180 kW right-sized array on an ambient regional store

This is an illustrative example based on typical projects - not a real named client. The figures are indicative planning numbers for a representative ambient regional store, shown to demonstrate how a load-led design plays out in practice.

The brief

A single-shift ambient storage operation ran a 60,000 sq ft regional store in the East Midlands, on the fringe of the logistics Golden Triangle. The building was a modern clear-span steel-portal shed with a roughly 4,000 m² trapezoidal metal roof - the kind of unbroken, uncluttered roofscape a roof-fill installer sees pound signs on.

The operator’s brief was simple: electricity had become the single biggest controllable line on the P&L, and with TNUoS network charges set to rise around 60% in April 2026, it was only heading one way. They wanted solar, but had been pitched a “maximum kWp, fill the roof” proposal by another firm and instinctively distrusted it. Their question was the right one: how much of this will we actually use, and how much will we just dump back onto the grid?

What the half-hourly data showed

We did what every warehouse solar project should start with: we pulled twelve months of half-hourly (HH) meter data before designing anything. The profile was textbook ambient storage.

The site ran a genuine 06:00-18:00 single shift. Lighting was already LED, and materials-handling was light - a handful of electric pallet trucks and one reach truck on charge. That combination produces the lowest daytime base-load in the whole warehousing sector. Between the morning put-away peak and the afternoon despatch push, consumption sat surprisingly flat and low, with almost no overnight or weekend load.

The implication was stark. A roof-fill array sized to the full 4,000 m² roof would have generated far more midday power than the building could ever use, and the surplus would have spilled onto the grid at a Smart Export Guarantee rate a fraction of the 25-45p/kWh the site pays to import. Every extra panel past a certain point would have earned pennies instead of pounds - lengthening the payback, not shortening it. On a low-base-load shed, filling the roof actively wrecks the return.

The design - sized to the load, not the roof

Working from the HH profile, we sized the array to the site’s real daytime demand rather than its roof area. That landed at 180 kW - roughly 335 panels - occupying only part of the available roof. The design target was annual generation of around 60-85% of daytime consumption, the band that maximises self-consumption.

At a UK yield of about 900 kWh/kWp, the 180 kW array produces roughly 162,000 kWh a year. Because it was matched to the load, the site self-consumes about 70% of everything it generates. That is the number that makes the economics work - self-consumed solar displaces expensive imported units one-for-one, including the fast-rising TNUoS element, whereas exported units earn a far lower supplier-set SEG rate.

The install used non-penetrative clip-fix mounting on the trapezoidal metal roof, preserving the roof warranty with no penetrations, and a structural loading assessment confirmed the array’s dead and wind-uplift load was within tolerance. At 180 kW the connection went in as a standard G99 application, well inside the site’s existing import capacity - so no reinforcement or export-limitation constraint was needed. The layout was set out to LPC / RISCAuthority RC62 clearances from the roof’s fire zones, with the insurer’s pre-design sign-off obtained before any fabrication.

Crucially, the design left the remaining roof clear and cabling-ready. As the operator electrifies - adding EV-van charging for last-mile deliveries and more MHE on charge - that new daytime load can be grown into the roof with a second phase, without reworking the first.

The outcome

• System size: 180 kW (~335 panels), sized to load, not roof
• Annual generation: ~162,000 kWh
• Self-consumption: ~70%
• Annual saving: ~£34,000
• Payback: ~5.4 years

The array was funded on a self-owned basis. As special-rate plant, the solar qualified for the £1m Annual Investment Allowance, so a profitable company writes most of the capex off against tax in year one (full expensing does not apply to solar). The commercial VAT of 20% was reclaimable, and the panels are exempt from business rates in England through to 31 March 2035. A lodged post-install EPC captured an SBEM uplift, helping protect lettability under MEES - where the minimum band to let remains E today.

A roof-fill of the same building would have generated more kilowatt-hours on paper but saved little more money, because the extra output would have exported at a loss - pushing the payback well beyond this figure. Right-sizing delivered the same clean ~5.4-year return on a smaller, cheaper system with room to grow.

Why it matters for similar operators

Most single-shift ambient stores look exactly like this one on the meter: LED-lit, lightly automated, low and flat daytime base-load, big empty roof. The instinct - yours or your installer’s - is to fill the roof. The data almost always says don’t.

Size to what you actually consume between 06:00 and 18:00, capture 60-75% self-consumption, and let fleet and MHE electrification grow your load into the rest of the roof over time. That sequence turns a warehouse roof from a cheap-export liability into 15-20 years of near-free daytime power on a payback you can defend to the board.

If your store fits this shape, the honest first step is the same one here: read your own half-hourly data. Our ambient and general storage page explains the load-led method in full, the cost page sets out indicative 2026 pricing by size, and you can get an instant saving and payback estimate on the quote page.