Solar & battery
Solar Panel Calculator
Works out how many modules a target output needs, where the roof usually decides.
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. Installed cost per watt varies widely by market, roof complexity and whether the roof needs work first. It is a planning figure, not a quote.
Get a Free Business AuditHow the calculation works
Target array size divided by the per-module rating gives the module count, with the roof area that count requires. The constraint in practice is frequently area rather than the energy target, because usable roof is reduced by setbacks, obstructions, orientation and the parts of the roof that are shaded or face the wrong way.
What moves the number
- Per-module rating and physical dimensions.
- Usable roof area after setbacks and obstructions.
- Orientation of each roof plane.
- Shading across the day and the year.
Where this one goes wrong
Total roof area overstates what is usable by a wide margin once fire setbacks, vents, chimneys, skylights and the planes facing the wrong direction are removed, which is why a module count that fits on paper frequently does not fit on the roof. Measure the usable planes rather than the footprint. Higher efficiency modules buy area rather than energy per dollar, which matters when the roof is the binding constraint and not otherwise.
Before you quote
- Measure usable roof planes, not total roof area.
- Allow for fire setbacks and obstructions.
- Consider higher efficiency modules only when area is the constraint.
Where the rules come from
Questions
About this calculator
How much roof does a solar array need?
More than the module footprint, because fire setbacks, vents, chimneys and skylights all remove usable area, and roof planes facing away from the sun contribute little. Total roof area overstates what is usable by a wide margin.
Are high efficiency modules worth paying for?
When roof area is the binding constraint, since they fit more capacity into the same space. Where there is ample roof they generally cost more per kilowatt for the same energy, which makes the decision an area question rather than a performance one.
Does module count or array size matter more?
Array size in kilowatts determines the energy; the module count determines whether it physically fits and how the strings are configured. Both matter, and a design that optimises one without checking the other tends to fail at the roof.
The formula
panels = daily kWh needed / (panel kW x 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.
- Panel rating (watts). Enter your own figure; the page starts at 400.
- Peak sun hours per day. Enter your own figure; the page starts at Mid-Atlantic, Ohio Valley (4.5).
- System derate (%). Enter your own figure; the page starts at 80%.
- Installed cost per watt ($). Enter your own figure; the page starts at $2.9.
The result updates as you type, so you can try a figure, see what it does to panels needed, 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 |
| Panel rating (watts) | 400 |
| Peak sun hours per day | Mid-Atlantic, Ohio Valley (4.5) |
| System derate | 80% |
| Installed cost per watt | $2.9 |
What comes out
| Result | Value |
|---|---|
| Panels needed main result | 22 |
| System size | 8.8 |
| Each panel produces | 525.6 |
| Installed cost before incentives | $25,520 |
22 panels at 400 W, a 8.8 kW system. Each panel makes about 525.6 kWh a year, and the whole thing costs around $25,520 installed before any incentive.
How sensitive the answer is
Holding everything else steady and changing only installed cost per watt, this is what happens to panels needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Installed cost per watt ($) | Panels needed |
|---|---|
| $1.45 | 22 |
| $2.17 | 22 |
| $2.9 example above | 22 |
| $4.35 | 22 |
| $5.8 | 22 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Electricity used per month (kWh)
- A number, between 0 and 200000, starting at 950.
- Panel rating (watts)
- A number, between 0 and 1000, starting at 400.
- Peak sun hours per day
- Choose from Pacific Northwest, northern UK (3.5), Northeast, Midwest (4.0), Mid-Atlantic, Ohio Valley (4.5), Southeast, coastal California (5.0), Texas, inland California (5.5), Arizona, Nevada, New Mexico (6.0).
- System derate (%)
- A percentage, between 40 and 100, starting at 80%.
- Installed cost per watt ($)
- A money amount, starting at $2.9.
What the results tell you
| Result | What it is |
|---|---|
| Panels needed | The headline result. A number. |
| System size | A decimal figure. |
| Each panel produces | A decimal figure. |
| Installed cost before incentives | A money figure. |
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.
Keep going
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