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Why a standard battery shuts down in a power cut
Most commercial batteries are fitted to cut bills, not to keep the lights on. They run in grid tied mode, which means they work alongside the mains supply.
For safety, every grid tied system must stop feeding power the instant the grid goes down. This protects engineers working on the network. It is called anti islanding, and it is a legal requirement.
The result is simple. A battery bought for peak shaving or night rate charging will not power your site during an outage unless it was built to. Backup is a design choice made at the start, not a feature you switch on later.
What backup actually requires
Three things turn a battery into a backup system.
First, an inverter that can run in island mode. This lets the battery make its own stable supply and power your site while the grid is down, then hand back to the mains when it returns.
Second, an automatic transfer switch. This sensing switch disconnects your circuits from the dead grid and connects them to the battery, usually in under a second. Sensitive equipment like servers barely notices the gap.
Third, a critical loads board. You rarely back up the whole site. We wire only the circuits that matter to a separate board, so the battery lasts longer and the sizing stays sensible.
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Which systems are worth backing up, and for how long
The loads that usually justify backup are the ones that cost you money the moment they stop. Refrigeration and freezers in food and hospitality. Servers and IT that hold trading, telephony and data. Access control, CCTV and security. Tills and payment terminals. Process lines where a sudden stop means scrap or a safety risk.
Runtime depends on the battery size against the load it carries. A rough guide is usable storage divided by the load. A 400kWh battery holding a 120kW critical load runs it for about three hours. Protect only the essentials and it lasts longer. Most sites design for one to three hours of cover, which handles the typical short outage. For multi day resilience, a battery pairs with a generator, and we can design that too.
For the full picture on capability, pricing and how we build commercial systems, visit our battery storage installation page. If your staff or fleet also charge EVs on site, we design EV charging and storage as one system.
