What formula is used to calculate solar energy production?

Annual Production (kWh) = System Size (kW) × Peak Sun Hours (hrs/day) × 365 days × System Efficiency (%). Example: 7 kW × 5 peak sun hours × 365 × 0.80 = 10,220 kWh/year. System efficiency of 80% accounts for inverter losses (3–5%), wiring losses (2–3%), temperature derating (5–10%), and soiling (1–3%).

How much electricity does a solar panel produce per day?

A standard 400W solar panel produces about 1.2–2.0 kWh per day depending on your location's peak sun hours (4–6 hours for most of the US). A 10-panel, 4,000W system produces roughly 12–20 kWh/day, or 440–600 kWh/month. NREL's PVWatts tool can give precise production estimates by zip code.

How do I calculate solar panel output?

Solar energy production = panel wattage × peak sun hours × system efficiency factor. A 400W panel in a location with 5 peak sun hours produces: 400W × 5h × 0.85 (efficiency) = 1.7 kWh/day. Multiply by your number of panels for total daily production. Annual production typically ranges from 1,100–1,800 kWh per installed kW.

How much electricity does a solar panel produce per day?

A standard 400W solar panel in average US conditions (4–5 peak sun hours) produces 1.4–2.0 kWh per day. A 20-panel, 8 kW system produces 28–40 kWh/day. Annual production is typically 1,200–1,600 kWh per kW of installed capacity depending on location, with Hawaii and the Southwest at the high end and the Northeast/Pacific Northwest at the lower end.

What reduces solar panel output?

Key production reducers: shading (even partial shading can reduce output by 20–50% with string inverters), panel soiling (dust, leaves, bird droppings can cut 5–15%), high temperatures (panels lose roughly 0.35–0.5% efficiency per degree Celsius above 25°C), incorrect tilt angle, and panel aging/degradation over time.

Solar Energy Production Calculator

Estimate how much electricity your solar system will generate — daily, monthly, annually, and over its full lifetime. Accounts for real-world efficiency losses and long-term panel degradation.

System Size (kW)

Peak Sun (hrs/day)

Years (yrs)

Electricity Rate (¢/kWh)

About This Calculator

The Solar Energy Production Calculator estimates the actual electricity output of a solar system over time, accounting for the real-world factors that reduce production below nameplate capacity. A perfectly efficient 8 kW system in full sun would produce 8 kWh per hour, but real systems lose 15–25% to inverter conversion, wiring resistance, heat, dust, and panel mismatch. This calculator applies your system efficiency factor to give a realistic production number you can compare against your actual electricity consumption.

Degradation rate is a critical but often overlooked factor in long-term production estimates. Most tier-1 solar panels degrade at 0.3–0.7% per year, meaning a system producing 10,000 kWh in year one will produce roughly 8,800 kWh by year 25 at a 0.5% degradation rate. Over a 25-year lifetime this means total cumulative production is about 11% less than if the panels never degraded. Premium panels from manufacturers like LG, Panasonic, and SunPower typically carry 0.25–0.4% annual degradation guarantees — worth asking about when comparing quotes.

To find your local peak sun hours, use NREL's PVWatts Calculator (pvwatts.nrel.gov) — enter your address and look for the "Solar Resource" value in kWh/m²/day. The Southwest U.S. averages 5.5–7.0 peak sun hours; the Northeast averages 3.5–4.5. System efficiency depends on your roof orientation and tilt: a south-facing roof at 30° tilt in a mild climate might achieve 85% efficiency, while a shaded or east/west-facing roof might only reach 70%. Your installer's simulation software accounts for these specifics more precisely.

Knowing your expected production allows you to cross-check installer proposals, verify utility net metering credits, and plan battery storage sizing. It is also the input that drives every financial calculation — savings, ROI, and payback all depend on an accurate production estimate as their foundation.

Assumptions apply NREL's PVWatts methodology and National Solar Radiation Database (NSRDB) solar resource data, EPA eGRID emissions factors, and Lawrence Berkeley Lab installation cost benchmarks — reviewed quarterly by solar industry professionals. Read our full methodology and data sources.

Estimates only — not financial, tax, or legal advice. Verify important results with a licensed solar installer or financial professional before making decisions.