Solar Panel kW to kWh Calculator
The number on a solar panel or array's spec sheet — the kW rating — is a laboratory maximum, measured under ideal "Standard Test Conditions" that essentially never happen in your actual backyard. What you'll actually generate day to day depends on your location's sunlight, the season, your roof's angle and shading, and losses in the inverter and wiring. This calculator translates a system's rated kW capacity into a realistic daily and monthly kWh estimate, so you're planning around real numbers instead of a sales-sheet best case.
The Formula
Peak Sun Hours (PSH): the daily equivalent of hours at "1,000 W/m²" of sunlight intensity — a way of compressing a whole day's variable sunlight into one comparable number. Ranges roughly from 3.0-3.5 in cloudier northern regions to 5.5-6.5+ in the sunniest desert climates.
Derate Factor: accounts for real-world losses — inverter conversion (~95-97% efficient), wiring resistance, dust/dirt on panels, temperature effects (panels lose efficiency as they heat up), and slight panel mismatch. A typical combined derate factor is 0.80-0.85, meaning the system delivers 80-85% of its theoretical output.
Worked Example
A 6 kW residential system in a moderate-sun region (4.5 PSH), with a standard 0.82 derate factor:
Why the Sales-Sheet Number Isn't the Real Number
If you've been quoted "a 6kW system," it's tempting to assume that means 6 kWh every hour it's sunny — but that's not how solar works. The 6 kW is the peak instantaneous output under perfect lab conditions (specific temperature, specific sun angle, brand-new clean panels). Real daily output is always meaningfully lower once you factor in the sun's actual angle changing throughout the day, clouds, panel heating, and system losses — which is exactly what the Peak Sun Hours and Derate Factor account for.
This is also why two identical 6kW systems installed in different cities produce very different annual totals — a system in Arizona and an identical one in Seattle will have very different Peak Sun Hour averages, even with the same panels.
Regional Peak Sun Hour Reference (Typical Ranges)
| Region Type | Typical PSH | Example |
|---|---|---|
| Low-sun / northern | 3.0–3.8 | Pacific Northwest, UK, Northern Europe |
| Moderate | 4.0–4.8 | Midwest/Northeast US, most of Central Europe |
| High | 4.8–5.5 | Southern US, Southern Europe, Australia (temperate) |
| Very high | 5.5–6.5+ | Southwest US, North Africa, Middle East |
(For an exact figure for your specific location, check your national solar irradiance database — e.g., NREL's PVWatts for the US, or your local meteorological/solar agency — rather than relying on regional averages alone.)
Seasonal Variation Matters Too
Peak Sun Hours aren't constant year-round — a location averaging 4.5 PSH annually might see closer to 6 PSH in peak summer and 2.5-3 in December. If you're sizing a system to cover winter heating loads or a specific season's usage, calculate against that season's PSH, not the annual average, or you'll undersize for your worst month.
Frequently Asked
Why is my actual solar output lower than what the installer quoted?
Quotes are often based on the system's rated kW × ideal assumptions. Confirm what PSH and derate factor were used in the estimate — a quote using an inflated PSH or a 0.90+ derate factor (unrealistically high) will overstate real output.
Does panel degradation affect this over time?
Yes — most panels degrade roughly 0.5% per year, so a system's output in year 15 will be meaningfully lower than year 1. For long-term planning, apply a small annual reduction to the derate factor.
How much of my home can a 6kW system actually power?
Multiply the daily kWh estimate above by 30 for a monthly figure, then compare that to your actual monthly usage (visible on your utility bill) to see the percentage covered.
For sizing the battery to pair with a solar system, see the battery Ah to kWh calculator.
Founder & Lead Renewable Energy Specialist
Abdul Rehman is a Renewable Energy Consultant and Solar Systems Specialist. He built kwtokwh.com to provide transparent, accessible calculation tools that help homeowners, electricians, and facility engineers accurately convert electrical power (kW) into billed energy consumption (kWh) calibrated against NIST and IEC metrology standards.