Business Solar System Size Calculator: kWp from Your Bill, kWh or Roof Area
The sizing rule in one line: annual electricity use, times the share of it you use in daylight, divided by the yield for your region, gives the system size in kWp; then check that size fits the roof. This commercial solar system size calculator does that from a bill, an annual kWh figure or a roof area, shows every assumption and sends nothing anywhere unless you choose to.
- No email gate
- Load-limited or roof-limited verdict
- PVGIS-checked yields
- 50 kW planning line
Your electricity use, your roof, your region
Fill in what you know. Annual kWh is best; annual spend works as a first pass; the roof area sets the ceiling. The result updates as you type.
The size ladder: what each sizing rule gives
Generation is size × yield. On-site use is capped at the electricity you use in daylight; the rest is exported. Year-1 value is on-site kWh at your unit rate plus exported kWh at the export rate, before any costs.
How the sizing calculation works
A solar system for a business is sized against the electricity the building uses while the sun is up, not against the roof and not against the annual bill as a whole. The three rules this page applies are deliberately simple, and each one is a single line of arithmetic you can check.
- Daytime-matched size. Annual kWh × the share used in daylight ÷ yield. Over a year the system generates roughly what the site uses in daylight, so most of the output is used on-site and the export share stays small. This is the size the calculator recommends when the roof allows it.
- Full-offset size. Annual kWh ÷ yield. Over a year the system generates as much as the site uses in total. It is the size people often have in mind, and the ladder shows the price of it: a larger slice of the output is exported at the export rate rather than displacing imports at the unit rate.
- Roof limit. Usable roof area ÷ 6.5 m² per kWp. If this is smaller than the daytime-matched size, the roof decides and the verdict reads roof-limited.
On-site use at any size is capped at the daytime load, which is a different rule from the business solar calculator: that tool applies a fixed share of generation (65% by default) because its job is the money, while this one is answering a size question. Both are planning approximations. Real on-site use depends on the match between the half-hourly load and the half-hourly generation across the year, and a weekday-only site with a summer shutdown will use less of its output than the annual arithmetic suggests. Yields come from the yields by region page, where the five regional defaults are checked against the European Commission’s PVGIS model for twelve UK cities.
Worked example: a 115,000 kWh site in the Midlands
With the example inputs (115,000 kWh a year, 65% in daylight, 950 kWh per kWp, no roof limit entered) the daytime-matched size is 78.7 kWp and the full-offset size is 121.1 kWp. At 78.7 kWp the system generates 74,765 kWh in year one, of which 74,750 kWh is used on-site (65% of annual use) and 15 kWh is exported. At the default 26p unit rate and 12p export rate that is worth about £19,437 a year before any costs. The same system needs about 175 450 W panels on roughly 512 m² of roof, and because it is above 50 kW the English planning route is Class J(c) prior approval rather than straightforward permitted development.
Push the size up to the full-offset 121.1 kWp and generation rises to 115,045 kWh, but the export share climbs to 35% of output. The extra kWp still earn, just at the export rate. Whether that is worth doing is a payback question, which is why the result panel hands the size to the commercial solar payback calculator and the commercial solar ROI calculator.
Three illustrative sizing cases
These are hypothetical sites, not customers, chosen to show the three verdicts the calculator can give. Every figure is produced by the same engine that runs the tool above.
| Illustrative site | Annual use | Yield | Daylight share | Usable roof | Recommended | Verdict |
|---|---|---|---|---|---|---|
| Office (Midlands) | 60,000 kWh | 950 | 70% | 500 m² | 44.2 kWp | Load-limited, 50 kW or under |
| Warehouse (North England) | 250,000 kWh | 850 | 60% | 2,500 m² | 176.5 kWp | Load-limited, over 50 kW |
| Workshop (South England) | 90,000 kWh | 1,050 | 65% | 250 m² | 38.4 kWp | Roof-limited, 50 kW or under |
Hypothetical scenarios at the regional default yields, 6.5 m² per kWp. Not measured data from any installation.
- Office, load-limited under 50 kW. 60,000 kWh a year with 70% in daylight at 950 kWh per kWp gives 44.2 kWp. The 500 m² roof could hold 76.9 kWp, so the load decides. At or below 50 kW this is microgeneration in England.
- Warehouse, load-limited over 50 kW. 250,000 kWh with a lower daylight share of 60% at a northern yield of 850 gives 176.5 kWp on a roof that could take 384.6 kWp. Above 50 kW, so the council’s prior approval applies in England; the size itself is not restricted by that.
- Workshop, roof-limited. 90,000 kWh at 1,050 kWh per kWp with 65% in daylight would want 55.7 kWp, but the 250 m² roof holds only 38.4 kWp. The roof decides; the system will offset less of the daytime load and export almost nothing.
How to read your bill for sizing
Three numbers matter and all three are on a business electricity bill or available from the supplier.
- Annual consumption in kWh. Annual statements show it directly. Monthly bills need twelve months added up, because a winter month and a summer month can differ by a third or more. Do not size from one bill.
- The unit rate. The pence-per-kWh figure, excluding VAT and the Climate Change Levy line. It values every kWh the system keeps off the grid, and it is the number to type into the calculator in place of the 26p default. Standing charges do not change with solar and are not part of the sizing.
- The daytime share. Half-hourly data (every site with a half-hourly meter can request it from the supplier) shows how much of the annual total falls between roughly 8am and 6pm. Without it, use the operating pattern: a Monday-to-Friday daytime business sits at the top of the range, a cold store or a 24-hour operation lower per kWh, a site that closes for the summer lower still.
The method page explains the same inputs in the context of the main calculator, and the worked examples carry 30, 100 and 250 kWp systems through to savings and payback line by line.
Why the right size is rarely the biggest size
Every kWh of solar generation lands in one of two places. If the building is using electricity at that moment, the kWh displaces an import worth the unit rate on the bill. If it is not, the kWh is exported and paid at the export rate. In Great Britain, export rates are set by suppliers under the Smart Export Guarantee; the 12p default here is a planning figure in line with the better fixed rates on offer in October 2026, and some tariffs pay far less. With a 26p unit rate, an exported kWh is worth less than half of an on-site one, so the first kWp on a roof is the most valuable and each additional kWp beyond the daytime load is worth progressively less.
That is the whole argument for sizing to the daytime load rather than the annual total or the roof. It is also why the calculator shows the ladder rather than a single number: if the export rate you can get is high, or if a battery or an EV fleet is planned that will absorb the export, the full-offset size can make sense, and the ladder lets you see the trade at a glance.
The 50 kW planning line
In England, solar on a building that is not a house or a block of flats is covered by Class J of Part 14 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015. Up to 50 kW the equipment counts as microgeneration under Class J(b) and is permitted development, subject to the class’s conditions: it must not protrude more than 0.2 metres from a pitched roof or sit more than one metre above a flat roof, must stay a metre from the roof edge, and listed buildings and scheduled monuments are excluded. Above 50 kW the installation falls under Class J(c) and the developer must apply to the council for its prior approval on the design and external appearance, including glare. The calculator tells you which side of the line your size sits on. Wales uses Part 43 Class A of its own order and Scotland Class 6J; check the current order for your nation before relying on permitted development.
Where the inputs come from
- Yields. The five regional defaults are the same planning values the main calculator uses, checked on the yields by region page against PVGIS v5.3 output for twelve UK cities at 10° and 35° tilt (queried 20 October 2026).
- Roof area per kWp. 6.5 m² is our default. A current commercial module of around 440 to 460 W occupies roughly 2.1 to 2.3 m², which is about 4.7 to 5.2 m² of module per kWp; the rest is row spacing on a flat roof, edge margins and access. A pitched roof with modules laid flush can do better; a flat roof with wide rows does worse. It is an input so you can use your installer’s layout.
- Daylight share and unit rate. Both are our defaults, both are inputs, and your half-hourly data and your bill replace them.
- Module rating. 450 W is a planning assumption for the panel count only; it does not change the kWp.
Planning references last checked on 20 October 2026. This page does not cover inverter sizing, string design or DC-to-AC ratios, which are the installer’s design decisions.
Common questions about sizing a business solar system
How do I work out what size solar system my business needs?
Start from annual electricity use in kWh (your bill or half-hourly data), decide what share of it falls in daylight, and divide by the yield for your region. 115,000 kWh a year with 65% in daylight at 950 kWh per kWp points to about 78.7 kWp. Then check the roof: at 6.5 m² per kWp that size needs about 512 m² of usable roof. The smaller of the two answers is the size to take to an installer.
How many solar panels does a business need per kWp?
It depends on the module. At the 450 W module this page assumes, 1 kWp is between two and three panels and takes about 6.5 m² of roof once row spacing, edge margins and walkways are included. A 100 kWp system is roughly 223 panels on about 650 m². Your installer’s datasheet gives the exact figure.
Is a bigger solar system always better for a business?
No. Each kWh you use on-site is worth your import rate; each kWh you export is worth your export rate, and the two can differ by a factor of two or more. Once generation passes the electricity you use in daylight, every extra kWp earns mostly export income, so payback lengthens. The size ladder above shows the export share rising as the system grows.
Does the size change if the system is over 50 kW?
The maths does not, but the planning route in England does. Up to 50 kW on a non-domestic building is microgeneration under Class J(b) of Part 14 of the General Permitted Development Order, permitted development subject to its conditions. Above 50 kW it is Class J(c), which needs the council’s prior-approval decision on design and external appearance, including glare. Wales and Scotland have their own orders. None of this stops a larger system; it adds a step.
Can I size a solar system from my electricity bill alone?
Yes, as a first pass. Enter the annual spend and the unit rate from the bill and the calculator estimates annual kWh from them. The kWh figure on the bill is better, and half-hourly data is better still, because it shows how much of your use actually falls in daylight.
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