What size solar system does a business need?

Most UK SMEs need a solar system sized at 60-80% of annual electricity consumption to maximise self-consumption while staying within roof and capex constraints. For a business spending £30,000/year on electricity, that's typically a 60-75 kW system. Sizing should be driven by half-hourly meter data, not roof area or rules of thumb.

Most UK SMEs need a solar system sized at 60-80% of annual electricity consumption. Sizing larger pushes self-consumption down and forces excess generation into low-rate SEG export, weakening overall economics. Sizing smaller leaves savings on the table. For a business spending £30,000/year on electricity at 38p/kWh — about 79,000 kWh annual consumption — the optimal system size lands at around 60-75 kW. Sizing should always be driven by your half-hourly meter data and a proper PVSyst or similar yield model, not by roof area or generic rules of thumb. Roof, electrical capacity, and capex can constrain below the optimum, in which case you size to the binding constraint.

How sizing actually works

A proper sizing exercise considers four constraints, picks the smallest, and that’s your system size:

Consumption ceiling — usually 60-80% of annual kWh consumed
Roof ceiling — what fits on usable roof area (6-7 m²/kW)
Electrical ceiling — what your switchgear and supply can take
Capex / budget ceiling — what your finance route allows

Whichever is smallest, that’s your size. Most SMEs are constrained by consumption (it’s economically suboptimal to size beyond ~80% of annual kWh).

Worked sizing examples

50-employee office, £18,000/year electricity

Annual consumption: 47,000 kWh
Target generation: 60-75% = 28,000-35,000 kWh
System size: 32-40 kW
Roof check: 200-250 m² needed; office has 320 m² usable. OK.
Electrical check: three-phase 200 A supply existing. OK.
Capex: £30,000-£37,500. OK for asset finance.
Recommended: 36 kW

Light-industrial unit, £58,000/year electricity, 24/5 operations

Annual consumption: 153,000 kWh
Target generation: 70-85% = 107,000-130,000 kWh
System size: 120-145 kW
Roof check: 720-870 m² needed; building has 950 m² usable. OK.
Electrical check: 400 A three-phase. OK.
Capex: £108,000-£130,500.
Recommended: 135 kW

Garden centre, £32,000/year electricity, 7-day-a-week daytime

Annual consumption: 84,000 kWh
Self-consumption profile: very high (weekend operations)
Target generation: 75-90% = 63,000-76,000 kWh
System size: 70-85 kW
Roof check: 420-510 m² needed. Garden centre has 480 m² usable across two greenhouse-adjacent buildings.
Recommended: 75 kW (constrained slightly by roof)

Why not size at 100% of consumption?

Two reasons:

Generation profile doesn’t match consumption profile. Solar generates middle of the day. SMEs use electricity all day. Generation peaks and consumption peaks rarely align — leaving export and import.
Diminishing economics. The first kW saves grid imports at 38p/kWh. The 100th kW (which exceeds your daytime demand) exports at 5-15p/kWh. Marginal value drops sharply.

A 60-80% target balances generation high enough to capture most savings while keeping export to manageable levels.

When to size below the optimum

Roof can’t fit the optimal size — maximise within available area
Three-phase upgrade would be required to scale up — sometimes cheaper to size below the upgrade threshold
Capex / finance constraint — pick the size that fits your budget
DNO export limit — DNO may impose an export cap below your optimal generation

When to size above the optimum

Future-proofing for EV charging or process electrification (heat pumps, induction cooking, etc.)
Battery storage retrofitting plan within 3 years
Strong sustainability commitment that values being net-positive
Available space and unrestricted capex

What good sizing looks like in a quote

Your installer’s quote should show:

12 months of half-hourly consumption profile
Modelled generation hour-by-hour (PVSyst or similar output)
Self-consumption percentage at proposed size
Comparison of three sizes (typically -25%, recommended, +25%)
Annual saving for each size
IRR and payback for each size

If a quote shows only one size and “trust me, this is right”, request the comparison data.

Common misconceptions about sizing

“Match the system to the roof” — wrong default. Match it to consumption, then check the roof.

“Bigger is always better” — wrong. Above the consumption ceiling, every additional kW earns export rates 4-7x lower than self-consumed kWh.

“You can change the size later” — partially. Adding panels later costs 30-50% more per kW than original install (mobilisation, inverter sizing, scaffolding). Plan correctly upfront.

“Three-phase is required for any commercial install” — only above ~17 kW. Small office solar can sometimes run single-phase.

Next steps

For a PVSyst-modelled sizing report from your meter data, request a feasibility study. See related FAQs: system sizing, roof space, panel output, cost guide, grants and funding.