Backup Power & Resilience: Battery Storage for Business

Why this use case matters for UK businesses

An unplanned outage rarely costs you the price of the lost kilowatt-hours. It costs you a spoiled production batch, a cold-chain breach, a halted packing line, lost trading hours, or a regulatory breach in a building that must stay conditioned. For a manufacturer, a data-dependent operation, a healthcare site, or a cold store, a single multi-hour interruption can run into tens of thousands of pounds before the lights are even back on.

The traditional answer is a standby diesel generator. It works, but it is an asset that does nothing for 8,700 hours a year, costs money to fuel-test and service, takes 10-30 seconds to pick up the load, and brings fuel storage, emissions, noise and planning questions with it. A commercial battery solves the same resilience problem differently: it holds critical loads live through an outage with a sub-second changeover, and then earns its keep the rest of the year by shaving peaks and shifting load rather than sitting idle.

That dual role is the point. A pure uninterruptible power supply (UPS) protects the load but is a sunk cost. A backup-capable battery protects the load and pays towards itself, which is why the economics work where a UPS or a generator alone cannot.

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

In normal operation the battery is grid-tied and working for you. It charges on cheap overnight power and discharges into your daytime peak, trimming the expensive 25-45p/kWh units and the red-band charges that sit on top of them. We size and schedule that behaviour against your half-hourly meter data, because the import profile tells us exactly when your peaks fall and how much energy sits under them.

When the grid fails, the system switches mode. An islanding-capable inverter and an automatic transfer switch (ATS) disconnect the site from the network and carry your critical loads from stored energy, with a changeover fast enough that sensitive equipment never sees the interruption. This is true ride-through, not the 10-30 second start-up gap of a generator. How long you can island depends on the energy you have stored versus the size of the critical load you choose to protect: a 500 kWh system carrying a 100 kW critical load runs that load for several hours, longer if you protect only essentials.

Because the battery is sized from your real load shape rather than a nameplate figure, the same asset that delivers resilience is the one already lowering your bill every working day. If you also have or plan solar, the battery lifts self-consumption from roughly 55% towards 85% or more, and a sister page on solar and battery storage covers that pairing in detail.

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

A battery is rated two ways and you need both. Power (kW) is how hard it can push at any instant, which sets the size of the critical load it can hold. Energy (kWh) is how much it stores, which sets how long it can hold it. A 250 kW / 500 kWh system is a “2-hour” battery. Backup duty tends to demand more power relative to energy than a pure arbitrage system, because you must instantly cover the critical load.

For this use case we typically design in the range of 150-800 kWh of energy and 75-400 kW of power, at a C-rate around 0.5C, with round-trip efficiency of about 89% and a cycle warranty of 6,000 cycles or 10 years. Cells are lithium iron phosphate (LFP) for thermal stability and cycle life. Project value usually lands between £75,000 and £400,000 depending on size and site, with simple payback around eight years and typical day-to-day savings of £15,000-£80,000 a year before you count the avoided cost of downtime.

What drives the cost is straightforward: total kWh, the power rating and inverter count, the electrical and civil works to connect, the islanding and ATS hardware, and fire/ventilation provision. Commercial-scale cells run roughly £200-450/kWh, falling towards £140-240/kWh on larger containerised systems; prices have come down from around £800/kWh in 2020. The economics only stack when value streams are stacked, so the cost page walks through the full build-up and what moves it. Note the avoided-downtime value is real but site-specific, so we keep it out of the headline payback and treat it as upside.

A worked example

Consider an unnamed Midlands chilled-foods business on a half-hourly commercial tariff, paying peak units around 38p/kWh and carrying available-capacity (kVA) and red-band DUoS charges through the working day. Its critical load, refrigeration plus controls, is about 120 kW, and a four-hour outage two years ago wrote off a full day of stock.

We model a 200 kW / 400 kWh LFP system from twelve months of their half-hourly data, with an islanding inverter and ATS protecting the refrigeration circuit. Day to day it charges overnight at off-peak rates and discharges across the afternoon peak, cutting peak units and clipping the kVA charge, for a modelled saving of about £52,000 a year. On a grid failure it carries the 120 kW critical load with a sub-second changeover, comfortably riding out the kind of outage that previously cost them a day’s stock.

At a project value of roughly £300,000, the day-to-day savings give a simple payback near eight years, with the avoided-downtime protection sitting on top as insurance the business already needed. The 100% Annual Investment Allowance means the full cost is deducted against profits in year one, and the standard-rated 20% VAT is recovered in full because they are VAT-registered. The figures here are illustrative; your own come straight from your meter data.

Compliance and grid connection

A grid-tied commercial battery needs a G99 connection application to your Distribution Network Operator (DNO); only the very smallest systems use the simpler G98 route. For a backup-capable installation the grid-tied mode must include anti-islanding protection so the battery cannot energise a faulted network, while a separate islanding-capable inverter and ATS handle the intentional, disconnected island when the grid is down. Those two requirements coexist by design.

The DNO may apply an export limitation or an Active Network Management (ANM) connection on a constrained part of the network. That rarely hurts a backup-and-peak-shaving system, because its job is behind-the-meter rather than exporting. G99 timescales run from 8 weeks to 12 months depending on capacity and local headroom, so we submit early and design around whatever the DNO grants.

The installation is designed to the IET Code of Practice for Electrical Energy Storage Systems and to BS EN/IEC 62933 and IEC 62619, with fire, thermal, detection, separation and ventilation provision appropriate to the cell chemistry and enclosure. CDM 2015 applies to the works, and we engage your insurer before energisation. The behind-the-meter angle is also a quiet bonus: because the battery can supply peaks from storage, a site can often add EV chargers or extra plant without a costly DNO reinforcement, which the EV charging and storage page explores further.

Who it suits, and who it doesn’t

It suits any business where an outage is genuinely expensive: cold stores and food processors, manufacturers with sensitive or continuous lines, healthcare and care settings, data-dependent operations, and sites already running, or dreading the cost of, a standby generator. It suits them best when there is also a meaningful daily peak to shave, because that is what turns the resilience asset into one that pays towards itself rather than sitting idle.

It suits you less if your only need is multi-day, whole-site backup with no daily peak to exploit. A battery sized to carry an entire site through days of outage is rarely economic, and a generator, or a hybrid of the two, may be the honest answer. We will say so. If your interest is purely shaving demand charges with no resilience requirement, a simpler grid-tied system covered on the peak-shaving page will likely give you a faster payback without the islanding hardware. The right recommendation comes out of your half-hourly data, not a brochure, and you can request a fixed-price quote once we have it.

Frequently asked questions

Can a battery replace my diesel generator entirely? Often it can for typical UK outage durations, which are minutes to a few hours, and it does so with a sub-second changeover instead of a 10-30 second start-up and with no fuel testing. For very long or whole-site backup the honest answer may be a battery-plus-generator hybrid, and we will tell you which your load and risk profile actually need.

How long can the battery keep my site running? It depends on the critical load you choose to protect versus the energy stored. A 400 kWh system carrying a 120 kW critical load runs that load for roughly three hours; protect only essentials and it lasts longer. We size the energy to the outage duration you want to cover, established from your meter data.

Does the backup function reduce the everyday savings? No. The same battery does both jobs. It earns through peak-shaving and arbitrage every working day and only switches to island mode during an outage, so the resilience capability sits on top of the day-to-day savings rather than competing with them. Some sites add grid-flexibility revenue too, covered on the grid services revenue page.