Free calculator
How many solar panels do I need?
The average US home, 899 kWh a month (EIA), needs 17 solar panels of 400 W in Texas: 6.48 kW on about 352 sq ft of roof, at 5.3 peak sun hours and 14% losses. A medium UK home (2,500 kWh a year, Ofgem) needs 6 in London. Type your own bill and place to see your figures.
Result
- Panels of 400 W
- 17
- System size needed
- 6.48 kW
- Installed size (whole panels)
- 6.8 kW
- Roof area for the panels
- 352 sq ft
- Electricity use per day
- 29.6 kWh
- Production per year (installed size)
- 11,313 kWh
- Share of the yearly use covered
- 105%
Sized for Texas at 5.3 peak sun hours a day and 14% losses. Roof area counts the panels only (1.922 m² each, Trina Solar), not the gaps, walkways or the unusable edges of the roof. Estimates from published averages (NREL, EIA, Ofgem, industry cost surveys), each with its source and date: an installer's site survey and shading analysis replace them. Not financial advice.
Want it on paper? Pro prints the sizing report.
Inputs, system size, panels and roof area, production by month, cost, payback and the 25-year cash flow, with every source. Print, save as PDF or export to Excel.
How many solar panels for your monthly bill?
400 W panels to cover 100% of a monthly use from the cloudiest UK city to the sunniest US state, 14% losses. Each cell shows panels (system kW). A medium UK home uses about 208 kWh a month (Ofgem), the average US home 899 (EIA); the tool takes any figure and any state or city.
| kWh a month | Glasgow | London | Texas | Arizona |
|---|---|---|---|---|
| 200 kWh | 7 (2.7 kW) | 6 (2.2 kW) | 4 (1.4 kW) | 3 (1.2 kW) |
| 300 kWh | 10 (4 kW) | 9 (3.3 kW) | 6 (2.2 kW) | 5 (1.8 kW) |
| 500 kWh | 17 (6.6 kW) | 14 (5.5 kW) | 10 (3.6 kW) | 8 (2.9 kW) |
| 750 kWh | 25 (10 kW) | 21 (8.2 kW) | 14 (5.4 kW) | 12 (4.4 kW) |
| 1,000 kWh | 34 (13.3 kW) | 28 (11 kW) | 19 (7.2 kW) | 15 (5.9 kW) |
| 1,500 kWh | 50 (19.9 kW) | 42 (16.5 kW) | 28 (10.8 kW) | 23 (8.8 kW) |
| 2,000 kWh | 67 (26.5 kW) | 55 (22 kW) | 37 (14.4 kW) | 30 (11.8 kW) |
How it works
Electricity use per day = monthly kWh × 12 / 365. System size (kW) = daily kWh to cover / (peak sun hours × (1 − losses)). A peak sun hour is one hour at 1,000 W per m², so a 1 kW array in a place with 5 peak sun hours makes about 5 kWh a day before losses. Panels = system kW × 1,000 / panel watts, rounded up to a whole panel. Roof area = panels × the area of one panel (1.922 m², 20.7 sq ft, from the data sheet below), panels only, no gaps or edges.
With the US starting values, 899 kWh a month is 29.6 kWh a day; at 5.3 sun hours and 14% losses that takes 6.48 kW, so 17 panels of 400 W (6.8 kW installed, 11,313 kWh a year). In the UK, 208 kWh a month in London (3.48 sun hours at the optimal tilt) needs 2.28 kW: 6 panels on 11.5 m². At 2,000 kWh a month and 4.5 sun hours the answer is 43 panels (16.99 kW); the guide below has the full grid. If you only know the house, not the bill: a home of 2,000 to 2,499 sq ft averages 12,173 kWh a year (EIA, 2020), which is 22 panels at 4.5 sun hours; see solar panel cost for a 2,000 sq ft house.
The sun hours are yearly averages for a well-oriented array (south facing, tilted near the latitude). An east or west roof, shade from trees or a chimney, and snow in winter all lower the real output, which is why the losses field exists: NREL’s default 14% covers soiling, wiring, mismatch and availability, not heavy shading. Every number is yours to change; the starting values only save typing. All calculations run in your browser and the inputs stay there, so the Pro report can pick them up.
Sources of the US starting values
- Monthly electricity use: U.S. Energy Information Administration, FAQ "How much electricity does an American home use?": "In 2022, the average annual amount of electricity sold to (purchased by) a U.S. residential electric-utility customer was 10,791 kilowatthours (kWh), an average of about 899 kWh per month." — https://www.eia.gov/tools/faqs/faq.php?id=97&t=3, read on 2026-09-20
- Peak sun hours by US state: Unbound Solar, "Peak Sun Hours by US State" (undated, read 2026-09-20), table built from NREL's PVWatts data (National Solar Radiation Database); "A peak sun hour is one hour of sunlight at an average intensity of 1,000 watts per square meter (1 kW/m²)" — https://unboundsolar.com/solar-information/sun-hours-us-map, read on 2026-09-20
- Panel wattage and area: Trina Solar, Vertex TSM-DE09 data sheet (version TSM_EN_2020_PA3), product range 390-405 W: module dimensions "1754×1096×30 mm (69.06×43.15×1.18 inches)", 21.0 kg, efficiency up to 21.1%, "25 year Power Warranty, 0.55% Annual Power Attenuation, 2% first year degradation". Area = 1.754 m × 1.096 m = 1.922 m² per panel — https://static.trinasolar.com/sites/default/files/EN_Datasheet_Vertex_DE09.pdf, read on 2026-09-20
- System losses: NREL, PVWatts Version 5 Manual (NREL/TP-6A20-62641, September 2014), section 11 System Losses, Table 6: soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, age 0%, availability 3%; "The default total system loss is calculated to be 14 %" (the losses multiply, they do not add) — https://www.nrel.gov/docs/fy14osti/62641.pdf, read on 2026-09-20
UK starting values have their own sources (Ofgem, PVGIS), shown under each field when you switch region.
Estimates from published averages (NREL, EIA, Ofgem, industry cost surveys), each with its source and date: an installer's site survey and shading analysis replace them. Not financial advice.
Frequently asked questions
- How many solar panels does it take to power a house?
- It depends on the kWh you use and where you are. The average US home buys about 899 kWh a month (U.S. Energy Information Administration); in Texas that is 17 panels of 400 W. The same use in a place with 4 sun hours needs about a third more panels, and a home that uses half the electricity needs half of them. Type your own bill and state.
- How many solar panels do I need for a 2,000 sq ft house?
- About 22 panels of 400 W (8.62 kW) at 4.5 sun hours, 18 at 5.5 and 28 at 3.5. EIA's 2020 survey puts homes of 2,000 to 2,499 sq ft at 12,173 kWh a year, about 1,014 kWh a month. Floor area is only a stand-in: electric heating, a pool pump or an electric car change the use, so type the kWh from your bill when you have it.
- What are peak sun hours?
- The daily energy the sun delivers to the panels, expressed as hours at full strength (1,000 W per m²). A place with 5 peak sun hours may have 12 hours of daylight, but the morning, evening and cloudy hours add up to 5 hours of full sun. The US figures come from NREL data by state; the UK figures from the European Commission's PVGIS for the city at the optimal tilt.
- Why 14% losses?
- NREL, PVWatts Version 5 Manual (NREL/TP-6A20-62641, September 2014), section 11 System Losses, Table 6: soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, age 0%, availability 3%; "The default total system loss is calculated to be 14 %" (the losses multiply, they do not add) Increase it if your roof is shaded or faces east or west; the Energy Saving Trust notes an east- or west-facing system gets around 15-20% less energy than one facing south.
- How much roof area do the panels take?
- One 400 W panel of the data sheet used here is 1.922 m² (20.7 sq ft). Ten panels take 207 sq ft or 19.2 m², panels only. Real layouts leave gaps, fire setbacks and skip the roof's shaded parts, so an installer's drawing will show more.
- Should I cover 100% of my use?
- 100% means the panels produce over a year what you use over a year; in most months you still buy from the grid at night and export in the day. Whether more or less than 100% makes sense depends on what your utility pays for exports (the payback tool has a field for it) and on the roof you have. Change the coverage field to see the size at 50%, 80% or 120%.
- Does the tool store anything?
- Your inputs stay in this browser's local storage so the payback tool, the output tool and the Pro report can start from them. Nothing is sent to a server and there is no lead form.
Guides
- How many solar panels for 2,000 kWh a month?
2,000 kWh a month is 65.8 kWh a day: 43 panels of 400 W (17.2 kW) at 4.5 sun hours, about $44,720 installed. Every US state and UK city.
- How many solar panels to power a house?
Panels for the average US home (EIA 899 kWh a month) from half to twice that use, every US state, and Ofgem's low, medium and high UK homes in four cities.
- How many solar panels to produce 30 kWh per day?
20 panels of 400 W (7.75 kW) make 30 kWh a day at 4.5 peak sun hours; 25 at 3.5 hours, 16 at 5.5. 5 to 50 kWh a day, every US state and UK city.
- How many solar panels for 1,000 kWh a month?
1,000 kWh a month is 32.9 kWh a day: 8.5 kW of solar, 22 panels of 400 W, at 4.5 sun hours. Sizes at 3 to 6 sun hours, every US state and UK city.
- How many solar panels for 1,500 kWh a month?
1,500 kWh a month is 49.3 kWh a day: 12.7 kW of solar, 32 panels of 400 W, at 4.5 sun hours. Sizes at 3 to 6 sun hours, every US state and UK city.
- How many solar panels for 2,500 kWh a month?
2,500 kWh a month is 82.2 kWh a day: 21.2 kW of solar, 54 panels of 400 W, at 4.5 sun hours. Sizes at 3 to 6 sun hours, every US state and UK city.