Single-Phase Primary Schools: How Much Solar Can You Fit?

Why some primary schools can’t just fill the roof

An SBM looks at a big primary-school roof, imagines it covered in panels, and asks a sensible question: how much can we actually fit? For many older primary schools the honest answer is “less than the roof would allow” — and the reason is not the roof at all. It is the electrical supply. A large share of older primaries run on a single-phase supply, and single-phase puts a hard ceiling on how much solar you can connect, regardless of how much roof you have. This guide explains that ceiling, the grid-connection rules that create it, and when it’s worth paying to remove it.

If your primary school was built or last rewired decades ago and has never needed heavy three-phase equipment, this almost certainly applies to you — so it’s worth understanding before you fall in love with a roof-filling design that can’t be connected.

Single-phase vs three-phase, in plain terms

UK electrical supplies come in two forms relevant here:

• Single-phase — one live conductor, the standard domestic-style supply, common in smaller and older buildings including many primary schools. Typical supply ratings are 60–100 amps.
• Three-phase — three live conductors, the standard for larger commercial and industrial buildings, and for secondary schools with significant kitchen, IT and workshop loads. It can carry far more power and export far more solar.

A single-phase supply is perfectly adequate for a small primary’s day-to-day needs. The problem only appears when you try to push a large amount of solar back through it. There is a limit to how much generation a single phase can accept, and for schools that limit bites well below what the roof could physically hold.

There is a quick way to find out which supply you have without waiting for a survey. Ask your caretaker or SBM to look at the main incoming supply and the meter: a single-phase supply has one live “tail” and a domestic-style meter, while a three-phase supply has three live tails and a larger meter, often with a rotary or multi-element display. Your electricity bill and your DNO can also confirm it. It is worth establishing this early, because it determines everything that follows — the size of system worth designing, whether you need a G99 application, and whether an upgrade even enters the conversation.

The ~13–17 kW ceiling

Here is the number that matters. On a single-phase supply, practical solar is capped at roughly 13–17 kW:

• The formal grid-connection framework allows up to about 17 kW per phase under the simpler notification route (more on G98/G99 below).
• In practice, a 60–100 amp single-phase supply — typical for an older primary — often caps usable PV nearer 13 kW once real-world headroom and the existing building load are accounted for.

So a primary with a generous roof that could physically hold 40 or 50 kW of panels may only be able to connect around 13–17 kW on its existing single-phase supply. The rest of the roof stays empty — not because of the roof, the funding or the planning, but because the supply can’t take the generation. This is one of the most common surprises in primary-school solar, and it is exactly why sizing must start from the electrical supply, not the roof area.

G98 vs G99: the grid-connection rules

The limit is set by the two connection standards the Distribution Network Operator (DNO) uses to manage generation on its network:

• G98 — the simpler, faster route for small-scale generation, broadly systems up to about 17 kW per phase. It is largely a notification process: the connection can proceed and the DNO is informed, so timelines are short.
• G99 — required for larger systems above the G98 threshold. It is a full application to the DNO, which assesses the network’s capacity to accept the generation. G99 takes longer and can occasionally require network reinforcement, which adds cost and time.

For a single-phase primary staying within ~13–17 kW, the project connects under G98 — quick and simple. The moment you want more solar than a single phase can take, you are into either a G99 application or a supply upgrade, and usually both. Most SBMs have never dealt with the DNO and don’t need to — we manage the G98 or G99 process end to end as part of every project.

When a three-phase upgrade pays

If your roof could hold significantly more than ~17 kW and you want to use it, the answer is usually a three-phase supply upgrade. Upgrading from single- to three-phase lifts the connection ceiling dramatically — allowing 50 kW, 80 kW or more, subject to the DNO’s assessment — and lets you use the full roof.

The upgrade is not free, and whether it pays comes down to a straightforward trade-off:

• The cost of the upgrade — this varies widely depending on the distance to the network, the DNO’s requirements, and any reinforcement needed. It can range from modest to substantial, which is why it must be quoted, not guessed.
• The extra generation it unlocks — a larger array generates more, saves more, and (with interest-free Salix funding) can still be cash-flow positive even after the upgrade cost.

The upgrade pays when the additional solar it enables saves enough over the system’s life to cover the upgrade cost and then some. For a school with a large roof, high electricity demand, and access to Salix funding, a three-phase upgrade plus a 40–60 kW array often stacks up better than settling for a 13 kW system on the existing supply. For a small school with modest demand and a small roof, staying single-phase and keeping the project simple under G98 is frequently the smarter call. There is no universal answer — it turns on your roof, your demand and the upgrade quote.

How we handle it in the feasibility study

Because the supply is the real constraint, we assess it first. Every primary-school feasibility study we run:

• Checks your existing supply — single- or three-phase, and its amperage rating.
• Establishes the connection ceiling — how much solar the current supply can take under G98.
• Models the three-phase upgrade — the likely cost, the extra generation it unlocks, and whether the larger system pays back after the upgrade.
• Sizes the array from your half-hourly meter data, not the roof area, so the design matches both your demand and your connection headroom.
• Manages the DNO end to end — G98 notification or G99 application, and any upgrade works.

That way you get a clear either/or: a simple, fast 13–17 kW system on your existing supply, or a larger system with a costed three-phase upgrade — with the numbers to choose between them, rather than a nasty surprise late in the project. For the wider cost picture, see our cost guide; for how primary economics differ, our primary schools page; and for the term-time factors that also shape a primary’s sizing, our guide to whether a school needs a battery.

Know your supply before you size your solar

The single most useful thing an SBM at an older primary can do before commissioning solar is find out whether the school is on a single- or three-phase supply — because that, more than the roof, decides how much solar is possible and whether an upgrade is worth it. Don’t design around roof area and discover the supply can’t take it. For the questions that come up most, see our FAQs and grants and funding page.

To find out exactly how much solar your primary school’s supply can take — and whether a three-phase upgrade pays — request a free feasibility. We will check your supply, establish the connection ceiling, cost any upgrade, and give you a clear choice built from your own meter data.