Battery savings calculator

The tool models one repeating tariff-shifting event on each operating day. Capacity means usable AC energy delivered, rather than nominal cell capacity. Power is assumed to be the continuous AC charge and discharge rating. Round-trip efficiency is AC output divided by AC input.

Daily delivered energyMinimum of:
  • Available usable AC output
  • Discharge power × peak hours
  • Peak-window net site demand × peak hours
  • Charge power × charging hours × round-trip efficiency

We assume non-overlapping charging and peak windows within one day, a constant net load during the peak window and sufficient connection capacity for the entered charging power. Real projects require interval data and a charging-headroom check.

Charging energy = delivered energy ÷ efficiencyAnnual gross benefit = operating days × (delivered energy × peak price − charging energy × charging price)Annual net benefit = annual gross benefit − annual operating costSimple payback = installed cost ÷ first-year annual net benefit

Prices entered in pence are converted to pounds. Non-positive annual net benefit produces no positive simple payback. Year-one payback ignores degradation; it is deliberately distinct from the ten-year illustration.

The ten-year illustration

For year n, available capacity is initial capacity × (1 − annual capacity reduction)n−1. We recalculate output and annual net benefit each year, keeping tariffs, demand, operating days and operating costs fixed. The displayed ten-year balance sums these benefits and deducts the entered installed cost once.

Excluded from the model

Finance, discounting, inflation, tax, VAT changes, replacements, residual value, export revenue, solar self-consumption, flexibility or trading revenue, capacity-charge savings, outage value and avoided network reinforcement. Prices, costs and degradation are your assumptions. The model does not assess warranty compliance or battery suitability.

Worked arithmetic

At 80% efficiency, delivering 100 kWh requires 125 kWh of charging electricity. Avoiding 30p/kWh imports and charging at 10p/kWh gives £17.50 per operating day. At 200 days, this is £3,500 before operating costs, or £3,000 after £500 of annual costs. These figures are illustrative arithmetic, not a product claim.

Grid headroom calculator

Combined demand = existing coincident demand + additional loadRequired power = max(0, combined demand − modelled import limit)Required energy = required power × event hoursAvailable energy = usable AC output × (1 − reserve)

A scenario is labelled supported only when battery power meets the required power and available energy meets the required energy. If combined demand is already within the limit, the output explains that no support is required for this event.

The battery is assumed to begin with the entered usable energy; this tool does not model recharge, repeated events, degradation or outages. It uses kW and kWh, not kVA, amperes, phase imbalance, voltage, fault level or protection. It cannot determine actual DNO capacity, G100 compliance or approval.

Sources & review

Our guides draw on network operator and ENA documents and the Faraday Institution’s battery research. Calculation defaults are clearly labelled illustrative; they are not quoted market averages. The Arcgrid editorial team maintains this resource. Report a correction to hello@arcgrid.co.uk.