How to Size Warehouse Solar From Your Half-Hourly Data (Load-Led, Not Roof-Led)

Warehouse solar should be sized to your daytime electricity load from twelve months of half-hourly meter data, not to how many panels fit on the roof. A low-base-load ambient shed that is simply “roof-filled” just exports power cheaply and wrecks the payback. Sizing the array to roughly 60-85% of daytime demand keeps self-consumption at 60-75% and lands payback at around five to six years. This guide explains the method: how to read your half-hourly data, why roof-fill fails on a single-shift shed, the planning rules for kWp per square metre, and how forklift and EV-van charging let you grow into the rest of the roof over time.

The core mistake: sizing to the roof instead of the load

Most warehouse solar quotes start with the wrong question. A salesperson looks at your roof on a satellite image, counts how many panels fit, and quotes the biggest number the roof allows. That is a roof-led design, and for a warehouse it is usually the wrong answer.

The problem is specific to how warehouses use power. A modern LED-lit, single-shift ambient shed has a surprisingly low daytime base-load. Between order peaks it might be drawing lighting, a handful of chargers, some office and welfare load, and not much else. If you cover the whole roof with panels, on a sunny midday in June the array generates far more than the building consumes. The surplus is exported to the grid.

Exporting is not worthless, but it is worth far less than self-consumed power. You save 25-45p per kWh on every unit you use on site; you receive perhaps a few pence to around 15p per kWh for what you export under the Smart Export Guarantee, and those rates are set by suppliers and vary. So a roof-fill array on a low-load shed spends capital generating power you are forced to sell at a discount. The payback stretches out, and the “bigger system” that looked impressive on paper delivers a worse return than a smaller, well-matched one.

The fix is to flip the question. Instead of “how many panels fit?”, ask “how much power do we actually use during daylight, and when?” That is a load-led design, and the only honest way to answer it is your half-hourly data.

What half-hourly (HH) data is and why it drives everything

If your warehouse has a maximum import capacity above 100 kVA, your electricity meter is almost certainly a half-hourly meter. It records your consumption in 48 readings per day, one for every half-hour, and your supplier or your Meter Operator can provide twelve months of this data as a spreadsheet (often called HH data or P272 data).

This dataset is the single most valuable input to a warehouse solar design, because it shows three things a roof survey never can:

• Your daytime base-load - the floor of consumption during working hours, the power you draw even when nothing much is happening.
• Your load shape - how demand rises and falls across the day, the week, and the seasons. A 06:00-18:00 single-shift operation looks completely different from a 24/7 automated fulfilment site.
• The overlap with the solar curve - solar generates in a predictable daytime arc that peaks around midday. The more your consumption sits under that arc, the more of your generation you self-consume.

We take twelve months so that seasonal swings are captured. A warehouse that runs hot in the Q4 peak and quiet in January has a very different profile month to month, and sizing off a single sunny week would badly mislead the design.

Reading the profile

When we overlay your averaged daytime load against a modelled solar curve for your roof orientation, the picture usually falls into one of three shapes:

1. Genuine day operation - consumption tracks the solar arc closely. Self-consumption can reach 90%+, and you can size the array aggressively because almost everything generated is used on site.
2. Modest, flat base-load (the classic ambient shed) - a low, roughly level daytime draw. Solar generation will overtop it around the middle of the day. This is the textbook case for careful sizing: match the array to the load, not the roof.
3. Peaky or 24/7 load (automated fulfilment, cold storage-adjacent, security-heavy sites) - a firmer round-the-clock base-load, often with sharp peaks. These sites can take a larger array and frequently justify a battery to shift midday surplus into evening demand.

The design target: 60-85% of daytime demand

For most warehouses, the sizing sweet spot is an array whose annual generation equals roughly 60-85% of your daytime consumption. Sit inside that band and you keep self-consumption high (typically 60-75%, and higher for genuine day ops), which is where the economics live.

Why not aim for 100% of daytime demand? Because solar is intermittent. To generate enough in winter to cover a large share of demand, you would have to massively over-generate in summer, and all that summer surplus gets exported cheaply. Sizing to 60-85% of the daytime load means the array is comfortably absorbed on most days and only spills over at the sunniest midday peaks. It is the point where each additional kWp is still mostly self-consumed rather than mostly exported.

This is also why the “we don’t use much power during the day, is solar even worth it?” objection is really a sizing question in disguise. A low-base-load site is not a reason to skip solar. It is a reason to size it properly and, as we cover below, to plan how the daytime load will grow.

The planning rule: ~100-140 kWp per 1,000 m² of usable roof

Once the load-led target sets how big the system should be, we sanity-check it against the roof. As a planning rule of thumb:

• About 100-140 kWp fits per 1,000 m² of usable, clear-span roof - roughly 1 kW per 6-8 m².
• Only about 40-60% of a gross roof is usable once you remove rooflights, plant, walkways, HVAC, sprinkler tanks, and the setbacks required for maintenance access and fire clearances.
• UK generation runs at roughly 750-1,050 kWh per kWp per year, with a national average near 900 kWh/kWp.

So a 10,000 m² gross warehouse roof might offer perhaps 5,000-6,000 m² usable, supporting somewhere around 500-840 kWp if fully filled. The load-led design will usually land below that ceiling for a single-shift ambient site, which is the whole point: the roof is not the constraint, your daytime load is.

These are indicative planning-grade figures. The firm numbers come after a roof survey (structural loading, roof type, orientation, shading) and the half-hourly analysis. Roof clearances around sprinkler zones and firewalls follow RISCAuthority RC62 guidance on rooftop PV, with insurer pre-design sign-off before anything is fabricated - that clearance requirement is one reason the “usable” roof area is always smaller than the gross figure.

Why self-consumption is the number that matters

Self-consumption is the share of what you generate that you use on site rather than export. It is the lever that decides the return on a warehouse array, because self-consumed units are worth several times more than exported ones.

• Higher self-consumption (day ops, automation, EV charging) → most generation offsets 25-45p/kWh grid import → strong payback, often 3-5 years.
• Lower self-consumption (roof-filled low-load shed) → much of the generation is exported at a few pence → payback stretches well beyond that.

This is compounded by rising network charges: transmission network use-of-system (TNUoS) charges are set to rise around 60% in April 2026 and keep climbing, and they apply to every unit you import. Solar cuts imported units directly, so high self-consumption hedges not just wholesale price but the fast-rising network element of your bill. Every self-consumed kWh is worth more in 2026 than it was in 2024, and worth more still by 2030.

Growing the daytime load into the roof

Here is the strategic part that competitors’ roof-fill pitches miss. A load-led design does not mean you are stuck with a small array forever. It means you build the array to today’s honest load and then grow your daytime consumption into the rest of the roof as you electrify operations.

Two shifts are already happening across UK logistics:

• Materials-handling equipment (MHE) - forklifts, reach trucks, and pallet movers are moving from LPG and diesel to electric, charged on site. Daytime opportunity-charging of an electric MHE fleet absorbs solar at close to 100% self-consumption. Every forklift you electrify lifts your daytime base-load and lets more of the roof pay its way.
• Last-mile EV vans - depots serving e-commerce and grocery are electrifying delivery fleets. Vans that return to the depot during the day can be charged from the array, again at very high self-consumption, with dynamic load management keeping the combined draw inside your grid connection.

So the right sequence for a low-load ambient shed is: size the array to today’s load, leave the mounting and cable routes ready for expansion, and add panels as MHE and EV charging raise the daytime demand. You capture the cheapest power on the estate now, and you grow into the roof deliberately rather than exporting cheaply from day one.

Worked example: a single-shift ambient shed

Consider a 60,000 sq ft (about 5,600 m²) ambient regional store, LED-lit, running a single 06:00-18:00 shift with light MHE. The figures below are illustrative planning-grade, not a quote.

Step 1 - read the half-hourly data. Twelve months of HH data show a modest, flat daytime base-load: lighting, a few chargers, office and welfare. Averaged across the year, daytime consumption is fairly low relative to the roof size. This is a classic shape-2 profile.

Step 2 - check the roof ceiling. At roughly 40-55% usable, the ~5,600 m² gross roof might carry a roof-fill of around 350-450 kWp. A roof-led quote would propose exactly that.

Step 3 - size to the load instead. Matching annual generation to roughly 60-85% of the daytime load points to a much smaller array - say 180 kWp (around 335 panels). At ~900 kWh/kWp that generates roughly 162,000 kWh a year.

Step 4 - compare the outcomes.

• Roof-filled ~400 kWp: generates far more than the shed uses at midday, so a large share is exported at a few pence. Self-consumption might sit near 35-40%, and payback stretches out because so much output is sold at a discount.
• Load-led 180 kWp: self-consumption around 70%, most units offsetting 25-45p/kWh grid import. Payback lands around 5.4 years - a materially better return from the smaller system.

Step 5 - leave room to grow. The mounting layout and cable routes are specified so that as the operator electrifies forklifts and adds EV-van charging, further panels can be added and the newly-raised daytime load absorbs them. The roof is left “ready to fill” - but filled in step with real demand, not ahead of it.

The lesson: the smaller, load-matched array beats the roof-fill on payback and leaves a clear upgrade path. Bigger is not better; better-matched is better.

How to get this right for your site

The method is straightforward but the inputs have to be real:

1. Pull twelve months of half-hourly data from your supplier or Meter Operator.
2. Overlay it against a modelled solar curve for your roof to find your true self-consumption at different array sizes.
3. Size to 60-85% of daytime demand, then sanity-check against usable roof area (~100-140 kWp per 1,000 m²).
4. Design the mounting and cabling to grow into the roof as MHE and EV charging raise your daytime load.
5. Confirm the funding route - cash, asset finance, or a PPA for shorter leases and tenanted roofs (see our grants and funding guide). Note that solar bought by a company is special-rate plant, so it qualifies for the £1m Annual Investment Allowance rather than full expensing, and commercial solar carries 20% VAT which a VAT-registered business reclaims.

If you run an ambient or single-shift operation, the “size to your load” approach is the difference between a five-year payback and a fifteen-year one - see how it plays out for ambient and general storage. If you are a 3PL or contract operator on a shorter lease, the funding structure matters as much as the sizing, which we cover for 3PL and contract logistics. For indicative costs by system size, see our warehouse solar cost guide.

Get a load-led sizing for your warehouse

We size every warehouse array from your half-hourly data, not a satellite roof-fill. Send us twelve months of your HH data and roof details and we will model your real self-consumption, size the system to your daytime load, and show you the growth path as you electrify MHE and vans - with cash, asset finance, and PPA options priced side by side.

Get your free load-led warehouse solar quote - a properly-sized system, not a roof full of panels you’ll export at a loss.

All figures in this guide are indicative planning-grade and for general information only; a fixed proposal follows a roof and meter survey. Tax and grant treatment depends on your circumstances - confirm with your accountant.