Backup Power & Resilience (Critical Load): Solar battery grants

Backup power and resilience: funding a battery that also protects critical load

For sites where a grid outage means real losses, cold-chain stock, data, life-safety systems, or a production process that is expensive to restart, a battery with an islanding or UPS-grade design can ride through outages cleanly and quietly where a diesel standby cannot. The funding angle that makes this configuration work is that the same battery does two jobs: it provides the resilience your operation needs and it stacks daily arbitrage and solar self-consumption value the rest of the time. That dual role is what lets the capital allowances and energy savings fund an asset whose primary purpose is risk reduction. A diesel generator earns nothing between outages and costs money to run and maintain, but a resilience battery pays its way every day, which transforms the case from a pure insurance cost into an investment with a payback. It is also cleaner and quieter than diesel, which matters on sites near homes or with their own environmental commitments.

This page is for operators who know they need resilience and want to understand how the funding stacks when the battery is sized around a critical load rather than purely around demand charges. There is no cash grant for the battery, but the capital allowances apply to the full system, the Smart Export Guarantee applies to any surplus, and where the building is residential or used solely for a relevant charitable purpose the 0 per cent VAT relief on standalone retrofit storage may apply. The right way to read the economics is that the everyday savings and arbitrage carry the financial case while the resilience effectively comes as a bonus, which is the reverse of a diesel generator, where the resilience is the only benefit and every other day it is dead weight. We size the resilience scope around your specific must-run loads, model the everyday value that funds it, and design it to the standards your insurer expects.

What "resilience" means in practice differs sharply by site, and the design and the funding both follow from that. A cold-storage facility may need only its refrigeration circuit to ride through an outage, because losing the chill on stock worth tens of thousands of pounds is the real risk, while the office lighting can go dark without consequence. A data or process site may need a genuinely seamless, zero-interruption transfer because even a momentary dip resets equipment, which points to a UPS-grade islanding design. A life-safety load has its own non-negotiable requirements. Pinning down exactly which circuits must keep running, and for how long, is the first thing we do, because it sets both the size of the battery and the standard it has to meet, and therefore the cost the funding has to carry. Specifying the whole site for islanding when only one circuit matters is the most common way these projects are over-priced.

What a typical install looks like and how we size it

A resilience battery usually lands between 50 kW / 100 kWh and 1 MW / 2 MWh, with the power and capacity set by the critical-load circuit you need to ride through an outage rather than by the whole site. The sizing question is twofold: how much power the critical load draws, which sets the kW, and how long you need to hold it, which sets the kWh. Where the design pairs with solar, the battery gets extended autonomy during a long outage and a daily self-consumption role between outages. Islanding or UPS-grade designs are available for loads that cannot tolerate any interruption at all, and these need careful sizing of the critical-load circuit so the battery carries exactly what must keep running and not the whole building. The CO2 saving varies with how the battery is charged and used, since the headline benefit here is resilience plus daily arbitrage rather than carbon. We pull at least 12 months of half-hourly data and walk and map your must-run loads before sizing, so the resilience scope is real and tested against your actual critical circuits rather than a guess at what matters. Under-sizing leaves a critical load exposed; over-sizing inflates the cost without adding protection, so the walk of the site is as important as the data.

Costs, payback and tax relief

A resilience project typically falls between £80,000 and £1.5m, with a simple payback near 8 years on the energy savings alone, before you count the avoided diesel running and maintenance cost and the de-risked losses an outage would otherwise cause. As special-rate plant, the battery draws 100 per cent Annual Investment Allowance on the first £1m of qualifying spend and a 50 per cent first-year allowance on the balance, which for a limited company can be worth up to around a quarter of the project value back as tax saved in year one, depending on how the spend sits against the £1m cap. The way to read the payback here is that the energy savings and arbitrage carry the financial case while the resilience comes effectively as a bonus, the reverse of a diesel generator that earns nothing between outages. The Smart Export Guarantee applies to any surplus the battery times into higher-priced windows. Our cost guide shows how the everyday savings and the avoided diesel cost combine, and how the de-risked stock or process loss, while hard to put a single number on, weighs in the board's decision.

Funding routes in detail

The funding routes are reliefs and income streams. The primary route is the Plant and Machinery Capital Allowances: 100 per cent AIA on the first £1m, then 50 per cent first-year allowance on the balance, applied to the battery, the islanding equipment and the transfer-switch arrangement as special-rate assets. The Smart Export Guarantee applies to any solar surplus the battery times into higher-priced windows rather than spilling. For the narrow case of a building used solely for a relevant charitable purpose or residential accommodation, the 0 per cent VAT on Energy Saving Materials covers standalone retrofit battery storage connected to the grid, running to 31 March 2027 and then set to move to 5 per cent, which is relevant for charity-occupied and residential-portfolio resilience projects but not for a general commercial site such as a factory or warehouse. For larger assets, NESO grid services can add income between outages, treated as upside given how volatile and saturated frequency-response prices have become rather than as the foundation of the case. We model capital, finance, lease and shared-savings routes side by side so the funding fits the balance sheet, because resilience projects are often approved on a risk basis where preserving capital for the core business matters as much as the headline payback.

Compliance and sector considerations

An islanding design requires anti-islanding protection compliant with G99, and the transfer-switch and changeover design must meet BS 7671. Fire detection and suppression must be integrated to your insurer's and the NFCC's expectations, with BS EN 62933 for system safety and BS EN 62619 for cells. For UPS-grade designs that carry a zero-interruption load, BS EN 62040 applies. Across the site level the wider framework holds: PAS 63100:2024 principles for fire protection, NICEIC or NAPIT electrical registration, and CDM 2015 where the work exceeds 30 person-days. Because this configuration touches life-safety and critical-process circuits, the insurer engagement matters more here than anywhere else, and we bring your insurer into the design conversation up front so the resilience scope is something they are comfortable underwriting rather than a surprise after commissioning. The same engagement that satisfies the insurer also makes the case to the board, since a standards-compliant, insurer-approved design removes the objection that lithium storage is a fire risk before it is raised.

How we approach this kind of project

We design the resilience scope around your specific must-run loads, mapped from your half-hourly data and a walk of the critical circuits, rather than around a headline site figure that would over-specify the battery. We size the everyday arbitrage and self-consumption role so the savings fund the asset between outages, and we specify the islanding or UPS-grade design only where the load genuinely needs zero interruption. We carry out roof, plant-room and asbestos checks before we quote, submit the G99 application and engage the DNO and your insurer on day one, and deliver a fixed-price proposal with the warranted cycle count, throughput and degradation curve stated, backed by a 10-year insurance-backed workmanship warranty. The full model is shared so your finance team can see how the energy savings, the avoided diesel cost and the de-risked losses combine, and so the resilience and the return can be judged together rather than the resilience taken on faith.

An illustrative example

As an illustrative composite based on typical UK resilience projects, and not a real named client or real project: a chilled-distribution site where a grid outage risked stock losses running into tens of thousands of pounds relied on an ageing diesel standby and had a 24/7 refrigeration baseload plus an existing 150 kW solar array. A 100 kW / 200 kWh lithium-iron-phosphate battery with islanding capability for the refrigeration critical load delivered seamless ride-through of grid outages, let the diesel standby be retired as the primary backup, and earned its keep day to day through solar self-consumption and off-peak arbitrage, for an indicative annual saving near £26,000 plus avoided diesel running and de-risked stock loss, and a payback close to 7.8 years. The system was designed to BS EN 62933 and BS EN 62619 with the insurer engaged up front, and the warranted cycle count and degradation curve were stated in the proposal so the late-life performance against the critical load was clear from the outset. The figures are illustrative and depend on your critical loads, outage risk, tariff and how much of the day the battery can earn its keep through arbitrage and solar self-consumption.

If the resilience need is tied to a cold-chain or constant baseload that also has demand peaks, see peak shaving and load shifting, and if you are pairing resilience with existing solar, see solar-plus-storage. When you are ready, read the cost and payback guide, review the funding routes, request a feasibility from your meter data, or work through the battery storage FAQs.