How to use this calculator
Enter the system’s DC rating (the sum of the panel ratings) and the average number of peak sun hours per day at your location — the equivalent number of hours at 1,000 W/m² of sunlight. Peak sun hours range from about 3 in the Pacific Northwest to over 6 in the desert Southwest; the free PVWatts tool (formerly from NREL) gives a site-specific figure. The system efficiency defaults to 82%, equivalent to PVWatts’ default 14% system losses with a 96% efficient inverter.
To size a system, enter the annual consumption from your electricity bills in the “energy per year” field together with your sun hours: the required kW and the number of panels appear. Divide your annual bill’s kWh by 365 to compare daily figures.
The formula
- P
- system size (dc rating)
- P_panel
- rating of one panel
- n
- number of panels
- PSH
- peak sun hours per day
- η
- system efficiency (after losses)
- E_day
- energy per day
- E_year
- energy per year
Worked example
A 6 kW system of 400 W panels is planned for a site that gets 4.5 peak sun hours per day on average.
- Panels = 6,000 ÷ 400 = 15.
- Daily energy = 6 kW × 4.5 h × 0.82 = 22.1 kWh.
- Annual energy = 22.14 × 365 = 8,081 kWh.
About 8,100 kWh a year — roughly three-quarters of an average US home’s consumption.
Assumptions and limits
- Annual-average sun hours; production in winter can be less than half of summer output.
- Panels are unshaded and at a reasonable tilt and orientation. Shading, east–west roofs or a flat mount reduce output.
- Efficiency includes wiring, soiling, mismatch, temperature and inverter losses; it does not include battery round-trip losses.
Frequently asked questions
How many kWh does a 1 kW solar system produce per year?
About 1,200–1,800 kWh in most of the US: 1 kW × 4–6 peak sun hours × 0.82 × 365.
How many solar panels do I need for 10,000 kWh a year?
At 4.5 peak sun hours and 82% efficiency, about 7.4 kW — nineteen 400 W panels.
What are peak sun hours?
The daily solar energy at a site expressed as the number of hours of full 1,000 W/m² sunshine. Five peak sun hours means 5 kWh/m² per day.
Sources
- PVWatts Calculator (National Laboratory of the Rockies, formerly NREL) — system losses and defaults
- US Energy Information Administration — Solar explained