Solar Panel Calculator
Enter what you use and how much sun you get, and this sizes the array in kilowatts and in panels. Both of those local numbers are fields, because they are yours rather than ours.
System size
8.00kW
- Panels
- 20
- Array size
- 8.00 kW
- Generates
- 930 kWh/month
- Roof area
- 420 sq ft
- Installed cost
- $24,000.00
- Peak sun hours is not hours of daylight. It is the day's total sunlight expressed as that many hours at full rated intensity, which is why somewhere with fourteen hours of summer daylight still counts four or five.
- The derate covers everything between the panel's laboratory rating and your meter — the inverter, heat, wiring, dust and shade. Panels are rated at 25°C and a roof in July is a great deal hotter.
- Cost per watt is what your installer quotes, before any credit or rebate. It varies enormously by region and by installer, so use a quote rather than the starting figure here.
- Roof area is the panels alone. A real array needs setbacks from the edges and a path for firefighters, so allow more than this.
About the solar panel calculator
The whole calculation turns on peak sun hours, and it is the number most often misunderstood. It is not hours of daylight. It is the day's total solar energy expressed as that many hours at full rated intensity — a thousand watts a square metre, the condition panels are rated under. That is why a place with fourteen hours of summer daylight still counts four or five peak sun hours: the early and late sun is real but weak, and it adds up to less than the clock suggests.
The derate factor covers everything between the panel's laboratory rating and your meter. Inverter losses, wiring, dust, snow, shading, and heat — panels are rated at 25°C and lose output as they get hotter, which a roof in July certainly does. Eighty-five per cent is a reasonable general figure; a shaded or hot roof is worse and a cool, clean, unshaded one is better.
Panels come in whole units, so the array is always slightly larger than the target. Nineteen and a bit panels is twenty panels, which at 400 watts each is an 8 kW array generating a little more than asked for. That is the right way round to be wrong.
Cost per watt is a field rather than a figure published here, because it varies enormously by region and installer and moves constantly. Put your own quote in. The result is the installed cost before any credit, rebate or tariff, none of which are in this calculation either.
What it works out
- Array size in kilowatts and in panels
- Roof area the panels themselves take
- What the array actually generates each month
- Installed cost at your own price per watt
- Partial offsets, if you only want to cover some of your usage
The formula
Array kW = (Monthly kWh ÷ 30.4) ÷ (Peak sun hours × Efficiency)
Turn the monthly figure into a daily one first. A month is taken as 30.4 days — the year divided by twelve — so that a monthly answer and an annual one agree with each other. Nine hundred kilowatt hours a month is 29.6 a day.
Then divide by what one kilowatt of array produces in a day, which is the peak sun hours multiplied by the efficiency. At 4.5 sun hours and 85 per cent, a kilowatt of panels gives 3.83 kilowatt hours a day. So 29.6 divided by 3.83 is a 7.74 kilowatt array.
Panels come in whole units. At 400 watts each, 7.74 kilowatts is 19.35 panels, which is twenty — an 8 kilowatt array producing about 930 kilowatt hours a month rather than the 900 asked for.
Roof area is the panels alone, at about 21 square feet each. A real array needs setbacks from the roof edges and a clear path for firefighters on top of that, so the roof has to be meaningfully larger than the figure here.
- Monthly kWh
- From your electricity bill. Use a whole year and average it if you can.
- Peak sun hours
- The day's sunlight as hours at full rated intensity. Not hours of daylight.
- Efficiency
- The derate: inverter, heat, wiring, dust and shade. 85 per cent is a fair default.
- Panel wattage
- The panel's rated output under test conditions.
- 30.4
- Days in an average month, so monthly and annual figures agree.
A worked example
A house using 900 kWh a month, at 4.5 peak sun hours, in 400 watt panels at three dollars a watt installed.
That works out to 8.00 kW.
- Panels
- 20
- Array size
- 8.00 kW
- Generates
- 930 kWh/month
- Roof area
- 420 sq ft
- Installed cost
- $24,000.00
Questions
How many solar panels do I need for a 900 kWh a month house?
Twenty 400-watt panels — an 8 kilowatt array — at 4.5 peak sun hours and 85 per cent system efficiency. That generates about 930 kWh a month, slightly more than the target, because panels come in whole units.
What are peak sun hours?
The day's total solar energy expressed as hours at full rated intensity — one kilowatt a square metre, the condition panels are tested at. It is not hours of daylight. Fourteen hours of summer daylight is typically four to six peak sun hours, because early and late sun is weak.
How do I find the peak sun hours where I live?
The US National Renewable Energy Laboratory publishes it, and its PVWatts tool gives a figure for any address. Across the US the range is roughly 3.5 in the Pacific Northwest and the Northeast to 6 in the Southwest.
What is the derate or efficiency factor?
Everything lost between the panel's laboratory rating and your meter: the inverter, wiring resistance, dust and snow, shading, and heat. Panels are rated at 25°C and produce less as they warm up, which a roof in summer does considerably. Eighty-five per cent is a reasonable general figure.
How much roof space do I need?
About 21 square feet a panel, so twenty panels is 420 square feet of panel. The roof itself needs to be larger — codes require setbacks from the edges and ridge for roof access, and any chimney, vent or dormer breaks up the usable area further.
Should I size the array to cover all my usage?
It depends entirely on how your utility treats exported power. Where net metering credits exports at the retail rate, covering all of it makes sense. Where exports are credited at a much lower rate, a smaller array sized to what you use during daylight often pays back faster. Set the share to cover to model that.
Does this include batteries?
No. This sizes generation against consumption over a month. Battery sizing is a separate question about how much you want to shift from day to night, or how long you want to run through an outage, and it does not change the array size.
Why is cost per watt something I have to enter?
Because it moves constantly and varies enormously by region, installer and system type. Any figure published here would be wrong somewhere and out of date everywhere within a year. Put your own quote in and the result is a real number rather than a national average.
Does the cost include tax credits?
No — it is the installed cost before any credit, rebate or incentive. Those vary by country, state and year, and are exactly the sort of figure that would be out of date the moment it was written down.
Will my array generate the same amount every month?
No. Peak sun hours vary through the year, typically by a factor of two between summer and winter. The figure here is an annual average, so summer will run well above it and winter well below.