Solar & battery
Solar System Size Calculator
Sizes an array from annual consumption, where the derate factor does the real work.
Result
System derate covers inverter losses, wiring, heat, soiling and panel mismatch. Around 0.80 is the figure most design tools use; a hot climate, a dusty site or a shaded roof pushes it lower. It is why a system never produces the sum of its panel ratings. Peak sun hours are not daylight hours. They are the day's total solar energy expressed as hours at 1,000 W per square metre, so a long overcast day can be worth fewer peak sun hours than a short bright one. Use a figure for your own location and month rather than a national average. Roof area assumes about 21 square feet per panel with no allowance for setbacks, obstructions or fire access pathways, all of which local code requires and which reduce usable roof substantially.
Get a Free Business AuditHow the calculation works
Annual energy consumption divided by the energy one kilowatt of array produces in a year at the site gives the array size. That per-kilowatt figure is peak sun hours times days times a system derate that accounts for temperature losses, soiling, inverter conversion, wiring and module mismatch, which together remove a meaningful share of what the nameplate rating suggests.
What moves the number
- Annual consumption in kilowatt hours, from actual bills.
- Peak sun hours at the site, which is location specific.
- System derate, which is where most of the error lives.
- Orientation and shading, which reduce production further.
Where this one goes wrong
Sizing from nameplate without a derate produces a system that falls short of the target by the whole loss factor, and the losses are real rather than conservative padding: cell temperature alone costs a substantial share on a hot roof. Use twelve months of actual consumption rather than an estimate, since households differ enormously, and check whether consumption is about to change through an electric vehicle, a heat pump or a new occupant, because sizing to last year's bills on a house whose load is about to double is the other common error.
Before you quote
- Use twelve months of actual metered consumption.
- Apply a realistic derate, not the nameplate rating.
- Ask whether the load is about to change.
Where the rules come from
Questions
About this calculator
Why does a 10 kW array not produce 10 kW?
Because the rating is measured at standard test conditions with a 25 degree cell temperature and a defined irradiance, which a real roof rarely provides. Temperature, soiling, inverter conversion, wiring and mismatch losses all reduce it, and together they are substantial.
What consumption figure should I size from?
Twelve months of actual metered usage, because households vary enormously and a single month or a national average will be wrong. Utility bills or the utility's online data give the real figure and it takes minutes to collect.
Should I size for future loads?
If they are known and imminent, yes. An electric vehicle or a heat pump can change consumption substantially, and sizing to last year's bills on a house whose load is about to rise produces a system that disappoints within a year of installation.
The formula
kW = daily kWh needed / (peak sun hours x derate)
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How to use this calculator
- Electricity used per month (kWh). Enter your own figure; the page starts at 950.
- Peak sun hours per day. Enter your own figure; the page starts at Mid-Atlantic, Ohio Valley (4.5).
- Share of the bill to cover (%). Enter your own figure; the page starts at 100%.
- System derate (%). Enter your own figure; the page starts at 80%.
- Panel rating (watts). Enter your own figure; the page starts at 400.
The result updates as you type, so you can try a figure, see what it does to system size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Electricity used per month (kWh) | 950 |
| Peak sun hours per day | Mid-Atlantic, Ohio Valley (4.5) |
| Share of the bill to cover | 100% |
| System derate | 80% |
| Panel rating (watts) | 400 |
What comes out
| Result | Value |
|---|---|
| System size main result | 8.7 |
| Panels needed | 22 |
| Annual production | 11,563 |
| Daily production | 31.7 |
| Roof area required | 462 |
You need about 8.7 kW, which is 22 panels at 400 W and roughly 462 square feet of unshaded roof. That produces around 11,563 kWh a year.
How sensitive the answer is
Holding everything else steady and changing only electricity used per month (kwh), this is what happens to system size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Electricity used per month (kWh) | System size |
|---|---|
| 475 | 4.3 |
| 713 | 6.5 |
| 950 example above | 8.7 |
| 1,425 | 13.0 |
| 1,900 | 17.4 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Electricity used per month (kWh)
- A number, between 0 and 200000, starting at 950.
- Peak sun hours per day
- Choose from Pacific Northwest, northern UK (3.5), Northeast, Midwest (4.0), Mid-Atlantic, Ohio Valley (4.5), Southeast, coastal California (5.0), Texas, inland California (5.5), Arizona, Nevada, New Mexico (6.0).
- Share of the bill to cover (%)
- A percentage, between 0 and 150, starting at 100%.
- System derate (%)
- A percentage, between 40 and 100, starting at 80%.
- Panel rating (watts)
- A number, between 0 and 1000, starting at 400.
What the results tell you
| Result | What it is |
|---|---|
| System size | The headline result. A decimal figure. |
| Panels needed | A number. |
| Annual production | A number. |
| Daily production | A decimal figure. |
| Roof area required | A number. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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