How these numbers are worked out
Every figure on this site is labelled with where it came from. Nothing is modelled from a guessed efficiency unless it says so.
Three labels, used consistently
| verified | Published by the manufacturer or the regulator, quoted directly. No arithmetic applied. |
|---|---|
| calculated | Arithmetic performed only on verified inputs. No assumptions introduced. |
| estimated | Depends on an assumption that could be wrong. Always disclosed at the point of use, never buried in a footnote. |
The mistake almost every charging calculator makes
The obvious way to work out charging time is to divide battery capacity by charger power. For a BYD Atto 3 on a 7 kW home charger that gives 60.48 ÷ 7 = 8 h 38 min.
BYD's own figure is 9 h 42 min. The difference is not rounding. Electricity is lost as heat in the onboard charger, in the battery management system and in thermal conditioning, so roughly 11% of what the meter records never reaches the battery. A calculator that divides capacity by power will tell you the car charges faster and costs less than it does.
What this site does instead
Rather than assume an efficiency figure, the calculation starts from the manufacturer's published charge time and works backwards to the energy that must have passed through the meter:
| Charger | Official 0–100% | Energy from the wall |
|---|---|---|
| 7 kW single-phase | 9 h 42 min | 67.9 kWh |
| 11 kW three-phase | 6 h 30 min | 71.5 kWh |
This matters more than it looks. BYD publishes 60.48 kWh as the battery capacity but does not say whether that is the gross pack or the usable portion. Deriving wall energy from charge times sidesteps the question entirely: both the unknown usable capacity and the unknown charging efficiency are absorbed into one number that comes straight from the manufacturer. The site never has to claim a usable capacity it cannot verify.
Vehicle limit, charger limit, supply limit
Three separate ceilings decide how fast a car actually charges, and the lowest one wins:
- What the car's onboard charger accepts — 11 kW for this model
- What the wallbox or charge point is rated at
- What the building's electrical supply can deliver
Quoting the first as if it were the answer is how specification sheets mislead people. The BYD Atto 3 accepts 11 kW, but that needs a three-phase supply, and most homes in the United Kingdom are single-phase. On a normal domestic supply the car charges at up to 7 kW, and the site defaults to that.
Peak power is not average power
Rapid charging is quoted as a peak: 88 kW for this car. That figure is held briefly at low state of charge and tapers as the battery fills. BYD publishes one verified interval, 30–80% in 29 minutes verified, which is an average of about 62.6 kW.
Only that interval is presented as fact. Any other range is scaled from it and labelled estimated, because publishing a full charge curve the manufacturer never released would be inventing data.
Time-of-use tariffs are modelled minute by minute
An off-peak tariff only applies during its window. Intelligent Octopus Goruns from 23:30 to 05:30, which is 6 hours, and at 7 kW that window can deliver about 42 kWh. A full charge needs 67.9 kWh, so it does not fit.
The calculator simulates the session at one-minute resolution, bills each minute at whichever rate applies, and shows the overflow separately. Treating the whole session as off-peak understates a 10–80% charge by roughly a quarter.
Prices are dated, not averaged
Domestic prices use the Ofgem price cap in force on the date the page was built, currently £0.2611/kWh from 2026-07-01. Every published cap period is stored, so the site changes over automatically when a new one takes effect.
Public charging prices are attributed to a named operator and never presented as national averages, because they are not. Rates vary by operator, by site and in some cases by local authority.
The standing charge is excluded from charging costs. You pay it whether or not you plug in, so it is not part of the cost of a charge. It belongs in an annual running-cost model, where it will appear when that tool is published.
What is measured, and what is not
The specifications and prices above come from manufacturers and regulators. Separately, this site logs real data from one BYD Atto 3: every charging session, sampled journeys, and a battery capacity check every two months. That logged data is presented as its own thing and never blended into manufacturer figures without saying so.
Corrections
If a figure here is wrong, it should be fixed rather than defended. Errors are corrected in place and the change is noted at the foot of the affected page. Get in touch if you find one.