Solar Panel Calculator
Work out how many solar panels you need from daily or monthly kWh, peak sun hours and system losses, plus a battery bank for days of autonomy and controller amps.
The interactive tool loads here when JavaScript runs.
About this toolWhat is the Solar Panel Calculator?
Tell this solar panel calculator how much electricity you use, in kWh per day, month or year, and how many peak sun hours your site gets, and it works out the array size in kW and the number of panels at the wattage you choose. It includes system losses (14% by default, the same default NREL uses in PVWatts) so the array covers real output rather than nameplate.
For off-grid cabins, boats and backup systems it also sizes a battery bank for the days of autonomy you want, using the depth of discharge of your battery type and the inverter efficiency, and gives the current rating for an MPPT charge controller at 12, 24 or 48 V.
A table shows how many panels the same need would take at two to seven peak sun hours, which is the quickest way to see why the same house needs far more panels in Seattle than in Phoenix.
- Daily, monthly or yearly kWh input
- User-supplied peak sun hours with guidance on PVWatts and PVGIS
- Panel count, array kW and expected production
- Battery bank for days of autonomy at 12, 24 or 48 V
- MPPT charge controller current with 125% margin
- Panels-versus-sun-hours table
How to use the Solar Panel Calculator
- Enter your energy use in kWh per day, month or year. A year of utility bills gives the best figure.
- Enter peak sun hours for your site from PVWatts or PVGIS (worst month for off-grid, annual average for grid-tied).
- Keep 14% losses unless you know better, and enter the panel wattage you plan to buy.
- For a battery system, enter days of autonomy, bank voltage, usable depth of discharge and inverter efficiency. Set days to 0 for a grid-tied system.
- Read the panel count, array kW, expected output, battery size and charge controller rating.
Worked examples
Typical home using 30 kWh a day
Input
30 kWh/day, 4.5 sun hours, 14% losses, 400 W panels, 1 day at 48 V, 80% DoD, 95% inverter
Output
20 panels (8 kW), about 31 kWh/day; 39.5 kWh battery (822 Ah at 48 V); 208 A of MPPT capacity
A controller that large would in practice be split across two or three MPPT units or a hybrid inverter.
Off-grid cabin in winter
Input
3 kWh/day, 3 sun hours, 20% losses, 200 W panels, 3 days at 24 V, 50% DoD, 90% inverter
Output
7 panels (1.4 kW); 20 kWh battery (833 Ah at 24 V); 73 A controller
Three days of autonomy with lead-acid makes the battery the expensive part of a small system.
Grid-tied, 900 kWh per month
Input
900 kWh/month, 5.5 sun hours, 14% losses, 430 W panels, no battery
Output
6.26 kW minimum, 15 panels (6.45 kW), about 11,100 kWh a year
900 kWh a month is 29.6 kWh a day. Sunnier sites need fewer panels for the same bill.
Background and insight
A solar panel's rating is its output at standard test conditions: 1,000 W/m² of sunlight at 25 °C cell temperature. Peak sun hours express a site's daily solar energy on the panel as the number of hours of that standard sun: 4.5 kWh/m²/day is 4.5 peak sun hours. The array size follows directly: kW = daily kWh ÷ (peak sun hours × (1 − losses)). 30 kWh a day at 4.5 hours and 14% losses needs 7.75 kW, which is 20 panels of 400 W.
System losses cover everything between the nameplate and your meter: soiling, shading, wiring, mismatch, inverter conversion, cell temperature and degradation. NREL's PVWatts uses 14% as its default total. Hot climates and dusty sites run higher; carefully designed systems with microinverters and clean panels can run lower.
Peak sun hours vary with location, tilt and season, so this tool does not guess them. Use NREL's PVWatts or the NSRDB for the US, or the European Commission's PVGIS for Europe, Africa and Asia, and read the value for your tilt. For grid-tied systems the annual average is right; for off-grid, design to the worst month you need to cover. Battery sizing is Wh = daily Wh × days ÷ (depth of discharge × inverter efficiency), and MPPT controller output current is array watts ÷ battery voltage, with a 25% margin in line with the NEC 690.8 practice of sizing PV circuits at 125% of their maximum current.
Common use cases
Quotes sanity check
Compare an installer's proposed system size with what your usage and sun hours call for.
Off-grid design
Size panels, battery and controller together for a cabin, van, boat or remote monitoring site.
Backup planning
Work out how much battery and solar keep essential loads running through a multi-day outage.
Roof space planning
Turn a kW target into a panel count to see whether it fits.
Tips and limitations
- Cut consumption first. Every kWh a day you save removes roughly a quarter to a third of a kW of panels.
- For off-grid, design to the worst month. A system that matches the annual average will run short all winter.
- Panel count is rounded up. Check string lengths against the inverter or controller voltage window before finalising.
- Lead-acid banks should be sized for 50% depth of discharge; LiFePO4 can use 80 to 90%.
- Shading on even one panel in a string can cut that string's output sharply unless you use optimisers or microinverters.
Frequently asked questions
How many solar panels do I need to power a house?
Divide your daily kWh by peak sun hours and by 0.86 for typical losses, then by the panel kW. A home using 30 kWh a day with 4.5 sun hours needs about 7.75 kW, or 20 panels of 400 W.
What are peak sun hours?
The daily solar energy on your panels expressed as hours of 1,000 W/m² sunlight. 5 kWh/m² per day equals 5 peak sun hours. It is not the number of daylight hours.
Where can I find peak sun hours for my location?
NREL's free PVWatts calculator gives monthly values for the US and many other countries; the European Commission's PVGIS tool covers Europe, Africa and most of Asia.
How big a battery bank do I need for off-grid solar?
Battery Wh = daily Wh × days of autonomy ÷ (depth of discharge × inverter efficiency). 3 kWh a day for three days on lead-acid at 50% and a 90% inverter is 20 kWh.
What size charge controller do I need?
For an MPPT controller, divide the array watts by the battery voltage and add 25%. A 1.4 kW array on a 24 V bank needs about 58 A, so choose at least 73 A of controller capacity.
How much power does a 400 watt solar panel produce per day?
Roughly panel kW × sun hours × 0.86. At 4.5 sun hours that is 0.4 × 4.5 × 0.86 = 1.55 kWh a day.
Is there an in-depth guide to the Solar Panel Calculator?
Yes. “How Many Solar Panels Do I Need? Sizing Arrays and Batteries” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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