How do I convert kWp to kWh?

In the UK, multiply kWp by approximately 1,000 for annual kWh output (south-facing). A 4kWp system generates ~4,000 kWh per year. Adjust: ×0.85 for east/west, ×0.58 for north, ×1.05 for southern England, ×0.88 for northern Scotland.

What does kWp mean on solar panels?

kWp (kilowatt peak) is the maximum power a panel can produce under ideal laboratory conditions (STC: 1,000 W/m² irradiance, 25°C, AM1.5). It is used to compare panels and size systems. Real-world UK output is always lower than the kWp rating.

How many kWh does a 1kWp solar panel produce per day?

In the UK, approximately 2.6–2.9 kWh per day averaged across the year. On a clear June day: 5–6 kWh. On a short December day: 0.5–1.0 kWh. The annual average works out to ~1,000 kWh per kWp.

Is kWp the same as kW?

kWp specifically means 'kilowatt peak' — the maximum under ideal conditions. kW is just 'kilowatt' — a general power measurement. In solar context, they are often used interchangeably, but kWp is technically correct when referring to panel ratings.

kWp to kWh: What the Difference Means

By Home Solar Guide Team|Updated 18 March 2026|educational

Independently written | Evidence-based | Sources cited

Solar system capacity (kWp) and actual generation (kWh) explained — kWp is capacity (what panels CAN produce). kWh is reality (what they DO produce). Both matter.

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What is the difference between kWp and kWh?

kWp (kilowatt peak) is the maximum power a solar panel can produce under ideal lab conditions. kWh (kilowatt hour) is the actual electricity generated over time. In the UK, each 1 kWp of solar generates approximately 950–1,050 kWh per year. So a 4 kWp system generates ~4,000 kWh annually. The simple conversion: multiply your system's kWp by 1,000 to estimate annual kWh output in the UK.

kWp Explained: Panel Capacity

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 Explained: Actual Energy

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 (26.11p 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.

Annual kWh output from a UK solar system — the number that matters for savings — kWh per year is what determines your savings. A 4kWp system generates ~4,000 kWh/year in the UK.

Find the right system size for your roof

Panel count depends on your electricity use, roof space and direction — not on a rule of thumb. Get a recommended size and what it should cost.

Size My System

The UK Conversion: kWp to Annual kWh

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.

kWp to kWh conversion varies by roof direction — south is best — Direction affects the kWp to kWh conversion: south = 100%, east/west = 85%, north = 58%.

Why kWp Is Always Higher Than Real Output

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.