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How to calculate how many solar panels needed for a house
This is the information you need to work out how many solar panels your home needs:
- Your home’s annual energy consumption
- The irradiance (sun’s energy) level for your location (AKA peak-sun-hours)
- What power rating in watts for the individual solar panels?
To run a house with energy consumption of 33kWh/day, 35 solar panels each rated at 300 watts would be needed if the average irradiance was 4.5 peak-sun-hours. The total calculated solar power rating needed will be 7333W. With solar PV system losses of 25% the actual total solar power required is 10566 watts.
Video – How many solar panels do I need to run my house?
How much energy does a typical home use?
US average home energy consumption is often quoted from government sources at about 900kWh/month or 30kWh/day. For the purposes of designing and sizing a solar panel system this value isn’t much use as it doesn’t take location into account.
Where you live is important for three reasons:
- Home energy consumption varies from state to state
- Solar power system output varies by irradiance according to geo-location
- Electricity costs (and therefore the solar payback period) varies by state
If installing an off grid or grid tie system on a newly built home, then you would be forced to add up all the appliances and estimate how many hours they would be used.
The power rating of appliances and the number of hours used will indicate how much solar power you require for your needs. This isn’t easy, particularly when dealing with fridges, freezers and AC units.
Table – Home energy consumption in 10 US States
State | Average kWh used per month | Ave kWh used per day | Solar required for 100% energy used (kW) |
Alabama | 1211 | 40 | 14.33 |
California | 557 | 18 | 6.59 |
Delaware | 944 | 31 | 11.17 |
Hawaii | 515 | 17 | 6.10 |
Louisiana | 1273 | 42 | 15.07 |
Maine | 551 | 18 | 6.52 |
Nebraska | 1034 | 34 | 12.24 |
New York | 602 | 20 | 7.13 |
Tennessee | 1245 | 41 | 14.74 |
Vermont | 569 | 19 | 6.73 |
How do I calculate solar panels for my home?
Home solar systems generally use larger solar panels between 200 watts and 300 watts. I’ll use 300 watts for my calculations.
Once you know how much energy your home needs each day, it’s easy to work out the number of panels you need. You need just one more thing …
A 300 watt solar panel will generate different amounts of power according to how much of the sun’s energy falls onto its surface. This is called ‘irradiance’ and is measured in kilowatt-hours per square meter per day or year (kWh/m2/day or year.)
It’s also known as peak-sun-hours and solar installers use it to calculate solar panel output levels. The value can be found in databases containing historical irradiance data for any location.
Example: Let’s assume a home in Greenville, Al using 32kWh of energy per day.
The image below shows a screenshot of Global Solar Atlas database for Greenville:
The peak-sun-hours for Greenville is 4.768.
The solar power needed to supply this home – calculation:
Solar power needed = 32000 watt-hours / 4.768 peak-sun-hours = 7611 watts or 7.611kW
How many solar panels needed for a 7611 watts solar system?
7611 solar watts / 300 watts = 25.37 (24) panels @300 watts each
So 24 panels should do it, in theory. Unfortunately, in the real world it’s a little different …
The losses associated with solar PV systems
All systems have losses and solar PV installations are not exception. The infographic below illustrates the average losses:
You can see from the image there are 10 main losses:
- Shading – 7%
- Thermal losses – 4.6%
- Dust and dirt – 2%
- Array mismatch – o.7%
- Reflection – 2.5%
- DC cable loss – 1%
- Spectral losses – 1&
- Inverter losses – 3%
- Irradiation – 1.6%
- AC cable losses – 0.5%
Some of the solar system losses we can’t change, while some we can improve a little. For example, cleaning the solar panels twice a year will reduce the losses due to accumulating dust and dirt.
Overall, it’s best to include the power lost by using a loss factor of 25% (round up from the average 23%), which is the same as multiplying the calculated power by 1.44.
This mean that you will only get 75% of the rated power output of a solar power system.
For our example of a home based in Greenville we would need to adjust the power requirements in the following way:
Actual power output = Calculated solar power 7611 watts x 1.4 loss factor = 10655 watts
Number of solar panels required = 35 panels @300 watts each
DIY solar panel calculator
Use the calculator below to work out how many solar panels you would need to power your home:
FAQ:
Can a house run on solar power alone?
A house running on solar power alone uses a type of solar power system known as ‘off-grid.’
The method for sizing solar panels is exactly the same, but extra panels may be required for battery charging during daytime hours. Batteries are required in there is no house connection to the utility grid.
The number of extra solar panels required will depend on the number of hours of autonomy needed from the batteries and how much energy the home uses, both in the day and at night.
How do I calculate how many solar panels I need?
- Find out how much energy your home used the previous year in kWh
- Divide the annual energy by 365 to find the daily energy requirements
- Use irradiance historical data website to find peak-sun-hours for your location
- Divide your home daily energy usage by peak-sun-hours to find total solar rating in watts
- Divide the total solar rating by the rating of each solar panel to find the number of panels
How long do solar panels last?
Solar panel manufacturers generally provide warranties for between 25 to 30 years for their fixed rigid panels.
Power output is guaranteed of around 90% is guaranteed after the warranty life (solar panel output reduces by about 0.8% each year.)
However, solar panels can go on generating electricity for up to 50 or 60 years, with progressively less output as the years pass by.
How many solar panels does it take to run a water pump?
A 1HP water pump is equivalent to 746 watts electrical. The actual watts it uses when running will depend on the load.
Normally, pumps are designed to run a little below their power rating, so I would expect such a pump to run comfortably at 500 watts.
So could you use 500 watts of solar panel to power it? Maybe, maybe not!
Pumps experience increased load when starting, call inrush or surge current, which can be several times more than it’s running current.
You could use 600 or 700 watts solar for a 1HP pump if you also used a battery to act as an energy storage reservoir to supply the extra current.
If not battery was going to be used, then it would be wise to use 1000 to 1200 watts of solar panels to make sure they can supply the pump motor surge current.
Is grid-tie better than off-grid solar?
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