Home energy
Solar panel counts: separate daily kWh, nameplate kW and usable storage
“How many panels?” sounds like one question. It usually contains a daily energy target, a site-production assumption, a panel power rating, and sometimes a separate storage requirement. Keeping those quantities distinct makes the estimate easier to inspect.
Turn an energy target into a power scenario
Take an illustrative target of 20 kWh per day, 4 peak sun hours and 20% overall production loss. Required DC nameplate power is 20 ÷ (4 × 0.8), or 6.25 kW. Dividing by 0.4 kW per panel gives 15.625, so the quantity rounds up to 16 panels.
Those 16 panels have 6.4 kW of nameplate capacity. That is a power rating, not a promise of 6.4 kWh per day or any other measured energy output. Peak sun hours are also not the same as the number of hours between sunrise and sunset.
Size storage as a separate energy calculation
For one illustrative day of 20 kWh demand, an 80% usable share and 90% storage delivery efficiency imply 20 ÷ (0.8 × 0.9), about 27.778 kWh of nominal storage. The production-loss input and storage-delivery input apply to different parts of this simplified calculation.
Daily energy capacity does not settle inverter output, appliance startup loads, allowable battery current or how loads overlap. A battery can contain enough energy for a day while being unable to supply a particular instantaneous load.
Ask where the sunlight assumption came from
The tool accepts your peak sun hours and loss assumptions. It does not infer them from a country, postcode or photograph. Site weather, orientation, shade, temperature, technology and season can affect production.
Use a suitable site model and professional review when comparing a real system. PVWatts itself explains that its estimates depend on assumptions and do not capture every site characteristic. Our simpler arithmetic should not be presented as a more precise site forecast.
Keep the unresolved installation decisions visible
Structural suitability, electrical design, permits, protective equipment, grid rules, export tariffs, financing, tax incentives and reliable off-grid operation require separate evidence. This page establishes none of those.
Keep several clearly labeled energy scenarios beside a normalized quote comparison. Change one assumption at a time, retain the source and date, and obtain qualified advice before ordering an installable system.
Primary references
Authored 2026-10-29. Arithmetic examples are covered by local regression tests. New workflow guides remain outside the indexing allowlist pending editorial and applicable specialist review. Examples use the assumptions stated here; authority guidance can change.