Solar array sizing
How Many Solar Panels Do I Need?
Enter daily or monthly energy use to estimate the solar array watts and whole-panel count for a home or off-grid system. Use peak sun hours for a quick planning estimate or PVWatts for location weather. Already know the panel count? Use the output calculator; need a system-wide load-to-battery plan? Use the off-grid system calculator.
Size the solar array
Your planning estimate
Local calculationEnter your values, then calculate. Results will show the formula inputs and rounding used.
Formula used
manual: required W = daily Wh × coverage ÷ (peak sun hours × efficiency); location: required kW = annual target kWh ÷ PVWatts annual kWh/kWBoth methods divide the theoretical array by the selected panel rating and round up. Output is then recalculated from the larger whole-panel array. The optional roof check compares panel footprint with the usable area you enter; it does not design a layout.
See the full assumptions and rounding policy.
Maintenance and verification
The calculator calls the shared, tested TypeScript calculation modules documented in the methodology. Automated tests cover published examples, invalid inputs, rounding, and result invariants.
SolarMathKit maintains this implementation. The review is an internal technical check, not an independent engineering certification.
Worked example: 10 kWh per day
Assume 5 peak sun hours, 80% system efficiency, 100% coverage, and 400W panels.
- Required array: 10,000 Wh ÷ (5 × 0.80) = 2,500 W.
- Theoretical panel count: 2,500 ÷ 400 = 6.25.
- Round upward to 7 panels, or 2,800 W installed.
- Installed daily output: 2,800 × 5 × 0.80 = 11,200 Wh.
Common mistakes to avoid
- Using daylight hours instead of peak sun hours.
- Entering a loss percentage where an efficiency percentage is expected.
- Rounding panel count down from a fractional result.
- Showing production from the theoretical array after a larger whole-panel array is installed.
- Treating the basic roof-area comparison as a structural or layout approval.
Questions people ask
Can I use this solar panel calculator for a home?
Yes. Start with recent household kWh from an electric bill rather than home floor area. Monthly energy is converted to a planning-day average, and the coverage input lets you size for less than 100% of use.
How many 400W panels do I need for 5, 10, or 20 kWh per day?
With 5 peak sun hours, 80% system efficiency, and full coverage, the planning examples are 4 panels for 5 kWh/day, 7 panels for 10 kWh/day, and 13 panels for 20 kWh/day. Change the sun hours, efficiency, coverage, and panel rating for your site; the installed output is recalculated after rounding up.
Where do I get peak sun hours?
Use a reputable solar-resource source for the site and season you want to plan around. The location mode is another option when PVWatts has weather data for the entered coordinates.
How does location mode size the array?
It first models annual AC energy for a 1 kW reference array with PVWatts V8. The calculator divides your annual energy target by that local kWh-per-kW yield, rounds up to whole panels, and models the installed array again.
Should I use the sizing or output calculator?
Use this page when you know the energy target and need array kW or panel count. Use the solar panel output calculator when you already know the panel count and rating and want daily, monthly, or yearly kWh.
What does the roof-area check include?
It compares the rounded panel count with the panel dimensions and usable area you enter. It does not account for roof shape, access paths, setbacks, shade, attachment points, wind, or structural limits.
Does this calculate cost or tax credits?
No. Prices, incentives, and policy change by location and date, so they remain outside this calculator.