kWp to kWh: What the Difference Means

kWp = kilowatt peak = maximum capacity

kWp is measured under Standard Test Conditions (STC): - Irradiance: 1,000 W/m² (bright midday sun) - Cell temperature: 25°C - Air mass: 1.5 (standard atmospheric conditions)

These conditions rarely occur simultaneously in the real UK — actual output is always lower than kWp rating. Think of kWp like a car's top speed: a car rated at 150 mph rarely does 150 mph, but the rating lets you compare cars.

kWp is used to: - Compare panels (a 425W panel is more powerful than a 375W panel) - Size systems (a 4kWp system = 10 × 400W panels) - Quote prices (installers price per kWp: £1,300–£1,800/kWp in the UK) - Determine grid connection requirements (G98 under 3.68kWp, G99 above)

Source: IEC 61215 Standard Test Conditions definition.

kWh = kilowatt hour = actual electricity generated or consumed

1 kWh = 1,000 watts for 1 hour. It is the standard unit on your electricity bill.

What uses 1 kWh? - Running a 2kW kettle for 30 minutes - Running a 100W laptop for 10 hours - Running a 10W LED bulb for 100 hours - Driving an EV approximately 3.5 miles

What generates 1 kWh from solar? - A 1kW panel in full sunshine for 1 hour - OR a 2kW panel in half-sunshine for 1 hour - OR a 500W panel in full sunshine for 2 hours

kWh is what matters for your bill: - You PAY for grid electricity in kWh (24.5p per kWh) - You EARN SEG income in kWh (4–15p per kWh exported) - You SAVE money based on kWh self-consumed

Source: Ofgem billing; energy measurement standards.

In the UK, each 1 kWp generates approximately 950–1,050 kWh per year (south-facing, 35° tilt).

Quick conversion table:

| System Size (kWp) | Annual Output (kWh) | Daily Average (kWh) | |-------------------|--------------------|-----------------| | 1 kWp | 950–1,050 | 2.6–2.9 | | 2 kWp | 1,900–2,100 | 5.2–5.8 | | 3 kWp | 2,850–3,150 | 7.8–8.6 | | 4 kWp | 3,800–4,200 | 10.4–11.5 | | 5 kWp | 4,750–5,250 | 13.0–14.4 | | 6 kWp | 5,700–6,300 | 15.6–17.3 | | 8 kWp | 7,600–8,400 | 20.8–23.0 | | 10 kWp | 9,500–10,500 | 26.0–28.8 |

Rule of thumb: multiply kWp by 1,000 for annual kWh.

Factors that adjust this: - East/west facing: multiply by 0.85 (15% less) - North facing: multiply by 0.58 (42% less) - Location: southern England ×1.05, northern Scotland ×0.88 - Shading: reduce by 10–30% depending on severity

Source: PVGIS UK yield data by location and orientation.

Your panels rarely produce their full kWp rating because:

1. UK irradiance is lower than STC — STC assumes 1,000 W/m² constant. UK average is ~100–150 W/m² (annual mean). Panels only hit 1,000 W/m² on the brightest summer days around midday.

2. Temperature losses — STC assumes 25°C. On hot summer days, panels exceed 25°C and lose ~0.4% per degree. On cold winter days, panels are MORE efficient but receive less light.

3. Inverter losses — 2–4% of DC power is lost in the DC-to-AC conversion.

4. Cable losses — 1–2% lost in wiring between panels and inverter.

5. Soiling — 2–5% lost to dust, pollen, bird droppings over time.

6. Night time — 0% output for 8–16 hours per day.

7. Seasonal variation — December daylight: 7–8 hours. June daylight: 16–17 hours.

Combined, these factors mean UK panels produce approximately 10–11% of their kWp rating as average output (950–1,050 kWh per kWp per year ÷ 8,760 hours per year ≈ 11% average capacity factor).

Source: PVGIS capacity factor data; system loss calculations.