Solar Panel Calculator – How Many Panels Do I Need? | CalcLoft
Find out how many solar panels you need for your monthly electricity bill — including panel wattage, peak sun hours and a yearly savings estimate.
How many solar panels do I need?
Solar panels are usually sized against your monthly electricity usage in kWh. The calculator above takes that number, your panel wattage and the average peak sun hours for your area, then works out how many panels a typical grid-tied system needs — including an estimated yearly output and yearly savings.
How the panel count is calculated
First, estimate what one panel produces in a day:
Daily output per panel = (panel watts ÷ 1,000) × peak sun hours × 0.8
The 0.8 factor covers typical system losses such as inverter efficiency, wiring, heat and dirt. A 400W panel at 4.5 sun hours therefore makes 0.4 × 4.5 × 0.8 = 1.44 kWh per day.
Then divide your daily usage by that figure:
Panels needed = (monthly kWh ÷ 30) ÷ daily output per panel, rounded up. Example: 1,000 kWh per month is about 33.3 kWh per day; 33.3 ÷ 1.44 ≈ 23.2, so you round up to 24 × 400W panels — a roughly 9.6 kW system.
Monthly usage vs. panels needed (400W panels, 4.5 sun hours)
| Monthly usage | Panels (400W) | System size |
|---|---|---|
| 500 kWh | 12 | ≈ 4.8 kW |
| 800 kWh | 19 | ≈ 7.6 kW |
| 1,000 kWh | 24 | ≈ 9.6 kW |
| 1,500 kWh | 35 | ≈ 14 kW |
| 2,000 kWh | 47 | ≈ 18.8 kW |
Tips before you buy solar
- Cut usage first. LEDs, insulation and efficient HVAC shrink the system you need — and the upfront cost.
- Mind orientation and shade. South-facing roofs with a clear sky perform best; east–west, north-facing or partly shaded roofs need more panels for the same output.
- Plan about 18 sq ft per panel. A typical panel measures roughly 5.5 × 3.25 ft, and racking plus access space takes the total to about 18 sq ft each.
- Use a 12-month average from your electric bill — summer AC and winter heating spikes make single months misleading.
Related guides
Solar panel calculator FAQ
How many solar panels do I need for 1,000 kWh per month?
About 24 × 400W panels — a roughly 9.6 kW system — at 4.5 peak sun hours per day. Fewer sun hours, shading or a higher bill all mean more panels; use the calculator above with your own numbers.
What if my area doesn't get much sun?
Enter your local peak sun hours and the math adjusts itself. A 400W panel makes about 1.44 kWh per day at 4.5 hours, 1.12 kWh per day at 3.5 hours, and 1.6 kWh per day at 5 hours. Divide your monthly kWh by 30, then by the per-panel daily output, to get the real count for your location.
How many solar panels fit on my roof?
Plan on about 18 sq ft per panel including mounting gaps — typical modules are roughly 5.5 × 3.25 ft. Measure the usable, shade-free roof area (usually the south-facing side) and divide by 18. For example, 700 sq ft fits about 38 panels, which at 400W is a ~15.2 kW system.
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