Solar Panel Count Calculator
Enter your energy target and local sun hours to get the required array Wp, the number of panels and the rough roof area needed.
Open calculatorLast updated: 2026-09-15
How the calculation works
- The target energy divided by (sun hours × derate) gives the array wattage that produces it on an average day.
- Panels round up to whole units; the installed size and monthly output follow.
Worked example
Interpreting the result
Use the worst-month PSH if the target is a hard requirement year-round; use the annual average if some seasonal variation is acceptable (grid-tied homes usually accept it). Roof space at ~6 m² per 400 W panel means 9 panels need ~54 m² (580 ft²) of usable roof — check shading and structural capacity next.
Assumptions
- Uniform panel orientation and no significant shading.
- Panel wattage at STC with the derate applied afterwards.
Limitations
- Does not account for inverter clipping when DC/AC ratio exceeds ~1.2.
- Roof area estimate excludes access walkways and setbacks required by code.
400 W panels needed by daily energy use and peak sun hours
| Daily use | 3 sun hours | 4 sun hours | 5 sun hours |
|---|---|---|---|
| 5 kWh/day | 6 | 4 | 4 |
| 10 kWh/day | 11 | 8 | 7 |
| 15 kWh/day | 16 | 12 | 10 |
| 20 kWh/day | 21 | 16 | 13 |
| 30 kWh/day | 32 | 24 | 19 |
Array size = daily energy ÷ (peak sun hours × system derate), with a derate of 0.80 here.
Worked scenario: 10 kWh per day at 3.5 peak sun hours
The array has to be 3,571 Wp, which means 9 panels of 400 W (3.6 kW installed). Averaged over a month that produces about 307 kWh.
Mistakes to avoid
- Confusing peak sun hours with hours of daylight. Peak sun hours are the equivalent hours at full intensity and are much fewer.
- Leaving out system losses. The derate covers wiring, inverter, temperature and soiling.
- Ignoring roof area, orientation and shading, which can limit how many panels fit or how well they perform.
Frequently asked questions
How many solar panels for 30 kWh per day?
At 4 PSH and 0.8 derate: 30,000/(4 × 0.8) = 9,375 W ≈ 24 panels of 400 W. Roughly 6 m² each, so ~145 m² of roof.
Should I use worst-month sun hours?
For off-grid or hard energy requirements, yes. Grid-tied systems that can draw from the grid can size to the annual average and accept winter shortfalls.