Solar & battery
Solar Power Calculator
Works instantaneous power against energy, which are different quantities.
Result
Covering a load on a daily total is not the same as running it. Solar produces in the middle of the day and a load running at night needs storage or the grid, whatever the daily arithmetic says.
Get a Free Business AuditHow the calculation works
Power in kilowatts is the rate at the moment; energy in kilowatt hours is power accumulated over time. An array's rating is a power figure and a utility bill is an energy figure, and conflating the two is the most common confusion in solar arithmetic.
What moves the number
- Array rating in kilowatts.
- Hours of useful production, which varies by season.
- Load power against load energy.
- Inverter capacity, which limits instantaneous output.
Where this one goes wrong
Sizing an array against a load's power rating rather than its energy consumption produces a wildly wrong answer in either direction, because a high-power appliance used briefly consumes little energy while a modest continuous load consumes a great deal. Work in energy for sizing and in power for equipment limits. The inverter's capacity is the one place power genuinely binds, since it caps the instantaneous output regardless of what the array could produce.
Before you quote
- Size from energy, not from power ratings.
- Use power only for equipment and circuit limits.
- Check the inverter cap against peak array output.
Questions
About this calculator
What is the difference between kW and kWh?
Kilowatts are a rate and kilowatt hours are an amount: power accumulated over time. An array's rating is a power figure while a utility bill is energy, and conflating them is the most common error in solar arithmetic.
Can I size an array from appliance ratings?
Not usefully, because a high-power appliance used briefly consumes little energy while a modest continuous load consumes a lot. Sizing needs energy consumption over time, which is what the utility bill already provides.
Where does power actually matter?
At equipment and circuit limits, particularly the inverter, which caps instantaneous output regardless of what the array could produce. It also matters for battery and backup design, where the question is what can run at once rather than how much energy is available.
The formula
daily production = kW x peak sun hours x derate; daily load = watts / 1000 x hours
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- 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%.
- Continuous load (watts). Enter your own figure; the page starts at 1,200.
- Hours that load runs per day. Enter your own figure; the page starts at 10.
The result updates as you type, so you can try a figure, see what it does to daily production, 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 |
|---|---|
| System size (kW DC) | 7.2 |
| Peak sun hours per day | Mid-Atlantic, Ohio Valley (4.5) |
| System derate | 80% |
| Continuous load (watts) | 1,200 |
| Hours that load runs per day | 10 |
What comes out
| Result | Value |
|---|---|
| Daily production main result | 25.9 |
| Daily load | 12.0 |
| Surplus or shortfall | 13.9 |
| Share of the load covered | 216% |
| Peak output | 5,760 |
Production covers the load with 13.9 kWh a day to spare, 216% of what it needs.
How sensitive the answer is
Holding everything else steady and changing only system size (kw dc), this is what happens to daily production. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| System size (kW DC) | Daily production |
|---|---|
| 3.6 | 13.0 |
| 5.4 | 19.4 |
| 7.2 example above | 25.9 |
| 10.8 | 38.9 |
| 14.4 | 51.8 |
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
- System size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- 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%.
- Continuous load (watts)
- A number, between 0 and 500000, starting at 1,200.
- Hours that load runs per day
- A number, between 0 and 24, starting at 10.
What the results tell you
| Result | What it is |
|---|---|
| Daily production | The headline result. A decimal figure. |
| Daily load | A decimal figure. |
| Surplus or shortfall | A decimal figure. |
| Share of the load covered | A percentage. |
| Peak output | 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.
Keep going
Related calculators
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.