SolarToolsHow This Tool Works
This solar panel sizing calculator answers the most common question homeowners ask when going solar: "How many panels do I actually need?" The answer depends on three things — how much electricity you use, how much sunlight hits your roof, and how powerful the panels you choose are. Get any of those wrong and you'll either overspend on a system that produces more than you can use, or undersize it and keep paying the utility for the shortfall.
The calculator takes your monthly kilowatt-hour usage (find it on your electricity bill), the average peak sun hours for your location, and the wattage of the panel you're considering, then works out how many panels you need to cover your consumption. It also factors in real-world system losses — inverter inefficiency, wiring resistance, dust, and heat — so the number you get is the number you'd actually need installed, not a theoretical best case.
Beyond the panel count, the tool shows you the total system size in kilowatts, the approximate roof area required, and projected production in year one and year twenty-five. The twenty-five-year projection matters because panels degrade over time — a 0.5% annual loss compounds, and by the end of a typical warranty your system is producing about 12% less than it did on day one. Knowing this lets you decide whether to slightly oversize now or plan to add panels later.
Use the result as a planning baseline, not a quote. A real installer will adjust for roof orientation, shading, local permitting, and the specific inverter topology (string vs. microinverter vs. optimizer). But coming into those conversations knowing roughly what you need prevents you from being upsold or undersold.
- Find your monthly kWh usage. Look at your last 12 electricity bills and average them. Using a single summer or winter month will skew the result.
- Look up peak sun hours for your location. The US National Renewable Energy Laboratory (NREL) publishes free maps. Most of the continental US ranges from 3 (Pacific Northwest, Northeast) to 7 (Southwest desert). South Africa ranges from 4.5 to 6.5.
- Pick a panel model. The dropdown lists real, currently-shipping panels from 400W to 540W. Higher wattage means fewer panels but the same total roof area.
- Leave system losses at 14% unless you have a specific reason to change it. That number captures inverter conversion loss, DC wiring drop, soiling (dust), and thermal derating.
- Read the result. The headline number is panels required. The breakdown shows system size in kW, daily energy need, per-panel daily output, roof area needed, and 25-year production.
If the roof area needed exceeds your available south-facing roof, you can either use higher-wattage panels, accept partial offset (cover 80% of usage instead of 100%), or consider ground-mount. Use the Solar Savings Calculator to see what that partial offset is worth.
When to Use This Calculator
The math behind the number
The core formula is straightforward: panels = daily_kWh_needed / per_panel_daily_output. The daily kWh needed is your monthly usage divided by 30. The per-panel daily output is (panel_wattage / 1000) × peak_sun_hours × (1 − losses). The 1 − losses factor is what separates a useful calculator from a naïve one — without it, you'd be sizing for an idealized system that loses nothing to heat, dust, or inverter conversion, and you'd come up 15–20% short in real life.
Peak sun hours, not daylight hours
A common mistake is to plug in the number of hours the sun is above the horizon. Peak sun hours (PSH) is different — it's the number of hours per day when solar irradiance averages 1000 W/m², which is the standard test condition for rating panels. A clear summer day in Phoenix might give 7 PSH even though the sun is up for 14 hours, because morning and evening sun is weaker. Annual PSH averages smooth out seasonal variation, so for sizing use the annual average, not the summer peak.
System losses explained
The default 14% loss figure breaks down roughly as: inverter conversion (3–5%), DC wiring resistance (1–2%), AC wiring (0.5–1%), soiling/dust (2–5% depending on rain frequency), thermal derating (3–7% in hot climates), module mismatch (1–2%), and availability/downtime (0.5%). Hot climates lose more to heat; dusty climates lose more to soiling. If you live in a cool, rainy area you might get away with 10%. In a hot desert you might need 18%.
Why we round up
The raw calculation rarely produces a whole number. Rounding up ensures you actually cover your usage; rounding down guarantees you'll import from the grid. The 1-panel difference is usually negligible in cost but meaningful in coverage.
What about battery storage?
This calculator sizes for energy production, not for backup. If you want the system to also keep the lights on during outages, you need a battery sized for your critical loads. Use the Battery Storage ROI Calculator for that, and the Off-Grid System Designer for a complete solar + battery + generator setup.
Frequently Asked Questions
The math is transparent and uses industry-standard assumptions (14% system losses, 0.5% annual degradation). The main sources of variance are your actual roof orientation, shading, and local weather variability year to year. Expect ±10% accuracy versus a professional shade report.
East- and west-facing roofs typically produce 80–85% of what a south-facing roof would in the northern hemisphere. Multiply the result by 1.2 to compensate, or use the calculator with a slightly lower peak-sun-hours value.
It depends on your net metering policy. With 1:1 net metering, sizing for 100% makes sense. With reduced export rates or time-of-use plans, it's often better to size for 70–85% of usage and avoid overproducing. Check your utility's policy first.
No. This calculator sizes for annual energy offset only. If you plan to charge a battery from solar, you need additional capacity — typically 20–30% more panels depending on your battery size and winter production.
Most US homes use 800–1,100 kWh per month, which translates to 15–25 panels in a 5–7 PSH climate with 400W panels. Small efficient homes might need 10; large homes with electric heat or EVs may need 30+.
Further Reading
Deep-dive articles and guides related to this calculator.
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Energy & Solar Glossary
Plain-English definitions of every term used in this calculator and across the site.