Solar Energy: kW, kWh, Watt-hours, and System Efficiency Conversions
Published April 24, 2026
Solar panels are rated in kilowatts (peak power output, kW); daily energy production in kilowatt-hours (kWh); system efficiency in percentages. Understanding the differences prevents undersizing a solar array for your needs. A 5 kW system does not produce 5 kWh per day (it depends on sunlight hours, season, and inverter efficiency).
Table of Contents
Understanding the Basics
Power (kW) = instantaneous output; energy (kWh) = power × time. A 5 kW solar array produces 5 kilowatts at peak sunlight—but only for a few hours daily. On a 6-hour peak sun day, that array generates 5 kW × 6 hours = 30 kWh. Seasonal variation: winter may yield 3 peak sun hours; summer, 7+ hours. Sizing requires averaging annual peak sun hours for your latitude.
System efficiency cascades: solar panel efficiency (17-22%), inverter efficiency (95-98%), wiring losses (2-5%), shading losses (varies). A 5 kW array rated at 18% efficiency under full sun = 5000 W ÷ 0.18 ≈ 27,800 W panel power needed (27.8 kW nameplate). If inverter is 97% efficient and wiring loses 3%, final output = 5 kW × 0.97 × 0.97 ≈ 4.7 kW actual.
Solar Energy Units
- Kilowatt (kW): Power; instantaneous output rating. 1 kW = 1000 watts.
- Kilowatt-hour (kWh): Energy; power × time. 5 kW × 2 hours = 10 kWh.
- Watt (W): Base SI unit of power. 1 kW = 1000 W.
- Watt-hour (Wh): Energy unit (rarely used in solar; kWh is standard).
- Peak Sun Hours (PSH): Equivalent hours of full sunlight per day. Varies by season/latitude; annual average 4-6 hours.
Conversion Table
| from | to | factor |
|---|---|---|
| kW (power) | kWh (energy) | kWh = kW × hours |
| Peak Sun Hours (PSH) | Daily kWh | kWh = system kW × PSH |
| Annual kWh | Monthly average | ÷ 12 months |
| System efficiency | Actual output | Rated kW × efficiency % |
Worked Examples
Daily Energy Production
5 kW solar system, 5 peak sun hours (annual average). Daily: 5 kW × 5 hours = 25 kWh. Annual: 25 kWh × 365 days ≈ 9,125 kWh/year. If electricity costs $0.12/kWh, savings = 9,125 × $0.12 ≈ $1,095/year.
System Efficiency Loss
5 kW array, 18% panel efficiency, 97% inverter efficiency, 96% wiring efficiency. Actual output: 5 kW × 0.97 × 0.96 × 0.18 ÷ 0.18 (panel efficiency already in nameplate) ≈ 4.66 kW accounting for inverter/wiring.
Practical Applications
Sizing for needs: Average home uses 25-30 kWh/day. If your area has 5 PSH: 25 kWh ÷ 5 hours = 5 kW system needed.
Battery storage: 1 kWh battery = 1 day's power if system produces 1 kW for 1 hour. Sizing depends on daily loads and peak sun hours.
Utility-scale farms: Gigawatt (GW, 1 billion watts) plants; annual production in gigawatt-hours (GWh). Same power-to-energy relationship scales.
Net metering: Excess production credited in kWh. If you produce 30 kWh on a sunny day but use 20 kWh, 10 kWh credits your account.
Best Practices
💡 Use peak sun hours (PSH, region-specific) to size systems. PSH accounts for seasonal variation and angle losses. A 5 kW system × 5 PSH = 25 kWh/day; × 365 days = 9125 kWh annual—use this for cost/ROI calculations.
Use peak sun hours (PSH, region-specific) to size systems. PSH accounts for seasonal variation and angle losses. A 5 kW system × 5 PSH = 25 kWh/day; × 365 days = 9125 kWh annual—use this for cost/ROI calculations.
Common Mistakes
⚠️ kW ≠ kWh. Don't confuse power rating (kW) with energy output (kWh). A 10 kW system producing for 1 hour = 10 kWh. The same system for 0.5 hours = 5 kWh. Production varies daily/seasonally.
kW ≠ kWh. Don't confuse power rating (kW) with energy output (kWh). A 10 kW system producing for 1 hour = 10 kWh. The same system for 0.5 hours = 5 kWh. Production varies daily/seasonally.
Tools and Resources
- PVWatts calculator (NREL): Input location and system size, get annual kWh estimate
- Online solar calculator: Peak sun hours by zip code or coordinates
- System design tool: kW sizing based on daily energy needs and local PSH
Key Takeaways
- kW = power (instantaneous); kWh = energy (power × time). A 5 kW system ≠ 5 kWh daily production.
- Daily kWh = system kW × peak sun hours (PSH). PSH varies by season/location (4-6 hours average).
- System efficiency cascades: panel + inverter + wiring losses. Actual output ≈ 95% of nameplate after accounting for all losses.
- Sizing formula: daily kWh needed ÷ PSH = system kW required. A 25 kWh/day home needs ≈5 kW system (at 5 PSH).
- Annual production = daily kWh × 365. Use this for cost/ROI (production × $/kWh).