Can a Business Run on Solar? How Much of Your Demand a Roof Supplies
Updated 23 October 2026 · SEO Dons Editorial
What does running a business on solar mean?
Two measures sit behind the phrase. Self-consumption asks how much of the solar output the site uses. Self-sufficiency asks how much of the site's demand solar meets directly. They pull in opposite directions as an array grows, so "can a business run on solar?" has no single answer until it is clear which is meant.
| Measure | Question it answers | Hypothetical example |
|---|---|---|
| Self-consumption | Of the electricity the panels make, how much is used on site? | Panels make 100,000 kWh and the site uses 70,000 of it: 70% |
| Self-sufficiency | Of the site's demand, how much does solar meet directly? | The site uses 200,000 kWh and solar meets 70,000 of it: 35% |
This page treats running on solar as meeting demand while the grid connection stays in place. A roof array on a connected site is not an off-grid supply, and what happens in an outage is covered on the power cut page.
How much can a roof supply per square metre of floor?
For a building with the same footprint on every floor, roof area is floor area divided by the number of storeys. The illustration assumes 0.1 kWp of panels for each square metre of roof, which is 60% of the roof covered at 6 square metres per kWp. PVGIS gives a Birmingham east-west flat roof 800 kWh per kWp a year, so each square metre of roof makes about 80 kWh. Spread over several floors, that output falls in proportion to the number of storeys.
The table shows what share of a year's electricity the whole roof's output would equal, by storeys and by the site's use per square metre of floor.
| Storeys | 50 kWh per m2 of floor | 100 kWh per m2 | 200 kWh per m2 | 400 kWh per m2 |
|---|---|---|---|---|
| 1 | 160% | 80% | 40% | 20% |
| 2 | 80% | 40% | 20% | 10% |
| 4 | 40% | 20% | 10% | 5% |
A single-storey building using up to 80 kWh per square metre of floor can match its annual use in total. A two-storey building matches only half of that intensity, and a four-storey one a quarter. So a low, wide building with a large roof for its floor area comes closest, such as a single-storey distribution warehouse, factory or retail unit, and a tall office comes least close, whatever its load shape.
Use the row that matches the building: the first for a single-storey warehouse, factory or retail unit, the second for a two-storey school or office block, the third for a four-storey office. To find a site's column, divide its annual kWh by its floor area. A building over 250 square metres that is occupied by a public authority and frequently visited by the public needs a Display Energy Certificate, and the DEC shows the building's total useful floor area. For sector comparisons, CIBSE publishes the TM46 energy benchmarks, which it describes as covering "what they are, how they were developed and how to use them"; the figures sit behind a purchase, so they are not reproduced here.
Why does a roof that matches annual use still leave the site buying from the grid?
Because the sun's timing is not the site's. Suppose an east-west roof makes as many kWh in a year as a Birmingham site uses, spread evenly over the months. In December it would cover 19% of an average month's use and in June 178%. Six months fall below 100%.
| Month | kWh per kWp, east-west | Output as % of an average month's use |
|---|---|---|
| January | 16 | 25% |
| February | 30 | 45% |
| March | 63 | 94% |
| April | 95 | 142% |
| May | 114 | 172% |
| June | 119 | 178% |
| July | 118 | 177% |
| August | 99 | 148% |
| September | 72 | 108% |
| October | 41 | 62% |
| November | 20 | 31% |
| December | 13 | 19% |
Nights and weekends add to the gap. The load profile page runs hourly PVGIS data against five load shapes. For an east-west array whose annual output equals the site's annual use, solar supplies directly between 38% and 59% of the load, with the lowest result for a round-the-clock load, whose night-time demand cannot use daytime output. The grid supplies the rest. Matching in total over a year is therefore not the same as running on solar.
Orientation shifts the winter end more than the summer end. In January the south-facing 35 degree array makes about 1.2 kWh per kWp on an average day against about 0.5 on east-west rows, while in June both make about 3.9 to 4.0 kWh.
Which buildings come closest, and what lifts the share?
- Low-rise, large roof, daytime load. The first row of the table and a weekday or seven-day daytime load give the highest shares.
- Steady round-the-clock load. Demand is constant, but most of it falls when the panels are idle, so self-sufficiency stays modest.
- Multi-storey buildings. The roof cannot reach annual use whatever the array.
Without a larger roof the share rises by sizing the array to the daytime load, moving flexible loads into daylight hours or storing surplus. The sizing guide covers sizing, and battery storage is covered separately. The commercial PV systems guide sets out how the parts fit together, and you can send an enquiry through the site's form if you want a designer to test a specific roof against your readings.
Which settings sit behind these figures?
- PVGIS 5.3 PVcalc, Birmingham, 1 kWp, 14% system losses, PVGIS-SARAH3 radiation data for 2005 to 2023. East-west is the mean of an east-facing and a west-facing 10 degree plane; south-facing is 35 degrees. The south-facing annual figure is 968 kWh per kWp against 800 for east-west.
- Roof assumption: 0.1 kWp per square metre of roof. Real roofs lose area to rooflights, plant, access and set-backs, so the figure is a planning assumption, not a result.
- Monthly demand is assumed flat. Real demand varies, and the load profile page models hourly demand instead.
All arithmetic is illustrative.
Sources
- PVGIS user manual, European Commission Joint Research Centre, user manual, checked 22 October 2026.
- PVGIS 5.3 PVcalc, Birmingham, 1 kWp, 14% losses, European Commission Joint Research Centre, checked 22 October 2026: south 35 degrees, east 10 degrees, west 10 degrees.
- A guide to display energy certificates and advisory reports for public buildings, Department for Energy Security and Net Zero and Ministry of Housing, Communities and Local Government, gov.uk guide, checked 22 October 2026.
- TM46 Energy benchmarks, CIBSE, publication page, checked 22 October 2026.
Common questions
Can a warehouse run on solar power?
Partly. A single-storey warehouse has a roof about equal to its floor, so in these illustrations the roof makes about 80 kWh per square metre of floor a year. If the building uses less than that, the roof can match its annual use in total, but not hour by hour, because nights, winter and weekends still need the grid.
Does a bigger array mean more self-sufficiency?
Yes, but with a falling share of the output used. In the load profile modelling a long weekday load meets about 17%, 34% and 51% of its demand directly with arrays making 25%, 50% and 100% of annual use, while the share of output used falls from 69% to 51%.
Where can a public building find its floor area?
On its Display Energy Certificate. The government's guide says a DEC shows the total useful floor area of the building, and DECs are required for buildings over 250 square metres occupied by public authorities and frequently visited by the public.
Do east-west rows or south-facing panels cover more demand?
South-facing panels at 35 degrees make about 21% more per kWp over a year in Birmingham, about three-quarters of the difference arising between October and March. East-west rows spread output further into the morning and evening, so the better layout depends on the load.