Solar-Plus-Storage for UK Commercial Sites

Why this use case matters for UK businesses

A commercial solar array generates most of its power in the middle of the day, which is rarely when a business needs it most. On a typical site, demand peaks in the morning before production ramps up and again in the late afternoon and evening once the array has stopped producing. The result is that a large share of the generation is exported to the grid at a low Smart Export Guarantee (SEG) rate, while the same business buys power back later at a higher peak unit rate.

A behind-the-meter battery closes that gap. It stores surplus midday generation and releases it when the array is idle, so the energy your panels produce offsets the units you would otherwise import at full price. How much of the array’s output is used on site without storage depends on the load profile, and a correctly sized battery raises that share, which is where the economics of solar-plus-storage come from.

This is not solar panel installation. It is the storage layer that makes an existing or planned commercial array pay back faster. If you already have PV, a battery is usually the single most effective upgrade you can make to its returns. For more on whole-project costs across system sizes, see our cost guide. Whether the battery is AC-coupled or DC-coupled changes the design for an existing array, as AC vs DC coupling explains, and our buying guide covers the wider decision.

How the battery does it (the mechanism, referencing half-hourly load)

Every commercial site with a half-hourly (HH) meter generates a settlement-grade record of consumption in 48 daily readings. Overlay your solar generation profile on that HH demand curve and the opportunity becomes visible: a midday block where production exceeds on-site load, flanked by morning and evening periods where load exceeds production.

The battery is dispatched against exactly this picture. During the midday surplus, instead of exporting at the SEG rate, the system charges from the array. As production tails off in the afternoon and on-site demand rises into the evening, the battery discharges to cover that load. In economic terms it converts low-value export into avoided import at the higher peak price. That arbitrage between export value and import cost, repeated daily, is the core return.

Two coupling methods are available. A DC-coupled battery sits on the same side as the panels and shares an inverter, which is efficient for new builds. An AC-coupled battery connects on the building side and retrofits to existing PV regardless of the original inverter, which is the more common arrangement when storage is added later. Start from your own half-hourly meter data: the peak-shaving calculator on this site applies the published charge bands to it. If the terms are new, what a BESS is explains the parts.

The same stored energy also covers weekend and bank-holiday baseload, when many sites still draw power but generation is poorly timed against occupancy, and it can be scheduled to avoid the most expensive Distribution Use of System (DUoS) red-band periods on weekday evenings.

Sizing and economics (power vs energy; what drives cost)

A battery is sized in two dimensions, and both matter. Power, measured in kW, is how fast it can charge or discharge. Energy, measured in kWh, is how much it can hold. For solar-plus-storage the energy figure usually leads, because the job is to soak up a midday surplus and release it over several evening hours.

Size a solar-plus-storage battery from the surplus your array spills and the evening load it has to cover, and quote its power (kW) and energy (kWh) separately: see kW vs kWh. Round-trip efficiency and cycle warranties differ by maker and are stated on different bases, so compare them on the datasheets: see round-trip efficiency on commercial battery datasheets and what eight makers publish on warranties.

Cost follows the published UK bands on the cost page. What drives cost is energy capacity first, then power rating, coupling method, switchgear and protection, any DNO works, and the complexity of integrating with existing PV.

Payback for solar-plus-storage depends on array size, tariff and how much generation was previously being exported. Payback is achieved by stacking savings, not relying on one: self-consumption uplift, avoided low-value export, evening and weekend bill cover, and DUoS peak avoidance.

Capital allowances improve the picture. The £1m Annual Investment Allowance (AIA) lets a business deduct 100% of qualifying cost against profits in the first year. A battery is plant and machinery, and the Annual Investment Allowance covers the first £1m a year; above that, a company claims the 50% first-year allowance on special-rate spend, which is how this site treats a battery, as our tax guide explains. Commercial battery storage is standard-rated at 20% VAT, which a VAT-registered business recovers in full. The 0% VAT relief applies only to domestic and charitable buildings, not commercial installations. Funding routes are set out on our grants and funding page.

A worked illustration

A worked illustration, not a customer result: the saving is the sum of what each stream is worth on your site, and the payback is the installed cost divided by that sum. Compare any figure you are given with the bands and calculator on our cost page. Site-specific figures come from your own meter data; send us your details and we reply by email.

Compliance and grid connection

A retrofit battery is a new generating and storage device on your connection, so it needs the right paperwork. Most commercial systems require a G99 application to your Distribution Network Operator (DNO); only the smallest devices fall under G98. If your original PV was connected under G98 or G99, adding storage usually triggers a G99 variation to the existing connection rather than a fresh application. Timescales vary widely, so the application should go in early.

The DNO may impose export limitation or require an Active Network Management (ANM) connection, restricting how much you can export. For solar-plus-storage this is rarely a problem, because the battery’s purpose is to keep energy on site rather than push it to the grid; export limitation often makes storage more valuable, not less.

Systems are designed to the IET Code of Practice for Electrical Energy Storage Systems and to BS EN/IEC 62933 and IEC 62619, with proper fire, thermal, detection and separation design. Work is managed under CDM 2015, and your insurer is engaged before energisation. Use an MCS-certified installer for battery storage: it also supports SEG registration for any associated solar export. Check NICEIC, RECC or TrustMark registration and an insurance-backed workmanship warranty before you sign.

A further benefit worth noting: because a behind-the-meter battery lets you draw more during peaks than your agreed import capacity, it can support added load, EV chargers, plant or extra shifts, without a costly DNO reinforcement. If that is your priority, our EV charging and storage and peak-shaving pages go into more detail.

Who it suits, and who it doesn’t

Solar-plus-storage suits businesses that already run a commercial PV array and are exporting a meaningful share of generation at a low SEG rate, or that are installing solar now and want to maximise its return from day one. It works best where there is a clear mismatch between when the sun produces and when the site consumes, such as offices, manufacturing and warehousing with strong evening or early-morning baseload.

It is a weaker fit where self-consumption is already high, for example a site that runs heavy daytime processes and uses almost everything the array produces in real time. With little surplus to store, a battery has less to do, and the case may rest more on resilience or grid-flexibility revenue than on solar arbitrage. It is also a poor fit if there is no PV and none planned; in that case energy arbitrage on overnight tariffs or pure peak-shaving may be the better starting point.

Work from your half-hourly data before anyone quotes; if storage does not stack up against your specific load and tariff, the data will say so.

Frequently asked questions

Can I add a battery to solar panels that are already installed? Yes. An AC-coupled retrofit connects on the building side and works with existing PV regardless of the original inverter. Adding storage usually triggers a G99 variation to your existing DNO connection, which the installer normally handles as part of the project.

How much will a battery improve my solar self-consumption? A battery raises the share of your own generation that is used on site; the uplift depends on your generation profile and demand curve, which is why it has to be worked out from your half-hourly meter data first.

Does commercial solar battery storage qualify for 0% VAT? No. The 0% VAT relief applies only to domestic and charitable buildings. Commercial battery storage is standard-rated at 20% VAT, but any VAT-registered business recovers that in full, and the £1m Annual Investment Allowance lets you deduct the cost against first-year profits.

Retrofitting battery storage to an existing commercial solar array

Most storage projects now begin with an array that is already generating. A large stock of UK commercial PV went up when export payments were worth having and batteries cost several times what they do today, so the storage layer was left out. With the gap between import and export prices wide, retrofitting a battery to an existing array is the common project shape. The roof, the connection and the generation are already there; only the economics of adding a battery energy storage system have moved.

On coupling, the honest default for a retrofit is AC. It leaves the existing inverter and its warranty untouched and lets you size the battery to your load, not the array. DC wins in narrower cases: where the inverter is hybrid-capable or due for replacement, or where an oversized array is clipping and that lost generation is worth capturing. The extra conversion step costs a little efficiency, but a well specified system still sits inside the 88-92% round-trip band.

Storage changes the registered capability of your connection, so the DNO has to be told. Where the original PV sat under G98, a commercial battery will almost always push the site into a full G99 application. Where it was already G99, the route is normally a variation to the existing agreement rather than a fresh connection. Applying G100 export limitation, which responds within about 15 seconds and 60 seconds at the outside, holds you to the capacity you already have and tends to get a variation approved faster. Allow a long lead time and treat it as the critical path.

Size from the export profile, not the headline kWp. What matters is the shape of the surplus: how many kWh actually leave the site in a given half hour, over how many hours, and how that changes between June and December. Pull twelve months of generation and half-hourly import data and read the spill directly; two sites with identical arrays can differ several times over. Set energy capacity against the weekday surplus you can redeploy the same evening, and power rating against the fastest spill rate plus any peak shaving duty the asset must also carry.

Storage does not end your SEG arrangement. The meter still records what is left, but you export less by design, so model the income you give up as a cost against the import you avoid. Check the tariff terms first, particularly any condition on grid-charged electricity being exported; charging from the array alone keeps that simple. Where the site can also earn from flexibility, grid services revenue through an aggregator is modelled separately and never guaranteed. Send us your details and we reply by email.

See also

• South West solar retrofits
• Milton Keynes distribution roofs
• Oxfordshire science park sites