Solar Panel Output: How to Estimate Your Annual kWh Generation

The standard formula

Solar panel output in kWh is estimated using a straightforward formula:

Annual kWh = System capacity (kWp) × Annual peak sun hours × Performance ratio

Where:

• System capacity is your total panel wattage in kilowatts-peak (kWp)
• Annual peak sun hours is the solar resource at your location (typically 950–1,650 for the UK)
• Performance ratio accounts for real-world losses (typically 0.72–0.82 for UK systems)

Worked examples for UK homes

Example 1: A 4 kWp system in Surrey (South East England)

• Annual peak sun hours: ~1,450
• Performance ratio: 0.78
• Annual output: 4 × 1,450 × 0.78 = 4,524 kWh/year

Example 2: A 6 kWp system in Yorkshire

• Annual peak sun hours: ~1,250
• Performance ratio: 0.76
• Annual output: 6 × 1,250 × 0.76 = 5,700 kWh/year

Example 3: A 3.6 kWp system in Cornwall

• Annual peak sun hours: ~1,600
• Performance ratio: 0.80
• Annual output: 3.6 × 1,600 × 0.80 = 4,608 kWh/year

What affects performance ratio?

The performance ratio (PR) is the gap between theoretical maximum output and real-world generation. It's influenced by:

• Inverter efficiency — typically 95–97% for modern string inverters
• Temperature losses — solar panels are less efficient when hot (a counterintuitive fact)
• Shading — even partial shading can significantly reduce output without micro-inverters or optimisers
• Cable losses — small but present, typically 1–3%
• Soiling — dust, bird droppings and leaf debris reduce output, especially in dry periods
• Panel degradation — panels lose around 0.5% efficiency per year on average

A new, well-installed system with good shading avoidance might achieve a PR of 0.82. An older system with some shading might be closer to 0.70.

The problem with estimates

Even a carefully calculated estimate can be significantly off if any of the inputs are wrong. Most homeowners don't know their exact performance ratio. Location data affects sun hours materially. Roof orientation makes a big difference — a south-facing roof at 35° tilt is optimal for the UK; an east or west-facing roof can reduce output by 15–20%.

This is why the Sun Hours app takes a different approach. Instead of asking you to input figures you might not know, it asks for something you do know: the most kWh your system generated on its best ever day.

That single real number contains all the complexity. It reflects your actual roof angle, your shading, your inverter's real-world efficiency, your panels' current degradation level — everything. And it's accurate to your specific installation, not a theoretical average.

Checking your estimate

If you want to verify any estimate, the Microgeneration Certification Scheme (MCS) has a standard methodology for UK solar output calculations that most installers use when sizing systems. Your original installation paperwork may include a yield estimate calculated using this method.

Cross-referencing that against your actual Smart Meter or inverter data after a year of operation is the most reliable way to understand your system's real performance ratio — which you can then use to make future estimates more accurate.

Or just use Sun Hours. ☀️