Energy stored is not energy bought

A battery’s change in state of charge gives energy stored: capacity × (target percent − starting percent) ÷ 100. The meter supplies more energy because a charger and battery lose some energy as heat and auxiliary use. Grid energy is stored energy divided by charging efficiency. This calculator applies the same grid energy to a home tariff and a public-station tariff, so the two costs are comparable. It does not assume that a battery should routinely be charged to 100%; the target is an input chosen for a particular trip or manufacturer guidance.

Home-versus-public example

A 64 kWh battery rises from 20% to 80%, so it stores 64 × 0.60 = 38.4 kWh. At 90% efficiency, the meter must deliver 38.4 ÷ 0.90 = 42.67 kWh. With home electricity at 0.28 per kWh, the session costs 11.95. At a public price of 0.62 per kWh, it costs 26.45. The difference, 14.50, is the price of the same estimated grid energy at two tariffs. It is not a claim that every public charge has that price or that charging speed is identical.

Tariffs need the full price

Enter the price actually charged per kWh, including VAT or tax where that is how bills are quoted. Some networks add connection, parking, membership, idle, or time-based fees. Those charges are outside a pure energy-price formula and should be added separately to compare a real receipt. A home bill can have time-of-use periods, standing charges, solar export opportunity cost, or demand charges. If an off-peak tariff applies only overnight, do not compare it with a daytime public price without also considering whether the vehicle can be connected during that period.

Efficiency is a charger-and-battery assumption

An efficiency of 90% means 10% of meter energy does not end up as stored battery energy. Actual losses vary with AC versus DC charging, power level, battery temperature, cabin conditioning, cable and onboard-charger design, and low-current charging. Use a measured session from the vehicle or smart meter when available. Setting efficiency to 100% makes the calculation look tidy but understates purchased energy. Conversely, an extremely low efficiency should prompt a check of what the meter and vehicle each counted before it is used for a cost decision.

State of charge is a percentage of usable capacity

The capacity field should represent the usable energy relevant to the displayed state of charge, not necessarily the headline gross battery capacity. Manufacturers may reserve a buffer at the top or bottom, and an ageing battery can store less energy than when new. The calculation is still valid with a current usable estimate because it needs only energy change. It cannot estimate range directly: consumption changes with speed, temperature, tyres, terrain, wind, heating, payload, and driving style, so a kWh cost page should not pretend to forecast kilometres.

Home charging has practical constraints

A local tariff comparison does not establish whether a socket, circuit, wallbox, cable, or installation is suitable. Electrical work, circuit loading, earthing, protective devices, ventilation, and local rules need a qualified assessment. Charging power affects time, but not necessarily the energy cost per kWh; an hourly fee changes that relationship. Avoid extension leads or improvised connections where prohibited by the vehicle or equipment instructions. The calculator answers a pricing question after a safe charging method has already been selected.

Common input errors

Do not type 80 and 20 in the reverse order: the target state of charge must exceed the starting state for a charging session. Enter tariff currency consistently; 28 cents must become 0.28 if the result is expected in base currency units. Do not confuse charger power in kW with energy in kWh. A 7 kW wallbox describes a rate, while 42.67 kWh in the example is the accumulated energy bought. If a receipt seems high, first compare its kWh, fees, and efficiency assumption rather than guessing that battery capacity changed overnight.

How this differs from the existing EV charging page

The existing EV Charging Cost Calculator estimates one session from battery capacity, percentage added, one electricity price, and efficiency. This tariff-comparison page requires starting and target charge and presents both a home and public-station cost for the same energy. It does not compare tyre sizes or fuel consumption. Its point is to make the tariff difference visible while retaining the energy-loss assumption, not to recommend a network or tell a driver when to charge.

Check a real charging record

Before relying on a budget, enter a recent session and compare the calculated grid kWh with a wallbox, vehicle, or station receipt. Record whether the tariff includes extra fees and whether the session occurred in the intended rate period. Recalculate when energy prices or driving needs change. All inputs remain in the browser, which is useful when comparing private bills, but the local calculation cannot validate a tariff contract, station availability, battery condition, or electrical installation.

One last practical check

For a monthly comparison, multiply a representative session cost by the number of sessions only after checking how many kilowatt-hours are actually charged away from home. Public charging may be worth more on a long trip because it saves time, while home charging may be constrained by parking or off-peak windows. Keep energy cost separate from vehicle depreciation, home charger installation, parking, and subscriptions; combining them into one unexplained number makes later tariff changes impossible to audit. A meter reading over several sessions is the best way to refine the efficiency input for a particular vehicle and charger.

EV Home and Public Charging Cost Comparison