How many solar panels a load actually needs
From appliance energy to module count, with the derating that stops an array from under-delivering.
The engineering question
How many panels are needed to run these appliances? What does the derating figure actually account for?
Worked example
The figures below were produced by running the Solar Array Sizing Calculator with the same inputs — during the build, from its authoritative engine.
| 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. |
- Energy per module is its rating times peak sun hours, after derating.
- The module count is the daily demand divided by that yield, rounded up.
- Derating covers soiling, cell temperature, inverter losses and mismatch — not shading, which must be assessed separately.
Sanity check
If shading is present, annual output can fall by 30 % or more. A derating figure cannot substitute for a shading study.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- NREL PVWatts — system loss assumptions
- Global Solar Atlas — peak sun hours
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