Start with the job, not the battery
A battery sized for moving cheap electricity into an expensive tariff window may be very different from one sized for a brief demand peak. Write down the service you need first: tariff shifting, solar self-consumption, peak support or backup. These can overlap, but a single unit of stored energy cannot be spent twice. Our public savings calculator models tariff shifting only.
Separate power from energy
Power, measured in kW, describes how quickly a battery can charge or discharge. Energy, measured in kWh, describes how much it can store. A system with 250 kWh of usable output and 125 kW of discharge power has a theoretical two-hour duration at that output. Actual duration depends on starting charge, operating limits, reserve and losses. Check whether a supplier quotes nominal DC capacity, usable DC capacity or usable AC output; they are different measurement boundaries.
Find demand inside the expensive window
Use interval electricity data, ideally covering a representative year. Identify the load that coincides with your expensive periods, rather than using annual consumption divided by the number of days. A weekend, shutdown or seasonal change can materially reduce the energy available to displace. Solar can already meet some daytime demand, so model the remaining net demand after generation.
Check that it can recharge
A battery needs time and connection headroom to charge. Charging can create a new peak, and a short cheap-rate window may limit how much energy can be shifted. In the calculator, delivered energy is bounded by usable output, discharge power, site demand and charging-window input after round-trip losses. For example, delivering 100 kWh at 80% AC-to-AC efficiency requires 125 kWh of charging electricity.
Ask for a site-specific simulation
A screening calculation helps narrow the question; it does not choose equipment. Ask an installer or engineer to use your actual interval data, tariffs, connection agreement and proposed control strategy. Include reserve, degradation, outages and end-of-life plans. Confirm inverter ratings, operating temperature and warranty conditions with the supplier. A battery serving backup duties may need energy held in reserve that is unavailable for daily tariff shifting.
Sources & further reading
Use current documents and confirm the requirements with your own network operator.
Created by Arcgrid. Educational information for commercial sites in Great Britain, not an engineering design or a guarantee of savings. Scenarios are illustrative. Northern Ireland has different connection arrangements.
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