also depends on energy consumption and your utility prices.
The example below shows the payback period for an installation in Houston, Tx:
Solar Payback Period Calculation For 5kW Solar Power In Houston, Texas (2021)
How long before solar pays for itself in Houston, Texas?
- Location: Houston, Tx
- Solar system power rating: 5kW
- Solar installation cost = $13700 – 26% tax credit = $10138
- Peak-sun-hours in Houston = 1552/year
- Power generated by solar system = 1552 x 5kw = 7760 kWhrs
- Houston home electricity price = 10.98 cents/kWh
- Yearly savings = solar generation x unit elec. cost = 7760 x 10.98 = $852
- Time to pay back solar installation = install cost/annual savings = 10138/852 = 11.9 years
The payback time might be less than the above – 9 to 10 years is possible. Professional solar installers can provide very accurate estimates using historical data for the irradiance in your city and the orientation and tilt angle of your proposed installation.
Key takeaways about calculating solar panel system size – solar calculator
- Sizing PV panels always starts with how much energy the home consumes
- Use the monthly average of last year’s utility bills to find energy that needs replacing with PV systems
- As a general rule, a 5000 kw system is adequate for the average American home
- The installed price of a solar energy system can almost double is a battery bank is needed
- Use your old electric bill to identify how you can reduce electricity usage to save on solar costs
- How much electricity is used by air conditioning? It’s a good idea to install soft-starts to spread the load (National Renewable Energy Laboratory)
- Next step is to know how many hours of sunlight per day i.e. how much solar energy can you generate per square foot of solar panels with the average solar radiation levels in your specific location?
- Peak sunlight hours are very important for calculating solar size. Sunny days come and go, so average solar hours across the seasons is the figure to use
- What is your roof area in square feet? This factor and the amount of sunlight determines the size of your solar PV system and the kw output
- Average peak sun hours and the solar system size gives you a rough estimate of how much solar power you will have available
Resources relating to home solar power system sizing and solar calculations:
What Size Solar System Do You Actually Need?
How Many Solar Panels Do You Need: Panel Size and Output
How Many Solar Panels Do I Need For My Home? | EnergySage
Solar Panel Calculator – How Many Solar Panels Do I Need Calculator
