Solar Battery Storage Sizing Calculator

The economics of commercial battery storage in the UK shifted sharply once export tariffs fell well below import tariffs and Capacity Market plus DSR revenues became more accessible. The dominant question is no longer whether to store solar, it is how much to store. Oversize the battery and you erode payback with kit that cycles too little. Undersize it and you spill expensive midday generation back to the grid at SEG rates that rarely beat 8p/kWh. The right kWh figure sits at the intersection of your half-hourly load profile, your generation curve and your import tariff structure.

For sites with on-site solar feeding into a battery, sizing also drives the conversation with the DNO. A battery that exports significant kW will trip a G99 application and may push you toward a higher MEC (Maximum Export Capacity), with a queue and capex tail attached. Conversely, a battery that only ever flattens internal peaks behaves like a load-side asset, simplifies the connection and lets you stack benefits across self-consumption, Triad-style peak avoidance under the current DUoS bandings, and a possible flexibility contract. Getting the kWh maths right makes those downstream conversations far cleaner.

Purely Energy sees the bill side of this equation across 2,000+ UK sites. The clients getting the strongest payback on battery storage are the ones who do the sizing arithmetic before talking to an installer, then verify the assumed self-consumption against monitored half-hourly data. Without that step, payback models routinely overstate savings by 20% to 40%, because they assume the load is on when the sun is up and the battery is full. Use this calculator as the conversation starter, then validate against actual demand from your monitoring platform before committing capex.