How many solar panels do I need for my barn? UK sizing guide 2026

The most common question we get from UK farm clients scoping their first solar install: how many panels do I actually need on the barn? The answer depends on three things — roof area available, on-farm load profile, and DNO export capacity. Here’s the sizing framework we use across every farm-building proposal.

The two-line rule for sizing

The simplest sizing rule for UK farm-building rooftop PV: roughly 6 square metres of south-facing or east-west-facing roof per kW of installed capacity, and roughly one panel per 1.85 sqm (at modern 540W rated output). So a barn with 600 sqm of suitable roof can carry around 100 kW (180–190 panels). A barn with 1,500 sqm can carry around 250 kW (450–460 panels). A large multi-bay structure with 3,000 sqm of clear-span roof can carry 500 kW+ (920+ panels). These are upper bounds — the right system size for your specific operation might be smaller depending on the load you can self-consume.

How load profile constrains optimal sizing

Solar self-consumption (the percentage of generated kWh used on-farm rather than exported under SEG) is the single biggest driver of payback economics. Self-consumed kWh save at grid retail prices (currently 24–28p/kWh); exported kWh earn the SEG tariff (8–15p/kWh). For most farm buildings, you want to size the PV system so that 70%+ of annual generation is self-consumed. That means matching system capacity to the building’s actual baseload pattern.

A dairy parlour with 24/7 cooling, vacuum and lighting baseload of 25 kW continuous can support 100–150 kW PV at 88–94% self-consumption. A grain store with seasonal drying load (5 kW year-round, 120 kW for 3 weeks in October) supports 200–400 kW PV at 35–55% self-consumption. A workshop with 8 kW daytime baseload (compressors, welder, lighting) supports 40–80 kW PV at 55–70% self-consumption. The dairy parlour is the easiest case — high baseload, simple sizing decision. The grain store is the most nuanced — too small misses the rooftop opportunity, too large dilutes payback.

Worked example — typical UK dairy barn

Take a representative example: a Cheshire dairy farm with a 1,800 sqm livestock shed roof on the south side and a 600 sqm parlour roof on the east. Annual electricity consumption 380,000 kWh across robotic milking, bulk-tank cooling, parlour washdown, cubicle housing ventilation. Available south-facing roof: 1,800 sqm parlour + 1,200 sqm of the livestock shed (rest is north-facing). Maximum PV capacity at 6 sqm/kW: 500 kW. Annual baseload pattern suggests 380,000 / 8,760 hours = 43 kW average, but with daytime peaks of 70–90 kW. Optimal sizing: 320 kW (590 panels) — captures full daytime baseload, exports moderate surplus during low-demand summer hours. Annual generation 294,000 kWh, self-consumption 92%, simple payback 5.2 years before AIA.

When to size below the roof’s maximum capacity

Several factors push optimal system size below the maximum the roof could carry: (1) DNO export capacity constrained — no benefit to generating more than can be self-consumed when export is limited; (2) low self-consumption profile — equestrian, seasonal-load arable, smallholdings where the building doesn’t use much daytime power; (3) capital constraints — better to deliver 100 kW well than 250 kW marginally; (4) future expansion planning — leave purlin and inverter headroom for a Phase 2 install in 2–3 years when capital becomes available.

When to size above the building’s individual baseload

Some scenarios justify sizing PV at the maximum the roof can carry, even if a single building can’t self-consume all generation: (1) multiple buildings on a single G99 application — surplus from one building feeds load on another, raising aggregate self-consumption; (2) EV charging or battery storage planning — generation excess can be absorbed by future load additions; (3) supplier-audit positioning — supermarket Scope 3 supplier requirements may reward maximum on-farm renewable capacity regardless of self-consumption ratio.

Battery storage and sizing decisions

For arable farms with seasonal drying loads, battery storage at 50–250 kWh scale changes the sizing calculation. A 400 kW PV install on a grain store with 200 kWh battery effectively time-shifts excess summer generation into autumn drying peak loads, lifting effective self-consumption from 35–45% to 60–75%. Battery capex is £400–£700 per kWh installed — economic for seasonal-load farms, less so for steady-baseload dairy and intensive livestock. We model PV-only and PV-plus-battery scenarios side-by-side in every proposal.

What to send us for a proper sizing answer

For a meaningful sizing recommendation for your specific barn, send us: half-hourly meter data for the past 12 months (most smart meters export this from the supplier portal); building dimensions including roof area, pitch, orientation, and shading; a brief note on your farm operation (what livestock or crops, when buildings are used, any planned changes); current annual electricity spend. We deliver a free desk feasibility study within 7 working days, including building-by-building system size recommendation, panel count, generation forecast, self-consumption ratio, and 25-year financial model.