How many kWh does a solar panel produce per day?

A single 400W panel produces approximately 1.0–1.2 kWh per day averaged across the year in the UK. On a sunny June day, it can produce 2.0–2.5 kWh. On a short December day, 0.2–0.4 kWh.

What does kWp mean on solar panels?

kWp (kilowatt peak) is the maximum power a panel can produce under ideal laboratory conditions (Standard Test Conditions). It is used to compare panels and size systems. A 400W panel has a kWp of 0.4. Real-world UK output is lower than kWp due to weather and conditions.

How much electricity does a 4kW solar system produce?

A 4kW system produces approximately 3,800–4,200 kWh per year in the UK, depending on location and roof orientation. This is roughly equivalent to the average UK household's annual electricity consumption of ~3,700 kWh.

Why is my solar output lower than expected?

Common causes: shading from new tree growth or construction, dirty panels, inverter fault, sub-optimal weather year, or gradual degradation. Check your monitoring app, compare to previous years, and contact your installer if output drops more than 10% without explanation.

Solar Panel Output: kWp, kWh & What It All Means

Last updated: 18 March 2026

Independently written

UK solar panel yearly energy production chart showing output over time — Understanding kWp and kWh is essential for comparing solar panels and estimating savings.

How much output do solar panels produce?

Solar panel output is described in two ways: kWp (kilowatt peak — the panel's maximum capacity under ideal lab conditions) and kWh (kilowatt hours — the actual electricity generated over time). A 400W (0.4kWp) panel in the UK generates approximately 380–420 kWh per year. A 4kW system (10 panels) generates 3,800–4,200 kWh per year. The gap between kWp and real output is due to UK weather, roof angle, and seasonal variation.

kWp vs kWh: The Key Difference

kWp (kilowatt peak): - The maximum power a panel can produce under Standard Test Conditions (STC) - STC = 1,000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum - This is a lab measurement — real UK conditions rarely match STC - Used to: compare panels, size systems, quote prices - Example: '400W panel' means 0.4 kWp

kWh (kilowatt hours): - The actual electricity generated over a period of time - 1 kWh = 1,000 watts for 1 hour (or 500 watts for 2 hours, etc.) - This is what your electricity bill measures and what you actually use - Used to: calculate savings, compare to your consumption - Example: 'Panel generates 400 kWh per year'

The conversion (UK): In the UK, each 1 kWp of solar capacity generates approximately 950–1,050 kWh per year (depending on location, orientation, and tilt).

So a 4kW system generates: 4 × 1,000 = ~4,000 kWh/year (UK average).

Source: PVGIS UK irradiance data; IEC 61215 STC definitions.

Solar system sizes showing kWp capacity for different homes — kWp tells you system capacity; kWh tells you actual generation — both matter.

UK Solar Output by Location

Solar output varies across the UK based on solar irradiance (how much sunlight reaches the ground):

Annual kWh per kWp installed (south-facing, 35° tilt):

| Region | kWh per kWp | 4kW System Output | |--------|------------|-------------------| | South West England | 1,050–1,080 | 4,200–4,320 kWh | | South East England | 1,020–1,050 | 4,080–4,200 kWh | | London | 1,000–1,030 | 4,000–4,120 kWh | | Midlands | 950–1,000 | 3,800–4,000 kWh | | North West England | 920–960 | 3,680–3,840 kWh | | North East England | 910–950 | 3,640–3,800 kWh | | Wales | 950–1,020 | 3,800–4,080 kWh | | Central Scotland | 870–920 | 3,480–3,680 kWh | | Northern Scotland | 830–880 | 3,320–3,520 kWh |

The difference between the sunniest and least sunny UK regions is approximately 20%. This affects savings and payback period but does not make solar unviable anywhere in the UK.

Source: PVGIS European Commission irradiance database.

UK solar suitability map showing output variation by region — Southern England generates 20% more than northern Scotland — but all regions are viable.

What Affects Solar Panel Output?

Factors that increase or decrease real-world output:

Roof orientation — south-facing = 100%. East/west = 80–85%. North = 55–60%.
Roof tilt — 30–40° is optimal. Flat (0°) = 85–90%. Very steep (60°) = 90–95%.
Shading — partial shade from trees, chimneys, or buildings can reduce output by 10–30%.
Panel efficiency — monocrystalline (20–22%) produces more per m² than polycrystalline (15–17%).
Temperature — panels lose ~0.4% efficiency per degree above 25°C. UK's cool climate is an advantage.
Inverter efficiency — string inverters convert at 96–98% efficiency. Micro-inverters are similar.
Cable losses — 1–2% lost in wiring between panels and inverter.
Soiling — dirt, bird droppings, pollen can reduce output by 2–5%.
Degradation — 0.3–0.5% per year. Year 25 output ≈ 85–90% of year 1.
Season — summer months generate 5–6x more than winter months.

Shading impact on solar panel output from trees and buildings — Shading is the biggest controllable factor — proper system design minimises its impact.

Daily and Monthly Output Patterns

A 4kW system produces different amounts depending on the time of day and season:

Daily pattern: - Sunrise to ~9am: gradual ramp up (5–20% of peak) - 10am–2pm: peak output (80–100% of capacity on sunny days) - 3pm–sunset: gradual decline (similar to morning) - Night: zero output

Peak daily output examples (4kW system, south-facing): - Clear June day: 22–25 kWh - Partly cloudy April day: 14–18 kWh - Overcast October day: 5–8 kWh - Short December day: 1–4 kWh

Monthly averages (4kW, Midlands): - January: 100 kWh | July: 500 kWh - April: 420 kWh | October: 220 kWh

The daily and monthly patterns are predictable — solar is not random. Your installer can estimate monthly output accurately based on decades of irradiance data.

Source: PVGIS hourly simulation data for UK.

UK sun path showing daily and seasonal output variation — Output follows predictable daily and seasonal patterns — peaking at midday in summer.

How to Monitor Your Solar Output

Every modern inverter includes monitoring capability:

Inverter display: Shows real-time output in watts and daily/total generation in kWh. Accessible without any app.

Manufacturer app: Most inverters connect via WiFi and provide: - Real-time power output - Daily, monthly, and annual generation history - Self-consumption vs export data - Performance alerts if output drops - Comparison to expected generation

Popular monitoring platforms: - GivEnergy app (for GivEnergy inverters/batteries) - mySolarEdge (for SolarEdge inverters) - Enphase Enlighten (for Enphase micro-inverters) - Solis Cloud (for Solis inverters) - FusionSolar (for Huawei inverters)

Monitoring is your best tool for ensuring your system performs as expected and catching issues early.

Source: Manufacturer monitoring platforms.

UK homeowner checking solar output on monitoring app — Monitoring apps show real-time and historical output — essential for tracking performance.

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