Quick answer: Divide your daily electricity use by peak sun hours × 0.8 to get the system size in kW, then divide by the panel wattage. A home using 900 kWh per month with 4.5 peak sun hours needs about 8.2 kW, or 21 panels of 400 W.
The number of solar panels a home needs depends on three things: how much electricity you use, how much sun your roof gets and how powerful each panel is. Here is how to estimate it from your utility bill.
Step 1: Find Your Electricity Use
Look at your utility bill for your monthly usage in kilowatt-hours (kWh). Use the average over a full year, because summer and winter use can be very different. Divide by about 30.4 to get daily use.
Step 2: Find Your Peak Sun Hours
Peak sun hours are the equivalent number of hours per day of full-strength sunlight (1,000 W per m²). They range from about 6–7 in the US Southwest to 4–4.5 in much of the Midwest and Northeast, and about 2.5–3 in the UK.
Step 3: Calculate System Size
System size (kW) = Daily kWh ÷ (Peak sun hours × 0.8)
The 0.8 accounts for about 20% of losses from the inverter, wiring, heat, dust and shading. For 900 kWh per month: 900 ÷ 30.4 = 29.6 kWh per day; 29.6 ÷ (4.5 × 0.8) = 8.2 kW.
Step 4: Count the Panels
Panels = System size (W) ÷ Panel wattage
8,220 W ÷ 400 W = 20.6, so 21 panels. With 430 W panels you would need 20.
Skip the math: enter your monthly usage, sun hours and panel wattage for the system size, panel count, roof area and yearly output. Open the free Solar Panel Calculator →
Solar Panels Needed by Monthly Usage
Assuming 4.5 peak sun hours, 20% system losses and 400 W panels:
| Monthly use | System size | Panels |
|---|---|---|
| 500 kWh | 4.6 kW | 12 |
| 750 kWh | 6.8 kW | 18 |
| 900 kWh | 8.2 kW | 21 |
| 1,000 kWh | 9.1 kW | 23 |
| 1,250 kWh | 11.4 kW | 29 |
| 1,500 kWh | 13.7 kW | 35 |
| 2,000 kWh | 18.3 kW | 46 |
How Much Roof Space?
A modern 400 W residential panel is roughly 1.7 × 1.1 m, or about 2 m² (21 sq ft). A 21-panel system needs about 42 m² (450 sq ft) of unshaded roof, plus space for setbacks required by fire codes in some areas. South-facing roofs produce the most in the Northern Hemisphere; east and west roofs produce less but still work well.
UK Example
A typical UK home uses about 2,700 kWh a year, or roughly 225 kWh a month. With about 2.7 peak sun hours, that needs a system of around 3.4 kW, or 9 panels of 400 W.
Other Things to Consider
- Plan for future loads such as an electric car or a heat pump.
- Batteries store daytime energy for evening use but do not change the panel count much.
- Incentives, tax credits and export rates change often and vary by location; check what is currently available.
- Solar installs require electrical work, permits and utility approval; use a certified installer.
- Re-roof first if your roof is near the end of its life. See how to measure your roof.
Frequently Asked Questions
How many solar panels do I need for 1,000 kWh per month?
With 4.5 peak sun hours, 20% losses and 400 W panels, about 23 panels (a 9.1 kW system).
How much does one solar panel produce?
A 400 W panel with 4.5 peak sun hours and 20% losses produces about 1.44 kWh per day, or about 525 kWh per year.
Can I run my whole house on solar?
Often yes, if you have enough unshaded roof space. Many homes are sized to cover 80–100% of annual use, with the grid or a battery covering nights.
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