Grain Store Solar: Designing for the Autumn Drying Peak

The grain store is the best roof and the hardest brief

Walk onto almost any arable farm and the grain store is the largest single roof on the holding — a clear-span steel portal building covering anywhere from 300 to 3,000 m², often facing close to south, with nothing to shade it. A roof that size can carry a 50–500 kW array generating roughly 45,000 to 460,000 kWh a year, which on paper makes it the most inviting solar site you’ll ever see.

The catch is the load. Unlike a poultry shed or a dairy parlour, a grain store does not hum away at a steady draw twelve months a year. For most of the year it is a quiet building. Then, for a few intense weeks after harvest, the drying and ventilation kit fires up and demand goes through the roof — at exactly the time of year the sun is fading. That mismatch is the whole design problem, and getting it wrong is how a £40,000-plus investment ends up underperforming.

Why the load and the sun pull in opposite directions

Solar PV generates most of its annual output between April and September, peaking around the long days of June and July. A grain store’s biggest electrical load — drying fans, ventilation, ambient-air conditioning and grain stirrers — typically runs from late August through October, often well into the evening and overnight as crops are pulled down to a safe moisture content for storage.

So the very weeks when your store draws the most power are the weeks when the array is already past its summer best, and much of the drying happens after dark when the panels produce nothing. Through high summer, when the panels are generating flat out, the store is often nearly empty. Left undesigned, you get a building that exports cheap surplus all summer and imports expensive grid power exactly when it matters in autumn.

This is why, for grain stores more than any other barn type, you cannot size from the roof — you size from the meter. Pulling half-hourly consumption data and overlaying it on a modelled generation profile shows you, hour by hour, how much of each kilowatt-hour you would actually use on site versus export. Everything else follows from that picture.

The three ways to design a grain-store array

There is no single right answer here — there are three legitimate strategies, and the best one depends on your drying load, your tariff and your appetite for capital.

1. Size for the roof and export the surplus under SEG

The simplest approach: fill the available roof, use what the store and the wider farm can absorb, and sell everything else back to the grid under the Smart Export Guarantee. An MCS-certified install with a smart export meter earns a per-kWh tariff (typically in the 4–15p range depending on supplier) for every unit you don’t consume.

For a building that exports a lot in summer, the SEG tariff matters far more than it does on a 24/7 site. The economics work best where you can negotiate a strong export rate, and where the rest of the farm — workshops, dryers, the farmhouse, EV charging — can mop up some of the daytime generation before it leaves the meter. The downside is that you’re selling power for a fraction of what you pay to buy it back in autumn, so every exported summer unit is a small missed opportunity.

2. Size for the daytime baseload and add a battery for the drying peak

The more sophisticated route is to size the array around what the farm reliably uses during daylight, then add battery storage to shift solar into the hours that drive your bill. A battery lets midday summer generation be stored and released into evening and overnight drying, and it smooths the gap between a sunny afternoon and an after-dark ventilation run.

A battery genuinely earns its place where the load is seasonal and evening-weighted — which describes a grain store almost perfectly. It is not free, though, and over-sizing storage for a six-week peak can waste as much capital as over-sizing the panels. The honest design question is how many kilowatt-hours of expensive autumn import a given battery actually displaces over a year, and whether that saving services the cost of the battery. Sometimes it does handsomely; sometimes a smaller battery plus a good export tariff wins.

3. Size for baseload only — self-consumption with no export

The third option deliberately keeps the system small: size it so that nearly every unit generated is consumed on site across the year, with little or no export at all. This is the route to take where the rural grid is the constraint rather than the roof.

Rural DNO networks are frequently capacity-constrained, and a large export-led scheme can trigger a lengthy G99 connection study or an outright export limit. A no-export or export-limited design — a smaller array matched to baseload, sometimes with a battery and an export limiter — can cut the connection timeline from many months to a few weeks. You leave roof space unused, but you get a faster, cheaper connection and a system where every kilowatt-hour offsets a unit you’d otherwise have bought at full price. For a store where the drying load is modest, this is often the quietly sensible choice.

A worked illustration

Take a typical 200 kW array on a large arable store, generating in the region of 180,000 kWh a year. Designed purely for the roof, perhaps 35–45% is used on site and the rest is exported. Add a battery and shift evening drying onto stored solar, and self-consumption can climb meaningfully — but only if the drying overlaps enough generation days to keep the battery cycling. The answer is rarely obvious until you see the half-hourly numbers, which is precisely why the meter data comes first and the panel count second. (This is an illustrative scenario, not a specific customer.)

The practical points that change the model

A few realities shape every grain-store design before the spreadsheet even opens:

• Permitted development usually applies. Rooftop PV on a modern agricultural store is normally Permitted Development under Class A, Part 14 of the GPDO 2015, so no planning application is typically needed — see the Planning Portal solar guidance for the headline rules.
• The roof has to take the load. A short structural appraisal confirms the purlins and frame can carry the modest PV dead load. Pre-2000 stores may have asbestos-cement sheeting, which can’t be drilled and needs a licensed strip-and-reclad first.
• Dust matters during install. Grain dust brings DSEAR considerations, so work on an operational store is scheduled and managed accordingly.
• Tax relief is real on a working store. A trading farm can usually claim 100% Annual Investment Allowance on the system in year one — see our grants and funding guide for how AIA and SEG stack together.

If your enterprise also runs livestock or general farm buildings alongside the arable side, it’s worth modelling the whole site together — our sister site solarpanelsforfarmbuildings.co.uk covers the mixed-building picture in more depth.

Get the design right before the roof goes on

A grain store is the most rewarding barn to put solar on and the easiest to get wrong. The roof is huge and the temptation is to fill it — but the autumn drying peak, the rural grid and your tariff all pull the answer in different directions. The right call between export, battery and baseload-only comes out of your own half-hourly data, not a rule of thumb.

If you’d like a free desk feasibility built from your meter readings — showing exactly how export, storage and self-consumption stack up on your store — request a quote and we’ll model all three before you commit a penny.