Solar Panel Calculator
Find out exactly how many solar panels you need, what the system will cost, how long before it pays back, and how much CO2 you will offset. Enter your monthly electricity use, local peak sun hours, panel wattage, and (optionally) your electricity rate and install cost to get a complete picture.
Formula
Worked example
A home using 900 kWh/month averages 900 / 30.4 = 29.6 kWh/day. With 4.5 peak sun hours, 80% efficiency and a 100% offset goal: 29.6 / (4.5 x 0.80) x 1.0 = 8.2 kW. At 400 W per panel: ceil(8,200 / 400) = 21 panels (8.4 kW installed). Year 1 production: 8.4 x 4.5 x 0.80 x 365.25 = 11,070 kWh. Roof area: ceil(21 x 21) / 0.75 = 588 sq ft. CO2 offset: 11,070 x 0.823 = 9,111 lbs/yr.
How solar system sizing works
Sizing a solar array starts with how much electricity you use, not how much roof you have. First convert your monthly kilowatt-hours into a daily average. Then divide that daily energy need by your location's peak sun hours and a system efficiency factor to get the raw kilowatts of panel capacity required. An offset goal lets you scale the system up or down: a 100% offset matches your current use, while 120-150% makes sense if you plan to add an electric vehicle or heat pump. Dividing the system size by a single panel's wattage and rounding up gives the number of panels you need. The area calculation adds a 33% overhead for the fire-code setbacks required around the perimeter of rooftop arrays (NFPA 1).
Cost, payback period and 25-year savings
Enable the cost section to move from sizing to economics. The gross install cost multiplies your system's kilowatts by a cost-per-watt figure; the U.S. residential median in 2026 is $2.58-$3.25 per watt according to Lawrence Berkeley National Laboratory's Tracking the Sun report. Year 1 savings equal the system's first-year output multiplied by your electricity rate. The simple payback divides the install cost by that annual saving. The 25-year projection models two real-world effects: panels degrade at roughly 0.5% per year (your 25-year output is about 92% of Year 1 on a linear average for tier-1 panels), and electricity rates have historically risen about 3% per year. The result is the cumulative value of the electricity you will generate over the system's warranted life, minus what you paid upfront.
Why peak sun hours and efficiency matter most
Two locations with identical electricity bills can need very different arrays because peak sun hours vary from roughly 3.5 in the cloudy Northeast to over 6 in the desert Southwest. Fewer sun hours means each panel produces less, so you need more panels to hit the same annual total. The efficiency factor accounts for unavoidable real-world losses: inverter conversion, wiring resistance, heat derating, dust and minor shading. NREL's PVWatts tool uses a default system derate of about 80%, meaning a panel rated at 400 watts delivers closer to 320 watts of usable AC power on average. Ignoring this derate would undersize your system by 20%. For the most accurate sizing, enter your location's specific peak sun hours from NREL's PVWatts map rather than using the default.
CO2 offset and environmental impact
Every kilowatt-hour your panels generate displaces a kilowatt-hour that would otherwise come from the grid, which produces carbon dioxide. This calculator uses the EPA's eGRID 2022 U.S. average emission factor of 0.823 lbs of CO2 per kWh. Actual displacement depends on your regional grid mix: coal-heavy grids in the Midwest produce more CO2 per kWh than the hydro-rich Pacific Northwest. As a rough benchmark, the EPA estimates a mature tree absorbs about 48 lbs of CO2 per year, so your annual offset in lbs divided by 48 gives a "tree equivalent." These numbers are for illustrative comparison and should not be used for formal carbon reporting.
Turning the estimate into a real install
This calculator gives a clean first-pass estimate, but a professional design refines several things it cannot see. Roof orientation and tilt change output: a south-facing roof at your latitude is ideal, while east or west faces lose 15-25% of output. Shading from trees, chimneys or neighbouring buildings can knock out whole strings of panels at certain hours. Usable roof area may cap how many panels physically fit. Net metering rules, battery storage, local utility incentives, and your appetite to offset 100% versus 80% of usage all shift the final number. Use this result to understand the scale of system you are looking at, then get three installer quotes who will conduct a shade and orientation analysis to confirm the panel count.
Peak sun hours and typical system sizes by U.S. region
| Region / Example city | Peak sun hours | 900 kWh/mo system | Approx. panels |
|---|---|---|---|
| Pacific Northwest (Seattle) | 3.5 | ~10.5 kW | ~27 |
| Northeast (Boston) | 4.0 | ~9.2 kW | ~23 |
| Midwest (Chicago) | 4.2 | ~8.8 kW | ~22 |
| Southeast (Atlanta) | 4.7 | ~7.9 kW | ~20 |
| Texas (Dallas) | 5.2 | ~7.1 kW | ~18 |
| Southwest (Phoenix) | 6.0 | ~6.2 kW | ~16 |
| Southern CA (Los Angeles) | 5.8 | ~6.4 kW | ~16 |
Estimates assume 80% efficiency, 400 W panels and 100% bill offset. Source: NREL PVWatts.
Frequently asked questions
How many solar panels do I need to power my house?
It depends on your electricity use, local sunlight, and panel wattage. Divide your daily kWh by your peak sun hours and an 80% efficiency factor to get the kilowatts needed, then divide that by your panel wattage and round up. A typical U.S. home using 900 kWh per month needs roughly 18 to 24 panels of 400 W each, depending on location.
What are peak sun hours?
Peak sun hours are the number of hours per day during which sunlight averages 1,000 watts per square metre, the standard intensity panels are rated at. It is not the same as daylight hours. Most of the United States receives between 3.5 and 6 peak sun hours per day depending on latitude and climate. You can look up the exact figure for your address at pvwatts.nrel.gov.
Why does the calculator apply an 80% efficiency factor?
Panels never deliver their full nameplate rating in practice. Energy is lost to inverter conversion, wiring, high temperatures, dust and small amounts of shading. NREL's PVWatts model uses a default derate of about 0.80, so this calculator assumes 80% by default. You can lower it for a hot, dusty or partly shaded site, or raise it slightly for a near-ideal install.
How long does a solar system take to pay back?
Without the federal residential tax credit (which expired at the end of 2025), simple payback periods for most U.S. homeowners run 9 to 14 years based on a $3/W install cost and a $0.18/kWh rate. Sunnier states with higher electricity rates, such as California, Hawaii, and Texas, often see paybacks under 10 years. After payback, the system generates pure savings for the remainder of its 25-30 year warranted life.
How much roof area do I need for solar panels?
A standard 400 W panel is about 21 square feet. Fire and building codes require clearance setbacks around the perimeter of rooftop arrays (typically 18 inches on all sides per NFPA 1), so usable roof area is roughly 75% of gross area. A 10-panel system (400 W each) needs about 21 x 10 / 0.75 = 280 square feet of usable south-facing roof.
What CO2 savings does solar provide?
The EPA's eGRID 2022 dataset puts the U.S. average grid emission factor at 0.823 lbs of CO2 per kWh. A system producing 10,000 kWh per year therefore offsets about 8,230 lbs, or 3.7 metric tons, of CO2 annually. The actual saving depends on your regional grid: coal-heavy grids produce more CO2 per kWh than hydro-rich regions.
Should I size my system to offset 100% of my electricity use?
A 100% offset is a common starting point. If you are planning to add an electric vehicle or an electric heat pump, sizing to 120-150% of your current use future-proofs the system. Conversely, if your utility offers poor net-metering rates for excess generation, staying at 80-90% offset avoids exporting cheap power. This calculator's "bill offset goal" slider lets you explore all of these scenarios.