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Solar Panel Calculator
Find out how big a solar system you need to cover your electricity use, how many panels that takes and how much roof space they occupy. Enter the kWh from your bill and your local peak sun hours; everything else has sensible defaults.
Your electricity use
kWh
Average from your bills; the US average home uses about 850–900 kWh.
%
Site and panels
h
Yearly average of full-sun-equivalent hours, roughly 3 to 6.
%
Inverter, wiring, heat, dirt and shading. 14% is a common default.
W
sq ft
A 400 W panel is about 1.7 m × 1.1 m ≈ 1.9 m² ≈ 20 sq ft (1 m² = 10.76 sq ft).
Optional cost
$
$
Leave blank to skip the cost estimate.
Enter your values to see the result.
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How to use the Solar Panel Calculator
- Enter your monthly electricity use in kWh from your power bill (a 12-month average is best).
- Enter your peak sun hours per day for your location.
- Set the panel wattage and panel area, or keep the 400 W defaults.
- Adjust losses and the share to cover if needed.
- Add your electricity price and cost per watt to see the yearly value and simple payback.
Solar sizing formulas
- Daily use = monthly kWh × 12 ÷ 365
- System size (kW) = daily kWh × offset ÷ (peak sun hours × (1 − losses))
- Panels = system size in W ÷ panel W, rounded up
- Yearly production = installed kW × sun hours × 365 × (1 − losses)
- Panel area = number of panels × area of one panel
Panels needed for common usage levels
With 400 W panels, 14% losses and 100% offset:
| Monthly use | System at 4 sun hours | Panels | System at 5 sun hours | Panels |
|---|---|---|---|---|
| 300 kWh | 2.87 kW | 8 | 2.29 kW | 6 |
| 500 kWh | 4.78 kW | 12 | 3.82 kW | 10 |
| 750 kWh | 7.17 kW | 18 | 5.73 kW | 15 |
| 900 kWh | 8.60 kW | 22 | 6.88 kW | 18 |
| 1,200 kWh | 11.47 kW | 29 | 9.17 kW | 23 |
| 1,500 kWh | 14.34 kW | 36 | 11.47 kW | 29 |
Tips
- Use 12 months of bills: air conditioning or electric heating can make some months much higher.
- Plan for future loads such as an electric car or heat pump by raising the offset above 100%.
- Your local solar resource and real losses can be checked with a modeling tool such as NREL PVWatts.
Frequently asked questions
How do I calculate how many solar panels I need?
Divide your daily electricity use by your peak sun hours and by (1 − losses) to get the system size in kW, then divide by the panel wattage in kW and round up. Example: 900 kWh/month is 29.6 kWh/day; 29.6 ÷ (4.5 × 0.86) = 7.65 kW; 7,650 W ÷ 400 W = 19.1, so 20 panels.
What are peak sun hours?
Peak sun hours are the number of hours per day the sunlight would need to shine at 1,000 W/m² to deliver the same daily solar energy. Most locations average between about 3 and 6 per day over a year, depending on latitude and climate.
What system losses should I use?
Losses cover inverter efficiency, wiring, panel temperature, dirt, mismatch and minor shading. 14% is the default in NREL's PVWatts calculator. Use more if your panels are partly shaded or face away from the sun.
How much roof space does a solar panel take?
A typical residential panel of 350–450 W measures about 1.7 m × 1.1 m, around 1.9–2 m² or 20–22 sq ft. Fire codes and racking need extra space for walkways and edges, so the usable roof area must be larger than the panel area.
Does panel direction matter?
Yes. In the Northern Hemisphere, south-facing roofs produce the most energy; east or west faces produce somewhat less. Steep or very flat tilts and shading also reduce output. This estimate assumes a reasonably good orientation.
Is this estimate accurate enough to buy a system?
It is a planning estimate. Production varies by season, weather, tilt and shading, and prices vary widely. Get a site-specific design and quotes from installers before buying.
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