How Many Solar Panels Does a Factory Need? (UK Sizing Guide 2026)
A factory needs roughly 5–6 sqm of unobstructed roof per kWp of solar PV. Sizing is driven by daytime baseload, not roof area. This guide walks through the full sizing method — with worked examples for 100 kW, 250 kW, 500 kW and 1 MW UK factory installations.
The 60-second answer
For a typical UK factory the answer lands between 180 and 1,800 panels depending on annual electricity consumption. The right number is the one that maximises self-consumption against your daytime baseload — usually 70–90% of peak daytime kW demand.
- Small factory (50–100 kW system, 90–180 panels) — £1,000–£2,000 sqm of unobstructed roof. Suits sub-300 MWh/year sites.
- Medium factory (100–250 kW system, 180–460 panels) — 600–1,500 sqm of roof. Suits 300–800 MWh/year sites.
- Large factory (250–500 kW system, 460–920 panels) — 1,500–3,000 sqm of roof. Suits 800–1,800 MWh/year sites.
- Industrial (500 kW–1 MW system, 920–1,840 panels) — 3,000–6,000 sqm of roof. Suits 1,800–4,000 MWh/year sites.
- Mega factory (1 MW+ system, 1,840+ panels) — 6,000 sqm+ of roof. Suits 4,000+ MWh/year sites.
The proper sizing method — step by step
Step 1 — Pull 12 months of half-hourly meter data
The single most important number is your half-hourly (HH) consumption profile. Every UK industrial site with a half-hourly meter (almost all > 100 kVA supplies) has this data sitting with their MOP (Meter Operator) and supplier. We pull 12 months of HH data via your supplier's online portal or via Elexon for HH metered sites — this gives us 17,520 data points showing kW demand for every 30-minute slot of the year.
From that we extract: average daytime kW (8am–6pm), peak daytime kW, weekend daytime kW, and seasonal variation. The system is sized to capture the highest possible % of daytime kWh while keeping the export ratio low.
Step 2 — Establish daytime baseload
Daytime baseload is what runs continuously through working hours: compressed air, conveyors, refrigeration, lighting, HVAC, IT, security. For a typical UK factory this is 30–60% of peak daytime demand. The PV system targets this baseload first — every kWh produced during daytime offsets grid imports at full retail (currently ~28 p/kWh industrial), every kWh exported at evenings or weekends earns SEG at 4–15 p/kWh.
Step 3 — Run the PVSyst yield model
PVSyst is the industry-standard yield modelling tool. We run it with your specific roof orientation, tilt, and shading (chimneys, AHUs, parapets, adjacent buildings). UK factory yields typically run 850–1,050 kWh per kWp per year, with the south-east corner of England achieving the highest yields and Scotland the lowest.
Step 4 — Stress-test against export tariff scenarios
SEG tariffs are supplier-set and can range from 4 to 15 p/kWh in 2026. We model the system economics against six tariff scenarios — from a 5p worst case to a 15p Octopus Outgoing Agile case — so you see how robust the IRR is to tariff volatility.
Step 5 — Optimise for self-consumption (not max kWp)
A common mistake is to size to fill the roof. The optimum size is one where the marginal panel produces enough self-consumed kWh to pay back faster than the marginal export. For most UK factories, this lands at 70–90% of peak daytime demand — not 100% of available roof.
Worked examples by factory size
Small factory — 80 employees
Single-shift engineering plant, 280 MWh/year electricity
- System size:
- 100 kW (180 panels)
- Roof area:
- 600 sqm
- Annual generation:
- 92,000 kWh
- Self-consumption:
- 75%
- Cost installed:
- £80,000–£90,000
- Year-1 saving:
- £21,000
- Payback:
- 3.8 years
Medium factory — 220 employees
24/5 production, 880 MWh/year
- System size:
- 250 kW (460 panels)
- Roof area:
- 1,500 sqm
- Annual generation:
- 232,000 kWh
- Self-consumption:
- 82%
- Cost installed:
- £175,000–£212,000
- Year-1 saving:
- £58,000
- Payback:
- 3.4 years
Large factory — 380 employees
Continuous production, 1,750 MWh/year
- System size:
- 500 kW (920 panels)
- Roof area:
- 3,000 sqm
- Annual generation:
- 462,000 kWh
- Self-consumption:
- 85%
- Cost installed:
- £350,000–£400,000
- Year-1 saving:
- £118,000
- Payback:
- 3.1 years
Mega factory — 750+ employees
Multi-shift, 4,000+ MWh/year
- System size:
- 1 MW (1,840 panels)
- Roof area:
- 6,000 sqm
- Annual generation:
- 920,000 kWh
- Self-consumption:
- 88%
- Cost installed:
- £650,000–£750,000
- Year-1 saving:
- £235,000
- Payback:
- 2.9 years
FAQs about factory solar sizing
How many solar panels does a 100 kW factory need?
A 100 kW commercial system typically uses 180 panels at 555 W each, occupying around 600 sqm of roof. With 84% self-consumption it generates ~92,000 kWh annually and saves £20,000–£25,000 vs grid retail at current Q1 2026 industrial tariffs.
How many solar panels for a 1 MW factory?
A 1 MW system uses 1,840 panels at 545 W each, occupying ~6,000 sqm of unobstructed roof. Suits factories drawing 4,000+ MWh annually, with payback in under 3 years on capital purchase.
What roof area do I need per kW of solar?
Approximately 5–6 sqm per kWp of installed PV in 2026, using 545–670 W panels in portrait orientation with optimised row spacing. Flat roofs need slightly more (6–7 sqm/kWp) due to ballast spacing.
Should I size to fill the roof or to my daytime load?
Always size to daytime load. Filling the roof leads to over 30% export ratio at most UK factories — exported kWh earn 4–15p but self-consumed kWh save 28 p. The best IRR comes from sizing at 70–90% of peak daytime demand.
Can solar panels power my entire factory?
Rarely on rooftop alone. Most UK factory rooftop installs offset 30–70% of annual demand. To get to 100% renewable you typically combine rooftop PV with a PPA on a remote solar farm or with a green energy supplier — both are common 100% Scope 2 reduction strategies.
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