Drop half-hourly CSV files exported from your energy supplier. One file per day or a single multi-day file — both work.
Timestamp, Electricity consumption (kWh),
optional Electricity cost (£)
Drop half-hourly CSV files exported from your energy supplier. One file per day or a single multi-day file — both work.
Timestamp, Electricity consumption (kWh),
optional Electricity cost (£)
Annual consumption is pre-filled from typical UK usage for this property type. Adjust if you know your actual figure from a bill.
| Size | Cost | Saving vs Today/yr | Payback | 10-yr ROI |
|---|
| # | Tariff | Battery | Cost | Saving vs Today/yr | Battery-Only Saving/yr | Payback | 10-yr ROI |
|---|
You upload one or more CSV files containing half-hourly electricity consumption readings (48 rows per day). Your data is sent securely to our server for analysis and is not stored beyond your session.
Each tariff is defined by a rate for every half-hour slot across the day. Time-of-Use tariffs have a cheap overnight window and a more expensive daytime or peak rate. Standing charges are included in all annual cost figures.
For each day, the server runs a half-hourly simulation: the battery charges during cheap slots (subject to your charge rate and capacity) and discharges during expensive slots to offset consumption. Round-trip efficiency (default 90%) is applied on the charging side.
Payback is calculated by compounding the annual saving at your chosen inflation rate until cumulative savings equal the installed cost. Ten-year ROI = (annual saving × 10) − installed cost.
Unlike FreePhase Static, FreePhase Dynamic sets new unit rates every day based on day-ahead wholesale electricity prices. The three-band structure (Green/Amber/Red) stays fixed; only the pence-per-kWh values change.
Because rates vary daily, this calculator uses representative recent averages: Green ~3.9p, Amber ~26.2p, Red ~43.1p. At 3.9p overnight the battery earns a healthy margin discharging into every non-green hour.
On low-price days the Green rate can approach zero (making charging nearly free). On high-demand days, Red can spike to the 75p cap. A smart battery that adjusts to day-ahead prices could capture significantly more value.
This tool is provided for general information and illustrative purposes only. Nothing contained herein constitutes financial, investment, legal, or energy advice.
Tariff rates and battery costs are indicative figures based on publicly available information (Q1 2026). Always verify current rates directly with your energy supplier before making any decision.
Modelled savings are estimates. Actual results will depend on future tariff changes, battery degradation, changes in consumption, installation costs, and other factors.
To the fullest extent permitted by law, the creators accept no liability for any loss arising from use of or reliance on this calculator.