Two questions decide whether tonight’s cheap window is worth using: will the battery reach your target before the rate flips back to peak, and what does that fill actually cost once you count the losses? The finish time is the usable energy you need to add divided by the rate that reaches the battery, checked against the window length. Tonight’s cost is the grid energy you import multiplied by your off-peak unit price. This home battery cheap-rate charging calculator works both from your own figures and shows the pounds you save for the night.
I run a Sunsynk 3.6 ECCO hybrid with a Fogstar 15kWh battery bank, charged to full from the grid every night since late 2024, so the defaults sit close to a real overnight setup rather than a spec sheet. The capacity default is a round 16kWh, so set it to your own bank before you read anything off the result, and change every other input to match your inverter and tariff.
How the calculator works out tonight’s charge
It takes the usable kWh between your current and target state of charge, divides that by your charge rate to get the time needed, and checks it against your cheap-rate window. Then it adds the charging loss to find the grid energy imported and prices it at your off-peak rate. Costs round up, savings round down.
Will the battery finish charging in the cheap window?
The answer is a single division. Work out the usable energy you need to add, then divide it by the rate that actually reaches the battery. Read that battery-side rate from your inverter or monitoring app while it charges, not the inverter’s AC nameplate. The AC figure is measured before the conversion loss, so entering it makes the finish time look a little quicker than it really is, by roughly 8% at 85% round-trip. If the time you get is shorter than the cheap-rate window, you finish with room to spare. If it is longer, you stop part-charged when the window closes.
Take a 16kWh battery with 90% usable capacity, so 14.4kWh you can actually cycle. Charging from 20% to 100% adds about 11.5kWh. A Sunsynk 3.6 is rated at up to 3.6kW on the AC side, and the battery sees a little less than that after the conversion, so the fill needs roughly three and a half hours. On an Octopus Go window of 00:30 to 05:30, five hours long, it finishes with time to spare. On a two-hour smart-charge slot it would only reach about two-thirds and leave close to 5kWh unfilled.
Two limits decide the outcome, so the calculator asks for both. The first is your charge rate, which is capped by the smaller of the inverter’s charge power and the battery’s own charge-current limit. The second is the window length, which is a property of your tariff, not your hardware. A bigger battery or a lower starting state of charge needs a higher charge rate or a longer window to still finish in time.
What does tonight’s charge actually cost?
Tonight’s cost is the grid energy you import times your off-peak unit price. The catch is that a battery does not store every kWh you put in, and it does not give every stored kWh back. Those losses are the round-trip efficiency, and they push the true cost of stored energy above the headline off-peak rate.
The honest figure to watch is the effective cost per usable kWh, which is your off-peak rate divided by round-trip efficiency. At 8.5p off-peak and 85% round-trip, each usable kWh you get back costs about 10p. That sits well below the 26.11p day rate in this example, the national average direct debit unit rate under the Ofgem July to September 2026 price cap, so at this spread overnight charging still pays after losses. Your own off-peak and day rates decide it: a narrow spread can wipe the margin out.
Put that in pounds, because pounds are what you actually quote. Filling that 16kWh bank from 20% to 100% pulls about 12.5kWh from the grid, since you import more than you store. At 8.5p off-peak that costs about £1.07. The 10.6 usable kWh it hands back would cost about £2.77 at the 26.11p day rate, so the fill saves roughly £1.70 for the night. The calculator shows this saving figure directly, next to the cost.
Round-trip efficiency is the input people guess worst, so it is editable. A lossless system would read 100%. Many hybrid AC systems land somewhere between 80% and 90% once you count the inverter conversion both ways and standby draw. Measure yours if you can; if you cannot, the 85% default is a cautious estimate. Lowering it in the calculator raises the effective cost, which is the cautious direction to be wrong in.
How long is the cheap-rate window?
The window length is set by your tariff, and it is the number people most often get wrong. Enter your own window in the calculator, because the common UK tariffs differ by hours, and the exact times shift by region and meter.
| Tariff | Typical off-peak window | Hours | Note |
|---|---|---|---|
| Economy 7 | Around 00:30 to 07:30, varies by region and meter | 7 | The seven hours are the source of the name. Times shift with your DNO region and with British Summer Time. |
| Octopus Go | 00:30 to 05:30 every night | 5 | Fixed nightly window. The off-peak unit rate varies by region. |
| Intelligent Octopus Go | 23:30 to 05:30 guaranteed for the whole home | 6 | This six-hour window covers a home battery. A separate six-hour cap that Octopus applies from 2026 limits cheap-rate EV smart charging only, not home-battery charging. |
| E.ON Next Drive | 00:00 to 07:00, no smart device needed | 7 | Whole-home off-peak. The Next Drive Smart variant uses a shorter 00:00 to 06:00 window at a lower rate. |
The practical takeaway is that a two-hour smart slot and a seven-hour Economy 7 night are very different jobs for the same battery. A large bank on a short window needs a higher charge rate to finish, so enter the window your own tariff actually gives a home battery, not an EV smart-charging figure.
Does grid charging still pay after losses and degradation?
Losses are only part of the real cost. Every full charge and discharge also wears the cells a little, and that wear is a cost per cycle the effective-cost figure above does not include. It is worth putting a number on before you cycle a battery hard every night for the arbitrage margin.
My Fogstar bank gives me measured evidence rather than a projection. After nearly three years and more than 800 real cycles, including nightly grid charging to 100% since late 2024, the original packs read about 90Ah against a 100Ah nameplate, roughly a 10% capacity drop. Fogstar rates the cells for 4,000 cycles to 80% retained capacity. On that curve the degradation cost per cycle is small next to a spread of 8.5p off-peak against 26.11p at the cap, but it is not zero, and a cheaper spread erodes it faster.
You can put a rough number on that wear yourself. Divide what the pack would cost to replace by its rated cycles, then by the usable kWh it moves per cycle. A bank that costs £2,000, rated for 4,000 cycles, moving 14kWh usable each night, wears at about 3.6p per usable kWh (2,000 divided by 4,000, then by 14, in pounds). Subtract that from the margin between your day rate and your effective off-peak cost to see the gain that is left. On the 10p against 26.11p example the margin easily covers it; on a narrow spread it may not.
So the order to check is simple. First confirm the battery fills in your window. Then confirm the effective cost per usable kWh sits well under your day rate. Only then is the remaining margin large enough to absorb degradation and still leave you ahead. If off-peak and day rates are close, the calculator will show a thin or negative margin, and grid charging is not worth the wear.
One seasonal check sits on top of the maths. From late spring to early autumn, if your solar already fills the battery on most days, there is nothing to gain from forcing a grid charge on top. Save the cycle. Overnight grid charging earns its place on the nights when solar has not done the job.
Assumptions, limits and what this does not answer
This is a screening tool for one overnight charge, not a payback model. It deliberately leaves out several things so the numbers it does show stay honest:
- It ignores the standing charge, which you pay whatever you charge.
- It ignores any house load running from the grid during the window, which adds cheap-rate spend but does not change whether the battery itself fills.
- It treats the round-trip loss as split evenly between charging and discharging. That split only shapes the intermediate grid-import and usable-output figures shown; the headline effective cost per usable kWh uses the full round-trip efficiency directly and does not depend on it.
- It does not price battery degradation per cycle, because that needs your battery price and rated cycle life. Treat the effective cost per usable kWh as a floor, then add the per-cycle wear figure worked above on top.
- It assumes a steady charge rate. Some batteries taper the last few percent near full, so a fill to 100% can take a little longer than the flat calculation suggests.
- It reads the charge rate as the power reaching the battery. Enter your inverter’s AC nameplate instead and the finish time comes out optimistic, because that figure is measured before the conversion loss. Use the battery-side rate your app shows while it charges.
Every default in the tool is editable, and none is set to flatter the result. The day rate defaults to the current Ofgem price cap unit rate, the off-peak rate to a mid Octopus Go figure, and round-trip efficiency to a cautious 85%. Put your own tariff, battery and inverter numbers in before you trust any output.
The next move
Find three numbers before you open the calculator: your battery’s usable capacity, your inverter’s charge rate, and your tariff’s off-peak window and price. Your inverter or monitoring app shows the charge rate live while the battery fills, and your bill shows the window and rates. Enter them, check the battery finishes in the window, and read the effective cost per usable kWh against your day rate.
If it fills with time to spare and the effective cost sits well under your day rate, overnight grid charging pays and the setup is exploiting the tariff window. If it stops part-charged, you need a higher charge rate, a longer window, or a lower target before the maths works.
Sources
- Ofgem, Energy price cap unit rates and standing charges: the 26.11p per kWh default day rate is the national average direct debit unit rate for July to September 2026, and your regional rate will differ.
- Octopus Energy, Octopus Go: the 00:30 to 05:30 nightly off-peak window, with rates that vary by region.
- Octopus Energy, Intelligent Octopus Go and Upcoming changes to Intelligent Octopus Go: the 23:30 to 05:30 guaranteed whole-home window, and the separate six-hour cap that from 2026 applies to EV smart charging only, not home-battery charging.
- E.ON Next, Next Drive: the 00:00 to 07:00 whole-home off-peak window, with a shorter Next Drive Smart window at a lower rate.
- Fogstar cell rating of 4,000 cycles to 80% retained capacity, and my own BMS readings after 800-plus cycles on a Sunsynk 3.6 ECCO, documented in the Fogstar 15kWh three-year owner review on this site.
