The workings
How we get our numbers.
The savings figure on our front page isn’t a marketing estimate - it’s the output of a simulation you can read the source code of. Here’s exactly what we ran, and what it found.
How the simulation works
We took a full year of real Octopus half-hourly prices (Agile, July 2025 - July 2026, London region, straight from Octopus’s public API) and replayed it, day by day, through the same software that runs on the Raspberry Pi in a member’s garage. It is one codebase, not a spreadsheet:
- Each morning, when the day’s prices publish, the optimiser plans all 48 half-hours as a small linear program: when to charge, when to power the house, when to sit still - minimising the whole home’s bill after efficiency losses and battery wear.
- A simulated inverter then executes that plan with real-world physics: ~5.2kWh of usable storage (one FoxESS EP6 module), 3.6kW charge/discharge, 88% round-trip efficiency, and battery-management cut-offs at the top and bottom of charge.
- Nothing is ever exported - the battery only displaces electricity the house would otherwise buy. No export tariff, no export paperwork, no selling.
- Safety rules run throughout - the same invariant checks the real controller enforces (power caps, state-of-charge floor, export limit) are applied every few simulated minutes.
- Wear is priced in: every kWh cycled is charged 1.5p against the day’s saving, so the optimiser never churns the battery for marginal trades.
- Savings are measured honestly - as the difference between the simulated home’s bill and the identical home, with identical usage, on a standard variable tariff (25.5p) with no battery. The battery’s state of charge carries over from day to day; nothing resets overnight.
Being open about the split: roughly £180 of the ~£410 comes from the tariff switch itself, the rest from the battery. We count both, because half-hourly prices carry real peak-price risk that makes them a gamble for a home without a battery - the battery is what makes the cheap tariff safe to hold (it never buys at the peak, in any year), so the two only work as a pair.
The real-world anchor: our first member site
Simulations need a reality check, so here is our one real datapoint - the York family home (our template install) behind the front-page figures. It has 4kWp of solar and an EV, and its real bill today is £105/month, standing charges and its 2p solar export included. We simulated that same home through the full year of half-hourly prices with the standard kit: the EV smart-charging into each night’s two cheapest slots, the battery grid-charging on the cheapest overnight slots while leaving headroom for the day’s forecast sun, storing ~1,100kWh/yr of solar surplus that earns pennies as export today, and never exporting.
| The same home, over a year | per month |
|---|---|
| Real bill today (flat tariff, solar, EV) | £105 |
| Simulated: kit + half-hourly pricing (same standing charge) | ~£61 |
| Saving | ~£44 |
That’s ~£530/yr - about £3,700 of kit paying back in ~7 years. The assumptions we can’t verify until it’s live: standing charges unchanged (~60p/day), ~9,000 EV miles/yr charged at home, and our weather model standing in for real York sunshine. It’s a simulation of a real baseline, not yet a measured result - the install is underway, and we’ll publish the live ledger, penny by penny, as it runs.
The prices, over the whole year
Half-hourly pricing makes no promises - it tracks the wholesale market. But look at the floor it actually delivered. This is the cheapest half-hour of each day across our simulation year, the slots your battery fills up on, against the flat ~25.5p a standard tariff charges around the clock:
The cheapest slot averaged ~10p against the 25.5p a standard tariff charges all day - and on 39 days the price went negative: the grid paid the battery to charge. The catch is that none of it is guaranteed: the cheap slots land at a different time each night, and the evening peaks carry real risk in a bad winter. It’s a market only a battery with software can navigate safely - the battery means you never buy at the peak, and finding each day’s floor is exactly the job of the optimiser, which re-plans every morning when the day’s prices publish.
What we’re not claiming
- These are projections from one backtest year, not guarantees. Half-hourly prices move with the wholesale market; a calm year flatters them and a crisis year would not. The battery’s protection - never buying at the peak - holds in any year, but the size of the saving will move.
- Part of the saving (roughly £180 of the ~£410) is the tariff switch itself - see the note above. The standard-tariff baseline is 25.5p flat, region C. Standing charges are excluded because the battery doesn’t change them.
- The simulation assumes the battery is available every day and a typical household load pattern (~9.9kWh/day). Your usage will shift the result - bigger users save more.
- The figures assume no solar. If you have panels already, the battery stores your surplus and puts it to work in the evenings (your FiT is unaffected), but a solar home’s bill is smaller to begin with, so the absolute saving is similar. We don’t sell or install solar panels.
- We compare ourselves to competitors honestly: Octopus’s own battery installs are fair deals, and homes that want export income under Octopus’s management can do well on their route. Ours is the simpler one - no export, no export paperwork, your battery under your control - see the cost breakdown.
The controller, the optimiser, the simulator and the raw price data are all in our repository -
the figure on the front page is kwc simulate’s output, nothing more.