Sizing a Commercial Solar PV System: How Many kWp Do You Need?

The single most common mistake in commercial solar

Most businesses ask the wrong first question. They ask how many panels fit on the roof. The right question is how much of your own generation you will actually use, because self-consumption, not roof area, is what decides your payback.

A commercial solar PV system is sized from your annual energy spend and the shape of your consumption across the day, not from a square-metre estimate. Get the sizing wrong in one direction and you leave money and roof space on the table. Get it wrong in the other and you export half your output at 5p when you are buying it back at 30p. This guide sets out how to work out the right commercial solar system size for your building, with the real numbers behind each step.

Start with kWp, not panel count

Capacity is measured in kilowatts-peak, or kWp. It is the output of the array under standard test conditions, and it is the number every quote, grant application and grid connection is built around. Panel counts vary as module wattage improves, so kWp is the stable unit.

Two rules of thumb anchor everything else:

• 1 kWp of PV occupies roughly 5 to 6 sqm of unshaded roof.
• 1 kWp generates about 900 to 1,000 kWh a year in the UK, and a well-oriented array can reach 1,050 kWh per kWp.

So a 1,000 sqm warehouse roof typically supports 150 to 180 kWp, and a 250 sqm office roof around 30 to 40 kWp. Those are ceilings set by the roof. The right size is usually below that ceiling, set by your consumption. For real-world pricing across the full range, see our cost guide.

Size from the meter, not the roof

The design target for a commercial system is annual generation equal to 60 to 85 percent of your current consumption. That band is deliberate. It captures the bulk of the daytime bill while avoiding a system so large that it spills low-value surplus onto the grid all summer.

To find that number properly you need your half-hourly meter data. Every commercial supply above the smallest tariffs records consumption in 48 half-hour slots a day, and your supplier can provide 12 months of it. That data shows the shape of your demand: when it peaks, how much runs overnight, how weekends differ from weekdays. A generation profile is then modelled against it in software such as PVSyst, and the overlap between the two curves is your self-consumption.

This is the whole game. A generic per-square-metre estimate cannot tell you whether you will use 55 percent or 85 percent of what you generate, and that difference can move payback by two years or more. If you want us to run the numbers from your data, start a free desk feasibility.

Self-consumption: the number that decides payback

Every unit of solar you consume on site is worth the full grid price you avoid, currently 25 to 45p per kWh on commercial contracts. Every unit you export earns the Smart Export Guarantee rate, typically 4 to 15p per kWh. That gap, often five or six times, is why self-consumption matters more than raw generation.

A daytime-occupied commercial building on 09:00 to 18:00 hours typically achieves 55 to 75 percent self-consumption with no battery at all. The type of building shifts that figure a long way:

Building type	Typical self-consumption (no battery)	Typical payback
Factory / manufacturing (steady process load)	80 percent or more	5 years
Warehouse with refrigeration or MHE charging	70 to 85 percent	6 years
Retail and showroom	60 to 75 percent	6 years
Office	60 to 80 percent	7 years
Hospitality and leisure (evening-weighted)	45 to 65 percent	7 years
School / public building (term-time daytime)	55 to 75 percent	7 years

Manufacturing sits at the top because a high, steady daytime process load soaks up almost everything the roof produces. That is why factories reach the best payback of any sector. Buildings with lighter weekend or evening use, offices and retail among them, export more and sit at the longer end. Explore how the numbers work for your sector on our manufacturing and factories and offices pages.

Worked examples by sector

Rules of thumb are useful, but real figures make sizing concrete. These ranges come from our sector modelling and reflect typical UK installs.

Sector	Typical size	Roof area	Annual generation	Typical project value
Offices	30 to 150 kW	200 to 900 sqm	27,000 to 138,000 kWh	£30,000 to £150,000
Warehouses / industrial	100 to 500 kW	600 to 3,000 sqm	92,000 to 460,000 kWh	£85,000 to £425,000
Manufacturing / factories	200 kW to 2 MW	1,200 to 12,000 sqm	184,000 to 1,840,000 kWh	£150,000 to £1.5m
Retail / showrooms	40 to 250 kW	240 to 1,500 sqm	37,000 to 230,000 kWh	£36,000 to £210,000
Agricultural	50 to 500 kW	300 to 3,000 sqm	46,000 to 460,000 kWh	£45,000 to £425,000

Take a mid-range warehouse. A 2,800 sqm distribution unit with £96,000 of annual electricity might carry a 180 kW array generating around 168,000 kWh a year. If daytime forklift charging and refrigeration drive 78 percent self-consumption, the annual saving lands near £38,000 and simple payback around 5.5 years. That is a real profile, not a rounded-up sales figure.

Contrast that with an office. A 900 sqm converted office and showroom paying £22,000 a year might take a 60 kW array producing 55,000 kWh. With lighter weekend load the saving is closer to £13,500 and payback around 6.5 years, still strong, and helped by lifting the EPC rating for MEES compliance. Sector-specific detail lives on our warehouses and industrial units and agricultural buildings pages.

When to add a battery

A battery does not generate power. It shifts it, storing daytime surplus for use in the evening, overnight or at the weekend. The question is whether enough of your demand falls outside generation hours to justify the cost.

For a factory running a steady daytime line, the answer is often no: the load already absorbs the output, so there is little surplus to store. For a hotel, a restaurant or a supermarket with evening and overnight refrigeration, the answer is frequently yes. Storage typically lifts self-consumption from 55 to 75 percent up to 80 to 95 percent and adds 25 to 40 percent to annual savings, at the cost of a longer payback on the extra capital.

Configuration	Self-consumption	Annual saving vs PV-only	Payback effect
PV only	55 to 75 percent	baseline	Shortest payback
PV plus battery	80 to 95 percent	+25 to 40 percent	Longer payback, higher lifetime return

The sensible approach is to model both side by side, and to design every system to be battery-ready even if you add storage later. Batteries are usually worth modelling above 100 kWp, or wherever evening, weekend or overnight baseload is significant. Hospitality is the classic case, covered on our hospitality and leisure page.

Where the panels go: rooftop, ground or carport

Most commercial systems are rooftop, because the roof is already there and does not compete with operational land. Where roof area falls short of demand, ground-mount or solar carport can top up the capacity.

Mounting	Best for	Trade-off
Rooftop	Almost every commercial building	Limited by roof size, pitch and shading
Ground-mount	Sites with spare land, factories short of roof	Needs land and usually full planning above PD thresholds
Solar carport	Car parks at retail, offices, leisure venues	Higher cost per kWp, adds EV-charging and shade value

Non-penetrative clip-fix mounting suits most metal warehouse and industrial roofs and preserves the roof warranty. Roofs over about 1,000 sqm generally need a structural survey for the added dead load and wind uplift, and any building constructed before 2000 needs an asbestos check first.

How sizing interacts with grid connection

System size drives the grid connection route, so it is not a purely financial choice. Small commercial systems, roughly under 50 kW, can often use the faster G98 or G99 fast-track. Above that, you need a full G99 application to your Distribution Network Operator, and for larger arrays export limitation (G100) is frequently used to cap what reaches the grid, which speeds the connection and avoids costly network reinforcement.

Sometimes the DNO decides your size for you. If the local network cannot accept much export, limiting export via G100 lets you install a larger array for self-consumption while keeping the connection simple. Rural sites in particular can hit capacity limits, so the grid application goes in early, usually before the site survey. Timescales run 4 to 12 weeks for small connections and 6 to 18 months for larger ones.

Funding does not change the size, but it changes the case

How you size the system and how you pay for it are separate decisions, but the funding route affects the return on a given size. Three points matter:

• 100 percent Annual Investment Allowance lets a profitable company deduct the full capex from taxable profit in year one, an effective saving of roughly 25 percent for a limited company.
• VAT is reclaimable for VAT-registered businesses. This is commercial relief, not the 0 percent rate that applies only to domestic installs.
• The Smart Export Guarantee pays 4 to 15p per kWh for surplus, which matters more for buildings that export a larger share.

Asset finance spreads the cost over 5 to 7 years and is usually cash-flow positive from month one, as the finance payment sits below the bill saving. A Power Purchase Agreement needs zero capex: a funder owns the system and you buy the power below grid rate. Full detail is on our grants and funding page.

Put a first number on your system today

You can estimate your own starting point in three steps. Take your annual kWh consumption from a recent bill. Multiply by 0.7 to hit the middle of the 60 to 85 percent target band. Divide by 950 to get an approximate kWp figure. A business using 200,000 kWh a year lands around 147 kWp, subject to roof area and load shape.

That is a sanity check, not a design. The real number comes from your half-hourly data modelled against a UK generation profile, which is what separates a defensible proposal from a clipboard estimate. Try the savings calculator for a quick projection, read the frequently asked questions for the detail behind each figure, or request a fixed-price quote and we will size it properly from your meter data.