The cost of charging an electric car at home is mainly the amount of electricity drawn from your home multiplied by the price you pay per kilowatt-hour (kWh). Your car’s energy use, charging losses, electricity plan and charging time all affect the result, so there is no single price that applies to every driver. To estimate your own cost, start with your recent driving distance and the vehicle’s real-world energy consumption, then use the relevant rate from your power plan. The worked example below shows how the calculation works; its figures are illustrative, not a guide to current New Zealand electricity prices.
For a quick estimate, use this formula:
Charging cost = distance driven ÷ 100 × vehicle consumption ÷ charging efficiency × electricity rate
Use distance in kilometres, consumption in kWh per 100 km, and the electricity rate in dollars per kWh. Charging efficiency accounts for energy drawn from the home that does not end up stored in the battery. If your charger or a separate home energy monitor reports the electricity delivered from the grid, use that reading instead and do not apply a second allowance for charging losses.
For example, suppose a car uses 18 kWh per 100 km at the battery, and you drive 1,000 km. That represents 180 kWh for driving. If, purely for illustration, charging efficiency is 90%, the home supplies about 200 kWh. At an assumed rate of 30 cents per kWh, the charging cost would be $60 for that distance. These assumptions are not a typical vehicle rating or a current tariff; replace them with your own figures.
| Illustrative rate | Grid electricity for 1,000 km | Estimated charging cost | Cost per kilometre |
|---|---|---|---|
| 20 cents per kWh | About 200 kWh | $40 | 4 cents |
| 30 cents per kWh | About 200 kWh | $60 | 6 cents |
| 40 cents per kWh | About 200 kWh | $80 | 8 cents |
This comparison holds the car, distance and charging efficiency constant, changing only the electricity rate. To estimate a monthly bill impact, use your usual monthly distance rather than 1,000 km. If you charge at home only part of the time, calculate the energy for that share rather than treating all your driving as home charging.
Check the vehicle’s trip computer, owner information or app for consumption in kWh per 100 km. A figure from your own driving is usually more useful for budgeting than a rating from a test cycle, because speed, hills, traffic, weather, heating or cooling, tyre condition and vehicle load can change energy use. Track consumption across several weeks if a single trip seems unusually high or low.
Make sure you know what the figure represents. A car’s display may report energy used for driving from the battery, while a home charger or electricity monitor may show energy drawn from the grid. The first needs an allowance for charging losses; the second already includes them. Applying both would overstate the cost.
Battery capacity is not the same as the cost of every charging session. The cost depends on how much energy you replenish, which varies with your starting charge and how far you have driven. If you want to estimate a particular session, multiply the energy added to the battery by the reciprocal of charging efficiency, then multiply by the applicable electricity rate.
For example, charging from a partially depleted battery to a chosen daily level will generally use less electricity than replenishing the full usable capacity. The car’s battery size matters most when estimating a large recharge; your regular distance and consumption matter more for routine running costs.
Use the per-kWh rate on your own New Zealand electricity plan, not an advertised rate from another provider or a general estimate. If your plan has time-of-use rates, identify the rate that applies during the hours the car is actually charging. If a session crosses rate periods, calculate the energy used in each period separately, if your charger or retailer provides that information.
With a single flat rate, charging at night does not automatically make each kWh cheaper. With a lower off-peak rate, shifting charging into that period can reduce the cost, but compare the whole plan rather than looking at the EV rate alone. A different plan may have other rates or a different daily fixed charge, affecting the household bill beyond the car.
Start with the usage rates and time windows shown on your bill or in your electricity retailer’s account. Then check whether the rate is available to your household and whether it applies to the hours you can realistically charge. A discounted overnight period is useful only if your charging schedule fits it and the overall plan works for your household’s other electricity use.
Separate daily fixed charges are not usually calculated per kWh of EV charging. They still appear on the household bill, however, and could change if you switch plans. For a fair comparison, consider the likely annual charging use as well as the rates and fixed charges for the entire home. A lower rate for a small share of consumption may not offset higher costs elsewhere on the plan.
Compare plans using your own consumption data where possible. Estimate how much of your charging would fall into each rate period, multiply those amounts by the corresponding rates, then add any differences in fixed charges and other household usage. Check plan conditions and the retailer’s current rate schedule before changing, since rates and terms can vary.
If your home has solar panels, charging while they are generating can reduce the amount of electricity you need to buy from the grid. The value depends on how much solar power is available at the time, how much the car can take, and whether the energy would otherwise have been exported or used elsewhere in the home.
Solar electricity is not necessarily cost-free for this calculation. If surplus power could earn an export credit, using it to charge the car means giving up that credit. If the panels are producing more than the home can use and there is no valuable alternative use, charging from that surplus may have a lower additional cost. Consider the export terms on your plan and avoid assuming that all charging can be supplied by solar, especially if the car is usually parked at home overnight.
A home battery adds another layer: account for its charging and discharging losses and consider the cost of the stored energy. For a practical estimate, compare the extra grid electricity used by the home with the EV charging period, or use monitoring data that separates solar, battery and grid supply.
A higher-powered home charger mainly changes how quickly the car can charge; it does not automatically reduce the energy needed for a given journey. Charger choice can affect installation cost and charging convenience, while the vehicle, electricity rate and energy supplied determine most of the running-cost calculation. Ask a qualified electrician to assess the installation, and follow the car and charger manufacturers’ instructions for home charging.
The calculation above estimates electricity consumption, not the full cost of setting up home charging. A dedicated charger, electrical work or changes to a home’s electrical installation can involve separate costs. Those are generally purchase or installation expenses rather than per-kWh charges, so include them separately if you are comparing the total cost of EV ownership or deciding whether to install equipment.
Likewise, the total household bill may include charges that are not directly caused by the car. To isolate the EV’s effect, compare the bill’s energy usage before and after home charging, allowing for changes in other household consumption and the electricity plan. A charger’s energy reading can help distinguish vehicle charging from the rest of the home’s use.
Not necessarily. The result depends on your home electricity rate and the price and fee structure of the public charger you use. Compare the cost per kWh and any session or parking-related charges, and make sure you are comparing the same amount of energy.
No. A larger battery can hold more energy, but a session’s electricity cost depends on how much charge you add and your rate. If two cars need the same grid electricity for a particular trip, their battery capacities alone do not determine that trip’s charging cost.
Use the car’s usable battery capacity and the percentage points added to estimate the energy stored. Divide that amount by charging efficiency to estimate grid electricity, then multiply by the rate. Treat the result as an estimate unless you have a charger or meter reading for the session.
Only if your electricity plan charges less during the hours when the car is charging, or if another relevant arrangement applies. On a flat-rate plan, the time of day by itself does not change the per-kWh price. Check the plan’s time windows and any conditions before setting a schedule.
The estimate may use a different driving distance, consumption figure, charging-loss assumption or electricity rate from your real use. Weather, speed, terrain and accessories can change the car’s energy consumption, while a time-of-use plan can change the rate during a session. Compare the estimate with charger or household monitoring data and update the inputs.
For a realistic estimate of the cost of charging an electric car at home, use your own driving distance, the car’s observed energy use and the rate that applies during charging. Treat battery capacity as a guide to how much energy a session can add, not as a fixed charge cost. If you are considering a new electricity plan, compare its effect on the whole household bill as well as the EV.
