Technical
What size solar system does my business need? (sizing methodology)
To size a commercial solar system, start with your annual kWh consumption from your bills, target 60-80% of that as annual generation, then check against roof area (6-7 m²/kW), electrical capacity (G98 vs G99 threshold), and budget. The right size is the smallest of these four ceilings. Insist on PVSyst modelling from half-hourly meter data, not generic estimates.
Sizing your commercial solar system properly is the single most important decision in the design process. Get it wrong and you’ll either overspend on capacity that exports at low SEG rates, or undersize and leave savings on the table. The methodology starts with your annual kWh consumption (from 12 months of bills or, better, half-hourly meter data), targets generation at 60-80% of that figure, then validates against three real-world constraints: roof area (6-7 m² of usable roof per kW), electrical capacity (G98 below ~17 kW per phase, G99 above), and capex/budget. The right size is the smallest of these four numbers. Insist on PVSyst or similar modelling from your actual data — generic kW-per-employee or kW-per-m² rules of thumb are unreliable.
The five-step sizing process
Step 1: gather consumption data
Pull 12 months of bills. For SMEs above 100,000 kWh/year you’ll have a half-hourly meter — request the data from your supplier (free, usually delivered within 5 working days). Below 100k kWh you’ll have aggregated monthly figures only.
What you need:
- Annual kWh consumption (total)
- Daytime/nighttime split if available
- Weekday vs weekend pattern
- Seasonal variation (manufacturing, retail, hospitality vary by season)
Step 2: model generation potential
Your installer plugs in:
- Postcode (for solar irradiance)
- Roof orientation, pitch, shading
- Proposed panel and inverter spec
- DC:AC ratio target (1.1-1.3)
PVSyst output: hourly generation forecast for 25 years, accounting for degradation.
Step 3: calculate self-consumption
Overlay generation profile on consumption profile. PVSyst calculates how much is self-consumed (replacing grid imports at 38p/kWh) and how much is exported (at SEG rates of 4-15p/kWh).
This is where sizing decisions get made: try 50, 75, 100, 125 kW and find the size where each marginal kW still adds meaningful value.
Step 4: validate against constraints
Check the proposed size against:
- Roof area available (6-7 m²/kW)
- Electrical capacity (DNO connection, switchgear, inverter)
- DNO connection viability (G99 timeline if applicable)
- Capex against budget / finance approval
Step 5: pick the smallest ceiling
The size you install is the smallest of:
- Consumption ceiling (60-80% of annual kWh)
- Roof ceiling
- Electrical ceiling
- Budget ceiling
The 60-80% target — why this range?
Below 50% of consumption: you self-consume essentially all generation. Maximum value per kWh, but you’re leaving roof space empty and unused capex on the table.
50-80%: most kWh are self-consumed; export portion grows with size. Sweet spot.
80-100%: significant export starts. Marginal kWh earn SEG rates only.
Above 100%: most additional kWh exported. Marginal economics weak unless future load growth is planned.
Per-employee and per-square-metre rules of thumb
These are okay for early ballpark estimates only:
| Rule of thumb | Value (UK SME 2026) |
|---|---|
| kW per employee (office) | 0.6-1.2 |
| kW per employee (light-industrial) | 1.0-3.0 |
| kW per 1,000 m² of office floor area | 30-50 |
| kW per 1,000 m² of warehouse floor area | 20-40 |
Use these for a 5-minute sense-check; don’t use them as a quote basis.
Sub-vertical sizing benchmarks
| Sector | Typical install size | Annual kWh generated |
|---|---|---|
| Small office (10-30 staff) | 15-40 kW | 13,000-36,000 |
| Mid office (30-80 staff) | 40-80 kW | 36,000-72,000 |
| Light-industrial unit | 80-200 kW | 72,000-180,000 |
| Retail showroom | 30-80 kW | 27,000-72,000 |
| Garden centre | 60-150 kW | 54,000-135,000 |
| Boutique hotel (20-50 rooms) | 50-120 kW | 45,000-108,000 |
| Larger industrial / logistics | 200-1,000 kW | 180,000-900,000 |
When to undersize on purpose
- Significantly capex-constrained — choose what your budget allows; oversize roof later
- Below G98 threshold to avoid 6-18 month G99 timeline — sometimes makes a 14 kW system more attractive than waiting 12 months for an 80 kW
- Pilot project to demonstrate ROI before scaling — 50% of “phase 1” then 50% in “phase 2” approach
- Tenant with short remaining lease — install matched to remaining lease tenor
Common misconceptions about sizing
“Bigger is always cheaper per kW” — true on £/kW, false on £/kWh saved. Above the consumption ceiling, additional kW earns SEG export rates.
“Match the system to the roof” — wrong default. Match it to consumption first.
“You can downsize during install” — too late. Inverter and DNO design are locked to nameplate. Downsizing means physical de-rating which usually doesn’t make financial sense.
“Battery lets you size larger” — partially. Battery raises self-consumption ceiling by 15-25 points. So you can size at 90-95% of consumption with batteries, vs 60-80% without.
Next steps
For a PVSyst sizing model from your half-hourly meter data, request a feasibility study. Related: sizing overview, roof space needed, panel output, cost guide, grants page.
Related questions
How much energy does a commercial solar panel produce?
A typical 540 W commercial solar panel in the UK produces 480-580 kWh per year, with the variation driven by location (south coast vs Scottish highlands), orientation, pitch, and shading. South-facing 35-40 degree pitch in the Midlands is the benchmark — about 530 kWh/panel/year. The whole system produces 850-1,050 kWh per kW of nameplate annually.
How much roof space do I need for commercial solar?
A commercial solar install needs roughly 6-7 m² of usable roof per kW. A 100 kW system fills 600-650 m². For SMEs, that's typically 50-70% of a light-industrial unit roof, 80-100% of an office roof, or one full bay of a large warehouse. Pitched roofs, shaded roofs, and roofs with skylights or HVAC plant lose effective area — usable area is rarely the full roof footprint.
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.