How do you size battery storage for a commercial site?

Size your commercial battery around the power you need and the energy you can regularly move, rather than annual electricity use alone. The Energy Technology List separates energy capacity and power testing [S1]. Its commercial battery category starts at 100 kWh, which is a listing criterion rather than a minimum for your site. Checked October 2026.

Last checked 16 October 2026

How do you size commercial battery storage?

Start by choosing its job: shift solar electricity into later hours, move purchases into a cheaper tariff window, support a short demand peak, or provide power to essential equipment during an outage. Storage holds electricity for later use; these objectives need different operating plans [S1], [S2].

Ask your installer to prepare separate designs if you need more than one job. Keep a written order of priority. A reserve held for an outage cannot also be treated as available for routine trading. Request an operating schedule showing which service has first call on the stored energy.

Read why businesses install batteries before comparing cabinet sizes. Decide what success means for your site before considering a particular product.

How do you calculate the battery capacity your site needs?

Use kilowatts (kW) for power: the rate at which equipment draws or supplies electricity. Use kilowatt-hours (kWh) for energy: power sustained over time. Battery specifications should separate usable energy, charging power, discharge power and efficiency [S5].

Request interval electricity data from your supplier or meter service. For a useful design comparison, provide a full operating year if available, with opening hours, shutdowns, refrigeration, heating and vehicle charging marked. This is a recommended evidence pack, rather than a prescribed minimum period.

Ask the designer to plot your demand alongside solar output and the proposed battery schedule. An annual bill cannot show whether demand coincides with generation. Check winter working days, sunny closure days and busy trading periods separately. Keep measured readings distinct from forecasts for new equipment.

The following is an illustration, not a quote. Every input is an assumption for a notional site; none is a market benchmark or a recommended product size.

Here, the usable fraction already includes the assumed reserve. Do not deduct it again. If a quote states usable capacity directly, compare it with the required stored energy instead of applying another nominal-to-usable conversion. Energy Saving Trust explains that batteries stop discharging at a set minimum level [S2].

Ask the supplier where each efficiency is measured. A battery-only figure and a complete system figure may cover different losses; the specification checklist explicitly asks for system energy loss information [S5].

Should you size for peak demand or daily consumption?

For a peak-shaving design, ask for a graph of demand above the proposed import limit. Calculate the energy under that graph and check the highest power shortfall. For tariff shifting, total the demand you intend to serve outside the charging window. Treat these as separate calculations before combining them.

For example, assume you have only 10 kW available for battery charging over a 3-hour window. That permits 30 kWh of input before losses: 10 × 3. It cannot refill the preceding example’s 44.4 kWh withdrawal. The design must change its charging window, charging power or service target. Buying additional capacity alone does not solve this arithmetic.

For solar storage, request a comparison between surplus generation and later consumption. Include the value of electricity you would otherwise export when calculating the cost of charging. Compare the same operating days across different battery options; do not count one unit of stored electricity against multiple benefits.

Use peak shaving and tariff arbitrage to prepare the operating brief.

What changes when you add EV chargers or backup power?

Separate direct vehicle charging from the building load in your proposed schedule. Ask the designer to compare managed charging against battery support before assuming every vehicle session should pass through storage. State which vehicles must leave by which time, and test repeated arrivals rather than one isolated session.

For backup, specify essential circuits and required duration. Request written confirmation of power-cut operation, changeover equipment and supported loads. The consumer code includes power-cut operation in its battery disclosure checklist [S5]. Ask for a commissioning demonstration of that function before acceptance.

Review solar, batteries and EV charging together when these systems share a connection. Give each installer the same site demand information and proposed controls.

What grid and space checks belong in the sizing exercise?

Your distribution network operator (DNO) manages the local electricity network. G99 is the connection standard used for relevant generation and storage connections in Great Britain. National Grid Electricity Distribution provides storage application forms and a separate export limitation application [S3].

Ask your installer to confirm import and export limits, existing generation and the application route before ordering. An export limit is not a substitute for checking the complete arrangement. Northern Ireland sites should confirm their local connection procedure rather than assume this GB process applies.

Allow space for servicing and emergency access. In England, planning guidance encourages early fire-service engagement for relevant storage planning applications of 1 MWh or more. That threshold is not an exemption from risk assessment below it. Confirm local planning requirements with the council and check lease provisions with your solicitor [S4]. See battery fire safety.

What should you do next?

Ask for alternative sizes against the same demand profile, with usable capacity, continuous power, losses, reserve settings and expansion terms stated. Require a separate cost for any grid work or backup function.

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