Solar System Size Calculator
Start a solar proposal from a single number on the electric bill. Monthly kWh, sun hours and panel wattage become a system size, panel count, yearly production and installed cost that installers and homeowners can both follow.
Free, no sign-up. Runs in your browser; nothing you type is stored unless you choose to email yourself the results.
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Email me these results. We'll send your numbers, plus how to turn this calculator into a lead magnet on your own website.
Three steps, about a minute
Step 1
Enter Average monthly usage in kWh, ideally averaged across 12 months of utility bills.
Step 2
Set Peak sun hours per day for the location and the System efficiency after losses.
Step 3
Add Panel wattage and Installed price per watt to see panel count and installed cost.
How the solar system size calculator works
Monthly kWh × 12 ÷ 365 gives daily use. Dividing that by peak sun hours, then by system efficiency, gives the kW of panels required. Panels needed is that size in watts ÷ panel wattage, rounded up, and yearly production is recalculated from that rounded-up array: panels × panel kW × sun hours × 365 × efficiency. Installed cost is panels × panel watts × price per watt, before any incentives. The default inputs (900 kWh a month, 5 sun hours, 80% efficiency, 400 W panels, $3 a watt) produce a 7.40 kW requirement, 19 panels, about 11,096 kWh a year and $22,800. Shade, roof direction and tilt are not modeled; a site survey sets the real numbers. Check local sun hours and incentives, and note that permits and interconnection follow local code and utility rules.
| Average monthly usage | 900 kWh |
|---|---|
| Peak sun hours per day | 5 |
| System efficiency | 80% |
| Panel wattage | 400 W |
| Installed price per watt | $3 |
| System size | 7.4 kW |
|---|---|
| Panels needed | 19 |
| Estimated annual production | 11,096 kWh |
| Estimated installed cost | $22,800 |
Daily usage ÷ sun hours ÷ efficiency. Check local sun hours and incentives for an accurate quote.
Using the numbers like a pro: tips for solar installers
Ask for 12 months of bills before you size anything. A single summer or winter bill can push the system well off, and an accurate size protects you from overpromising production.
Show price per watt and the total side by side, then any incentives as a clearly labeled estimate. Clear math helps your proposal stand up against high-pressure door-to-door pitches.
Check that the panel count fits the best roof plane. If 19 panels won't fit the south face, show the trade-off between a smaller array and using a less ideal plane.
Put a calculator like this on your own website
Solar shoppers tend to compare several installers before they sign anything. A sizing calculator on your homepage gives them a system size and panel count in seconds, which keeps them on your site, and an offer of a custom production estimate gets you their address and utility details. Those leads arrive ready for a design. We can build one that uses your own price per watt and panel options.
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Solar System Size Calculator questions
How many solar panels do I need for my house?
Divide daily electricity use by peak sun hours and system efficiency to get kW, then divide by panel wattage. A home using 900 kWh a month with 5 sun hours and 80% efficiency needs about 7.4 kW, which is 19 panels at 400 W each. Roof space, shade and local sun change the final count.
What are peak sun hours?
Peak sun hours count how many hours a day would deliver the location's total sunlight if it arrived at 1,000 watts per square meter, the intensity panels are rated at. Five peak sun hours can be spread across a whole day of weaker light. The number changes by location and season, and public solar resource maps list typical values.
What does system efficiency mean in a solar estimate?
It is the share of the panels' rated output that actually reaches the home after losses in the inverter, wiring, heat, dirt and shading. This calculator defaults to 80%, a common planning assumption. Lower efficiency means more panels for the same usage, so shaded or hot sites need a site-specific production estimate.
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