Key numbers
- 855 kWh/month: average US residential use (EIA, 2023 data)
- System kW = monthly kWh ÷ 30.42 ÷ sun hours ÷ derate
- 14%: NREL PVWatts default system losses; this calculator uses 20% to be conservative
- 18.2¢/kWh: US residential average price (EIA, August 2026)
On this page
How to use the Solar Panel Calculator
- 1Enter your numbersFill in your monthly electricity use (kWh), peak sun hours per day (h), panel size (W), system efficiency (derate) (%) and share of usage to cover (%). Every field starts with a typical value you can overwrite.
- 2Check the assumptionsPeak sun hours: about 3–4 in the cloudy north, 4–5 across most of the US, 5.5–6.5 in the desert Southwest (NREL). The 80% derate covers inverter, wiring, heat and dirt losses.
- 3Read the resultsThe calculator shows solar panels needed, system size, expected production, roof area (approx. 21 ft² per panel) and bill value at the us average price, and updates as you type.
- 4Use the resultCompare it with your bill, a quote or a product spec sheet before you buy or install anything.
Results update as you type. Back to the calculator ↑
This solar panel calculator answers the first question most homeowners ask: how many solar panels do I need to cover my electric bill? Enter your monthly kWh, your peak sun hours and the panel wattage you are considering. It returns the panel count, system size in kilowatts (kW), expected production and approximate roof area.
How the solar panel calculator works
The calculator divides your monthly energy use by the energy one panel makes in a month, then rounds up to a whole panel.
- Target energy = monthly kWh × share to cover ÷ 100
- One panel per month (kWh) = panel watts × peak sun hours × derate × 30.42 ÷ 1,000
- Panels = target ÷ one panel’s monthly output, rounded up
- System size (kW) = target ÷ 30.42 ÷ sun hours ÷ derate
The 30.42 is the average number of days in a month (365 ÷ 12).
Worked example with the default inputs
The defaults describe an average American home: 855 kWh a month (the 2023 US residential average in EIA’s Average Monthly Bill table), 4.5 peak sun hours, 400 W panels and an 80% derate.
- One 400 W panel: 400 × 4.5 × 0.80 × 30.42 ÷ 1,000 = 43.8 kWh per month
- Panels: 855 ÷ 43.8 = 19.5, rounded up to 20 panels
- System size: 855 ÷ 30.42 ÷ 4.5 ÷ 0.80 = 7.81 kW
- Output of 20 panels (8 kW of nameplate): 876 kWh a month, 10,512 kWh a year
- Roof area: 20 × 21 ft² = 420 ft² (about 39 m²)
- Value at 18.2¢/kWh (EIA, August 2026): about $1,913 a year
The 20 panels slightly overshoot the target because you can’t install half a panel.
How many solar panels do I need? Reference table
Panel counts below use 400 W panels and an 80% derate, computed with the same formula as the calculator.
| Monthly use | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours |
|---|---|---|---|
| 500 kWh | 15 panels (5.87 kW) | 12 panels (4.57 kW) | 10 panels (3.74 kW) |
| 855 kWh (US average) | 26 panels (10.04 kW) | 20 panels (7.81 kW) | 16 panels (6.39 kW) |
| 1,000 kWh | 30 panels (11.74 kW) | 23 panels (9.13 kW) | 19 panels (7.47 kW) |
| 1,500 kWh | 45 panels (17.61 kW) | 35 panels (13.70 kW) | 29 panels (11.21 kW) |
| 2,000 kWh | 59 panels (23.48 kW) | 46 panels (18.26 kW) | 38 panels (14.94 kW) |
The same 855 kWh home needs 26 panels where the sun is weak and only 16 where it is strong. That spread is why the sun-hours input matters more than any other.
What size solar panel system do I need?
System size is your daily energy use divided by peak sun hours and the derate: 855 kWh a month ÷ 30.42 days ÷ 4.5 h ÷ 0.80 = 7.81 kW. Panel wattage only changes how many panels make up that size, as the table shows for the default inputs.
| Panel rating | Output per panel per month | Panels for 855 kWh | Installed size |
|---|---|---|---|
| 350 W | 38.3 kWh | 23 | 8.05 kW |
| 400 W | 43.8 kWh | 20 | 8.00 kW |
| 450 W | 49.3 kWh | 18 | 8.10 kW |
Installed size is slightly above 7.81 kW because panels come in whole units. A 1 kW system makes about 110 kWh a month at 4.5 sun hours, a handy rule of thumb for scaling.
What affects the result
Peak sun hours. This is the biggest variable. Use NREL’s free PVWatts calculator to get the solar resource for your exact address, then enter its annual average here.
System losses. NREL’s PVWatts model applies 14% total losses by default (Dobos, 2014), covering soiling, shading, wiring, mismatch, availability and similar factors. Our 80% derate assumes 20% losses, which leaves margin for inverter conversion and hot climates. If your roof is unshaded and kept clean, try 85%.
Tilt and orientation. The estimate assumes a roof facing roughly south at a sensible tilt. East- or west-facing roofs produce less. Check your best angle with the solar panel angle calculator.
Share of usage to cover. Net-metering rules differ by utility. Where exported power earns little, covering 70–90% of use can give a better return than 100%. Lower the “share to cover” input to see the smaller system.
Future loads. An electric vehicle or heat pump can add hundreds of kWh a month. Estimate them with the electricity usage calculator and add them before sizing.
Grid-tied or off-grid? Grid-tied sizing works from yearly averages. A cabin with no grid connection must be sized for the worst month and needs batteries, so use the off-grid solar calculator instead.
History of the solar panel calculation
The arithmetic behind this calculator is only as old as practical solar cells. In 1954 Daryl Chapin, Calvin Fuller and Gerald Pearson at Bell Telephone Laboratories described a silicon p-n junction photocell that converted sunlight to electricity at about 6% efficiency (Chapin, Fuller & Pearson, 1954). Once panels were sold by their rated watts, sizing a system became a matter of matching watts to sunshine and losses.
The “peak sun hours” shortcut used here treats a day’s solar energy as hours of 1,000 W/m² sunlight, the same irradiance used to rate panels at standard test conditions. Modern tools such as NREL’s PVWatts replaced rough tables with hourly weather data; its Version 5 model, documented in 2014, set the 14% default loss figure quoted above (Dobos, 2014).
| Year | Milestone | Source |
|---|---|---|
| 1954 | Bell Labs silicon solar cell, about 6% efficient | J. Appl. Phys. 25:676 |
| 2014 | PVWatts Version 5 documents 14% default system losses | NREL/TP-6A20-62641 |
Tips before you get installer quotes
- Use 12 months of bills, not one. Summer and winter use can differ a lot.
- Compare quotes on cost per watt and on estimated annual kWh, not on panel count. See how much solar panels cost for current benchmarks.
- Then see what the solar permit process requires before installation.
- Ask installers which loss assumption they used. A quote with very low losses looks better on paper than it performs.
- The US federal §25D residential clean energy credit ended for systems placed in service after December 31, 2025, so check current state and utility incentives instead.
References
- Chapin, D. M., Fuller, C. S., & Pearson, G. L. (1954). “A New Silicon p-n Junction Photocell for Converting Solar Radiation into Electrical Power.” Journal of Applied Physics, 25(5), 676–677. doi:10.1063/1.1721711
- Dobos, A. P. (2014). “PVWatts Version 5 Manual.” National Renewable Energy Laboratory, NREL/TP-6A20-62641. PDF
For the physics behind the numbers, read how solar panels work, or see where rooftop solar fits in the wider picture of renewable energy.
FAQ
Frequently asked questions
How many solar panels do I need for an average house?
At the 2023 US average of 855 kWh a month, with 4.5 peak sun hours and an 80% system efficiency, you need about 20 panels of 400 W, a 7.81 kW system. Sunnier sites need fewer panels and cloudier sites need more. Enter the kWh from your own bills for a personal answer.
How do I find my monthly kWh usage?
Look at the usage line on your electric bill, or download 12 months of usage from your utility account and divide the total by 12. A full year smooths out the summer air-conditioning and winter heating peaks that a single bill would hide.
What are peak sun hours?
Peak sun hours are the number of hours per day that sunlight would need to shine at 1,000 watts per square meter to deliver the day's total solar energy. A site with 4.5 peak sun hours gets as much energy as 4.5 hours of full noon sun. NREL's PVWatts tool gives the figure for any US address.
What does the derate or system efficiency mean?
The derate covers real losses between the panel label and your meter: inverter conversion, wiring, heat, dirt, shading and mismatch. NREL's PVWatts model uses 14% total losses by default. This calculator uses 20% losses, an 80% derate, to stay on the safe side.
Does panel wattage change the system size?
No. System size in kW depends only on your usage, sun hours and losses. Larger panels just mean fewer of them. The default 7.81 kW system is 20 panels of 400 W, or 18 panels of 450 W once you round up to whole panels.
How much roof space do solar panels need?
This calculator assumes about 21 square feet per residential panel, so 20 panels need roughly 420 square feet. Leave extra room for fire-code setbacks, vents and chimneys, and check exact dimensions on your panel's spec sheet.
How much power does one solar panel produce?
A 400 W panel makes about 1.44 kWh a day at 4.5 peak sun hours and an 80% derate (400 × 4.5 × 0.80 ÷ 1,000), or about 43.8 kWh a month and 526 kWh a year. The label wattage is its output in full noon sun at standard test conditions, so real daily output depends on your sun hours.
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Last updated October 25, 2026. Part of our renewable energy toolkit. Found an error? Tell us.