What Size Solar System Does My Barn Need?

Start with the load, not the roof

The first instinct of most barn owners is to look up at that big empty roof and ask how many panels would fit on it. It is the wrong question. A modern steel-portal shed can carry far more solar than the building below it will ever use, and a system sized purely to fill the roof tends to spill most of its generation onto the grid at a low export price. The right question is the opposite one: how much electricity does this barn actually use, when does it use it, and how much of the sun’s output can it consume on site?

That distinction matters because of a single number — self-consumption. Every kilowatt-hour you use directly under the panels is worth the full retail price you would otherwise pay your supplier, perhaps 25–30p. Every kilowatt-hour you export under the Smart Export Guarantee is worth far less, typically 4–15p. So the most valuable system is not the biggest one your roof can hold; it is the one whose output most closely matches your daytime demand. Get that match right and the difference in payback can be years.

The one piece of data that decides everything

Before any sensible sizing conversation, pull your half-hourly meter data. If your barn has a smart meter or a half-hourly commercial supply, your DNO or supplier can provide twelve months of consumption in half-hour intervals. That single spreadsheet tells you the three things sizing depends on:

• Your daytime baseload — the steady demand that runs while the sun is up. This is the floor your solar should comfortably cover.
• Your seasonal pattern — whether demand is flat all year (a poultry shed) or spikes in a few weeks (a grain store at harvest).
• Your peak — the largest sustained draw, which tells you whether to design for it directly or let the grid cover the spikes.

A rough sizing rule helps frame the conversation — solar PV needs around 7–8 m² of clear roof per kW of panels, so a 100 kW system wants roughly 700–800 m² of unshaded roof. Almost every portal-frame barn has far more than that, which is exactly why the roof is never the constraint. The load is.

How sizing changes by barn type

Because barns do such different jobs, no single rule of thumb works across them. Here is how the decision tree splits.

24/7 working barns — size aggressively

Poultry sheds, pig units and dairy barns carry a high, near-constant electrical load: ventilation, heating, lighting, feed systems, milk cooling, vacuum pumps. The demand runs day and night, all year. On buildings like these, self-consumption routinely exceeds 85% — almost everything the panels make is used on site immediately, so there is no penalty for going big. These are the barns where payback can dip below five years, and where the only real ceiling is the roof area and the grid connection. Size up.

Seasonal-peak barns — baseload plus a battery decision

A grain or crop store is the classic mismatch. Its biggest load is the drying and conditioning fans that run for a few intense weeks after harvest in late summer and autumn — exactly when the sun is fading. The rest of the year demand is modest. Filling that roof with panels to cover the autumn peak would leave you exporting almost everything for ten months. The smarter design sizes to the year-round daytime baseload and then decides, separately, whether a battery or export is the better home for the surplus. We cover that trade-off in detail in our guide to grain-store seasonal load — it is the single most important design call on an arable holding.

Low-load and heritage barns — design to the envelope

A field barn, a stable block or a converted barn dwelling has a smaller, gentler demand. Here the right size is led by the load you can actually shift into daylight hours — EV charging, a heat pump, water heating, a home battery — plus whatever the Smart Export Guarantee pays for the rest. On a barn conversion the heating load from an air-source heat pump is often the anchor the system is sized around. On listed or traditional barns the consenting envelope can cap the array size before the load does.

Export or battery? The surplus question

Once the array is sized, you have to decide what happens to generation your barn cannot use at the moment it is produced. There are two answers, and the right one depends on your load shape.

Export is the simpler route. Surplus flows to the grid and earns the SEG tariff. For a working barn with high daytime self-consumption, exporting the modest leftover is perfectly sensible — there is little surplus and a battery would rarely pay back on it.

A battery earns its place where there is a real timing gap between generation and demand: a grain store’s autumn-evening drying, a barn-conversion home that draws most heavily after dark, or a site where the DNO has capped your export and you would otherwise spill electricity for nothing. Storage lets you bank midday sun and spend it in the evening at retail value instead of exporting it cheaply. A battery is an optional extra, not a default — we size one only where the numbers justify it, as set out in our barn solar battery storage guide.

A worked illustration

Consider a typical 100 kW system on a steel-portal livestock barn — clearly illustrative, not a specific customer. With a steady daytime load from lighting, scrapers and water heating plus a robotic feeder, perhaps three-quarters of the generation is used on site at retail value and the rest exported. That self-consumption ratio, not the panel count, is what makes the case work. Put the same 100 kW on a grain store with the same roof but a seasonal load and self-consumption can fall sharply without a battery — so the right answer becomes a smaller array sized to baseload, or storage to soak up the surplus. Same roof, same kilowatts, completely different design, because the load set the brief.

Don’t forget the roof structure

Sizing is a load question first, but the roof still has to carry the panels. PV adds a modest dead load of around 10–15 kg/m² plus wind uplift, and a short structural appraisal confirms the purlins and frame can take it. Modern steel portal frames almost always can; older or modified barns occasionally need minor strengthening, and any asbestos-cement roof must be dealt with first. None of this changes the sizing logic — it just confirms the chosen system can be safely fixed to the building.

How many panels — and how to find out for sure

For a quick check on whether your roof can host the system your load justifies, count on roughly 7–8 m² per kW and compare it against your clear roof span; our companion piece on how many solar panels fit on a barn roof walks through the arithmetic. But the panel count is the output of the sizing exercise, not the input — the proper answer always starts with your meter data and your demand profile. A 200 m² roof on a 24/7 poultry shed and a 200 m² roof on a seasonal grain store want very different systems, which is exactly why a desk feasibility on your half-hourly data beats any calculator.

If you would like us to size your barn properly from your own consumption data rather than a rule of thumb, request a quote and we will model the options — array size, self-consumption, battery or export — and tell you honestly where the line falls for your building.