What this calculator does
Once you know how much energy you need each day, the array follows from the sun you actually get. This calculator divides the daily demand by the peak sun hours and a derating factor, then rounds up to a whole number of modules, which is the figure every installer will ask you for.
How to use it
- Enter the daily energy demand from the load audit.
- Enter the peak sun hours for your site — 4 to 5 in most of Europe, 5 to 6 in North Africa and the Gulf.
- Set the derating factor; 14 % is the PVWatts default for a fixed tilted array with soiling, temperature and inverter losses.
- The result rounds up to a whole number of modules of the size you entered.
Governing formula
P_array = E_day / (PSH × (1 − d)) Where:
-
E_day= Daily energy demand [Wh] -
PSH= Peak sun hours [h/day] -
d= Derating fraction [–]
Derived Equations:
n = ⌈P_array / P_module⌉ Worked example — 7.4 kWh a day with 4.5 peak sun hours
The figures below were produced by running this page's own engine with the example inputs (Daily energy demand (Wh), Peak sun hours per day (h), System derating (%), Module rating (W)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Array size | 2000 W |
|---|---|
| Number of modules | 5 no |
| Expected daily production | 7740 Wh |
| Unrounded array size | 1912 W |
| Assessment | 5 × 400 W panels (2.0 kW array) produce ≈ 7.74 kWh/day in 4.5 peak-sun-hour conditions. |
- After derating, each 400 W module yields about 1,548 Wh a day.
- 7,400 / 1,548 needs 4.78 modules, so 5 modules are needed.
- The rounded 2,000 W array produces about 3,096 Wh a day.
Assumptions & limitations
Engineering calculation. Array size from daily energy, peak sun hours and a system derating factor. It ignores shading analysis, array-to-battery voltage window and winter design month.
- No shading assessment — even partial shading can cost 30 % or more and must be checked with a shading analysis.
- No winter design month, so a system sized on annual average may not meet demand in December.
- Assumes batteries carry the difference between generation and load; without storage the array must meet the daytime load directly.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
Schedule mode — module options
Module choice changes how many panels you need and how much they actually deliver. Enter the modules you are weighing at the same site and demand and compare the daily production directly.
The schedule totals are the sum of every item's own engine result, so they match running this calculator once per item. An item the engine rejects is reported in place and contributes nothing to the totals.
- Options are compared only at the peak-sun-hours value entered above. A site with seasonal variation is not represented by a single figure.
- A larger array is not automatically better: the extra production may exceed the load and be curtailed or exported at a value this tool does not assess.
References & standards
- NREL PVWatts Calculator — system loss assumptions.
- Global Solar Atlas — peak sun hours by location.