Solar output calculator: kWh per day per kW

What 1 kW, 4 kW and 10 kW of solar panels produce per day, month and year

In London, a 4 kW solar system produces about 12.0 kWh a day, 364 kWh a month and 4,369 kWh a year (PVGIS sun hours, 14% losses); 1 kW makes 3.0 kWh a day (91 a month, 1,092 a year) and 10 kW makes 29.9 kWh a day (910 a month, 10,924 a year). In Texas the same sizes make 4.6, 18.2 and 45.6 kWh a day. A June day in London gives 17.5 kWh from 4 kW and a December day 5.1 kWh, so the yearly average hides a 3.4-to-1 swing; the solar output figures for US states are yearly averages.

The 4 kW UK guide runs every UK city, and month by month in three cities, and the 3 kW and 5 kW guides do the same for those sizes; the kWh-per-panel guide has one 400 W panel and a 10 kW system in every state. Going the other way, from the kWh you need to the panels: 30 kWh a day takes 20 panels of 400 W at 4.5 sun hours, and the panel calculator takes any bill. For what the kWh are worth, the payback calculator prices the same sizes.

How much electricity does 1 kW of solar panels produce?

2.6-5.6 kWh a day in the US and 2.5-3.4 kWh a day in the UK, yearly averages after 14% losses: 78-170 and 75-102 kWh in an average month. A 400 W panel is 0.4 kW, so it makes 0.4 of the figure. Every state and UK city is in the tables below.

kWh per day per kW installed, by state and UK city

One kW of panels makes sun hours × 0.86 kWh a day after 14% losses. Multiply the figure for your place by your system size in kW: 7 kW in Texas is 7 × 4.56 kWh = 31.9 kWh a day. The top of the US table is Arizona and New Mexico at 5.59 kWh per kW a day, the bottom Alaska at 2.58 kWh; in the UK it runs from 2.48 kWh in Glasgow to 3.35 kWh in Brighton.

Where the 14% loss factor comes from

NREL’s PVWatts Version 5 manual (2014) lists ten default losses between the panel rating and the meter: soiling 2%, shading 3%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, availability 3%; snow and age are 0%. They multiply rather than add: 1 − the product of (1 − each loss) = 14.1%, which NREL rounds to 14%. So every kW of panels delivers 0.86 of what the sun hours alone suggest. The tool uses the same figure; raise it for a shaded or dirty roof.

Worked examples: what 3 kW, 4 kW and 5 kW systems generate in the UK, and how many panels you need for 1,000 kWh, 1,500 kWh, 2,000 kWh and 2,500 kWh a month.

How it works

Output per day (kWh) = system size (kW) × peak sun hours × (1 − losses). Per year = per day × 365; per month = per year / 12. Per panel = per day / (system kW × 1,000 / panel watts). The size is the DC rating of the panels; the losses turn it into what reaches your meter.

With the US starting values, 7 kW × 5.3 hours × 0.86 = 31.9 kWh a day, 970 kWh a month, 11,646 kWh a year. One 400 W panel alone makes 1.82 kWh a day there. In London, 4.5 kW makes 4,916 kWh a year, but not evenly: June 590 kWh against December 178 kWh, because PVGIS publishes the sun hours by month for UK cities and the tool uses them. For US states the source gives a yearly figure, so the month table splits the year by days per month; the real curve peaks in summer.

The yearly sun hours assume a south-facing array tilted near the latitude; the European figures are at the optimal tilt PVGIS computes (about 40°). PVGIS’s own production estimate for the same 1 kWp runs about 5-6% below this formula because it also models the panels’ temperature; the 14% losses here do not include that. An east or west roof gives 15-20% less (Energy Saving Trust). Every number is yours to change; the starting values only save typing.

Sources of the US starting values

• Peak sun hours by US state: Unbound Solar, "Peak Sun Hours by US State" (undated, read 2026-09-20), table built from NREL's PVWatts data (National Solar Radiation Database); "A peak sun hour is one hour of sunlight at an average intensity of 1,000 watts per square meter (1 kW/m²)" — https://unboundsolar.com/solar-information/sun-hours-us-map, read on 2026-09-20
• Panel wattage and area: Trina Solar, Vertex TSM-DE09 data sheet (version TSM_EN_2020_PA3), product range 390-405 W: module dimensions "1754×1096×30 mm (69.06×43.15×1.18 inches)", 21.0 kg, efficiency up to 21.1%, "25 year Power Warranty, 0.55% Annual Power Attenuation, 2% first year degradation". Area = 1.754 m × 1.096 m = 1.922 m² per panel — https://static.trinasolar.com/sites/default/files/EN_Datasheet_Vertex_DE09.pdf, read on 2026-09-20
• System losses: NREL, PVWatts Version 5 Manual (NREL/TP-6A20-62641, September 2014), section 11 System Losses, Table 6: soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, age 0%, availability 3%; "The default total system loss is calculated to be 14 %" (the losses multiply, they do not add) — https://www.nrel.gov/docs/fy14osti/62641.pdf, read on 2026-09-20

UK sun hours come from PVGIS (European Commission), shown under the field when you switch region.

Estimates from published averages (NREL, EIA, Ofgem, industry cost surveys), each with its source and date: an installer's site survey and shading analysis replace them. Not financial advice.