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How to Calculate Solar Panel Energy Output
The rated wattage of a solar panel tells you its peak output -- but real-world generation depends on your location, orientation, tilt, and shading. Here is how to estimate actual annual output.
A 400W solar panel does not generate 400W continuously. It generates 400W only at Standard Test Conditions (1,000 W/m2 irradiance, 25°C). Real output is always lower -- but by a predictable amount.
The Peak Sun Hours Method
Annual Energy Output (kWh) = Panel wattage x Peak Sun Hours/day x 365 x System Efficiency
Peak Sun Hours: average daily solar irradiance in your location
(not hours of daylight -- hours equivalent to full 1,000 W/m2)
UK average peak sun hours by region:
South England: 2.8-3.2 hours/day
Midlands: 2.5-2.9 hours/day
North England/Scotland: 2.2-2.7 hours/day
System efficiency factor: 0.75-0.85
(Accounts for inverter losses, wiring, temperature, soiling)
Example: 400W panel, South England, 3.0 peak sun hours, 0.80 efficiency
Annual output = 400 x 3.0 x 365 x 0.80 = 350,400 Wh = 350 kWh/year
Typical 4kW system (10 x 400W panels), South England:
Annual output = 4,000 x 3.0 x 365 x 0.80 = 3,504 kWh/year
Orientation and Tilt Factors
South-facing at optimal tilt: 100% (reference)
South-facing, flat roof (10 degrees): ~95%
South-east or south-west facing: ~95%
East or west facing: ~80-85%
North-facing: ~60-65% (generally not recommended in UK)
Optimal tilt angle in UK: 30-40 degrees from horizontal
Horizontal (flat): loses approximately 10-15%
Vertical (wall-mounted): loses approximately 25-30%
Combined adjustment for east-facing, 20-degree tilt:
Orientation factor: 0.82
Tilt factor: 0.97
Combined: 0.82 x 0.97 = 0.80 of south-facing optimal output
Shading Impact
Even partial shading causes disproportionate loss in string inverter systems
(where panels are wired in series -- one shaded panel limits the whole string).
Shading loss estimates:
No shading: 0%
Light shading (1-2 hours at certain times of day): -5 to -10%
Moderate shading (chimney, tree): -15 to -25%
Heavy shading: -30 to -50%
Microinverters or power optimisers per panel:
Each panel operates independently
Shaded panel does not pull down others
Used when any shading is present
Financial Return Calculation
4kW system, South England: 3,504 kWh/year
Self-consumption (used directly): 40% = 1,402 kWh
Export to grid: 60% = 2,102 kWh
Value of self-consumption (saves import at 24p/kWh):
1,402 x £0.24 = £336.48/year
Export income (Smart Export Guarantee, approx 15p/kWh):
2,102 x £0.15 = £315.30/year
Total annual benefit: £651.78/year
Typical 4kW system cost: £6,000-£8,000 installed
Payback period: £7,000 / £652 = 10.7 years
System lifetime: 25+ years -- significant profit after payback