Solar Panel Calculator
How many solar panels your house needs, from kWh usage, peak sun hours and panel wattage — with system size, annual production and roof area.
Solar Panel Calculator: with the default inputs, panels needed is 20.
From your bill. The US residential average is about 900 kWh a month (EIA).
100% offsets your whole bill on an annual basis. Go higher if you're adding an EV or a heat pump.
Annual daily average kWh/m² on the array. Roughly 3.5–4 in the Pacific Northwest, 4–4.5 in the Northeast and Midwest, 5–5.5 in the South, 6+ in the Desert Southwest.
Modern residential modules are 380–450 W. The rating is at standard test conditions, not in your backyard.
NREL's PVWatts default is 14%: soiling, shading, wiring, mismatch, connections, inverter and light-induced degradation.
- System size
- 8Panels × rating, in kilowatts DC.
- Annual production
- 11,300kWh a year after losses.
- Average monthly production
- 942
- Production per panel per year
- 565kWh
- Roof area required
- 559 ft²Panel area × count × the roof multiplier.
- Installed cost before incentives
- $0.00
- First-year bill offset
- $1,836.00
Assumptions
- Production = nameplate DC watts × peak sun hours × 365 × (1 − losses); the PVWatts default loss figure is 14%.
- Peak sun hours are an annual daily average for a reasonably oriented array; orientation, tilt and shading are not modelled.
- Panels are sized against annual energy, so monthly surpluses and deficits are assumed to net out under net metering.
- Roof area includes a 1.3× allowance for setbacks and racking gaps; local fire code may require more.
| Location | Peak sun hours | Panels for this usage |
|---|---|---|
| Phoenix, AZ | 6.3 | 14 |
| Denver, CO | 5.5 | 16 |
| Los Angeles, CA | 5.6 | 16 |
| Austin, TX | 5.2 | 17 |
| Atlanta, GA | 4.9 | 18 |
| Kansas City, MO | 4.8 | 18 |
| Chicago, IL | 4.3 | 21 |
| New York, NY | 4.3 | 21 |
| Boston, MA | 4.3 | 21 |
| Portland, OR | 3.8 | 23 |
| Seattle, WA | 3.6 | 24 |
| Anchorage, AK | 3.0 | 29 |
Same house, same panels, same losses — only the sunshine changes. Look up your exact address in PVWatts before you buy anything.
How this is worked out
The formula
Panel output per year (kWh) = panel watts ÷ 1000 × peak sun hours × 365 × (1 − losses) Panels = (monthly kWh × 12 × offset) ÷ panel output per year, rounded up System size (kW) = panels × panel watts ÷ 1000 Roof area = panels × panel area × roof multiplier
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Electricity used per month
- From your bill. The US residential average is about 900 kWh a month (EIA).0 or more · defaults to 900
- Share of usage to cover
- 100% offsets your whole bill on an annual basis. Go higher if you're adding an EV or a heat pump.a percentage · from 1 to 200 · defaults to 100
- Peak sun hours
- Annual daily average kWh/m² on the array. Roughly 3.5–4 in the Pacific Northwest, 4–4.5 in the Northeast and Midwest, 5–5.5 in the South, 6+ in the Desert Southwest.from 0.5 to 9 · defaults to 4.5
- Panel rating
- Modern residential modules are 380–450 W. The rating is at standard test conditions, not in your backyard.from 50 to 800 · defaults to 400
- System losses
- NREL's PVWatts default is 14%: soiling, shading, wiring, mismatch, connections, inverter and light-induced degradation.a percentage · from 0 to 60 · defaults to 14
- Area per panel(under More options)
- A 400 W module is about 1.13 × 1.76 m = 21.5 ft².in ft², m² · 1 or more · defaults to 21.5
- Roof area multiplier(under More options)
- Setbacks, walkways and racking gaps. 1.3 is a reasonable planning figure for a simple roof.from 1 to 3 · defaults to 1.3
- Installed price per watt(under More options)
- Before incentives. Leave 0 to skip; get a real quote rather than trusting a national average.in dollars · 0 or more · defaults to 0
- Electricity rate(under More options)
- A number.in dollars · 0 or more · defaults to 0.17
What you get back
- Panels neededmain answer
- System size
- Panels × rating, in kilowatts DC.
- Annual production
- kWh a year after losses.
- Average monthly production
- Production per panel per year
- kWh
- Roof area required
- Panel area × count × the roof multiplier.
- Installed cost before incentives
- First-year bill offset
What this assumes
- Production = nameplate DC watts × peak sun hours × 365 × (1 − losses); the PVWatts default loss figure is 14%.
- Peak sun hours are an annual daily average for a reasonably oriented array; orientation, tilt and shading are not modelled.
- Panels are sized against annual energy, so monthly surpluses and deficits are assumed to net out under net metering.
- Roof area includes a 1.3× allowance for setbacks and racking gaps; local fire code may require more.
About this calculator
Sizing a solar array is one division: the energy you use in a year, divided by the energy one panel makes in a year. Everything difficult is hidden inside that second number, so this calculator makes each part of it visible and lets you change it.
How to use it
Take the monthly kWh off your electricity bill — the annual total, divided by twelve, is better than one month. Set the peak sun hours for where you live: that's the annual daily average of solar energy hitting the array, in kWh per square metre per day, and the table under the results has figures for a dozen US cities. Panel wattage comes off the module's datasheet; 400 W is typical for a residential module today.
Leave losses at 14% unless you have a reason. That is NREL's PVWatts default and it bundles soiling, shading, wiring and connection losses, module mismatch, inverter efficiency, nameplate tolerance and light-induced degradation. It is not conservative — it's what a competent installation on a clean roof actually loses.
What this calculator does not know
Be clear-eyed about this. The answer here is an energy budget, not a design. Real output depends on:
- Orientation and pitch. A due-south array at latitude tilt is the reference. East or west facing costs 10–20%. North-facing in the northern hemisphere is usually not worth doing.
- Shading. A single chimney shadow crossing one panel can drag a whole string down. Optimizers or microinverters mitigate it; nothing eliminates it.
- Local irradiance. Coastal marine layer, summer haze and winter snow cover are all in the NREL data for your address and not in a national average.
- Net metering. If your utility credits exports below retail, an array that covers 100% of your kWh will not cover 100% of your bill.
Get a PVWatts run for your actual address and roof plane before you sign anything. This tool is for deciding whether the project is roughly a 6 kW or a 12 kW conversation.
Reading the results
Panels needed is rounded up to whole modules. Roof area required includes a 1.3× multiplier for fire setbacks, walkways and racking gaps — a real roof rarely packs tighter than that. Annual production is after losses, so compare it directly with your bill's yearly total.
Frequently asked questions
▸How many solar panels do I need for a 2,000 sq ft house?
Square footage doesn't determine it — usage does. At the US average of 900 kWh a month, 4.5 peak sun hours and 400 W panels, you need about 20 panels (8 kW). Pull your own kWh off the bill; identical houses differ by a factor of three.
▸What are peak sun hours?
The number of hours per day the sun would need to shine at exactly 1,000 W/m² to deliver the same energy your array actually receives. It is an energy figure, not a daylight-hours figure — Seattle gets plenty of daylight and about 3.6 peak sun hours.
▸Why subtract 14% for losses?
It's NREL's PVWatts default and covers soiling, shading, mismatch, wiring, connections, inverter efficiency, nameplate tolerance and light-induced degradation. A panel's nameplate watts are measured in a lab at 25 °C; your roof is not a lab.
▸How much roof space do I need?
About 21.5 ft² per 400 W module, times roughly 1.3 for setbacks and walkways — so a 20-panel array wants around 560 ft² of usable, unshaded roof plane, not 430.
▸Will this cover my whole bill?
It covers your whole kWh on an annual basis at 100% offset. Whether that zeroes the bill depends on your utility: fixed connection charges stay, and if exports are credited below retail you'll still owe something.
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