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.
Last 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.
Formula
Wp = daily kWh × 1000 ÷ (PSH × derate) Panels = Wp ÷ panel W, rounded up
| Symbol | Meaning | Unit |
|---|---|---|
Wp | Required array peak wattage | W |
PSH | Peak sun hours | h/day |
Worked example
10 kWh/day target, 400 W panels, 3.5 PSH worst month, 0.8 derate: Wp = 10,000/(3.5 × 0.8) = 3,571 W → 9 panels = 3.6 kW. Monthly average: ~3.6 × 3.5 × 30.4 × 0.8 = 307 kWh/month.
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.
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.