Why this happens
The headline price per unit on your bill is not the whole story: domestic tariffs often use slabs, fixed charges and taxes, and the charger loses some energy as heat. Measuring the energy you actually draw from the wall and dividing by the distance you ride gives a true running cost that already includes charger losses and your riding style.
What you need
Tools
- Plug-in energy meter rated for your charger
- Calculator or spreadsheet
- Odometer or trip meter
Materials
- Your latest electricity bill
- Your battery log (optional)
Before you start
- Use an energy meter rated above your charger's current and plug it directly into a wall socket.
- Do not tamper with or try to read inside your home's electricity meter; that belongs to the electricity company.
- Do not open the charger to fit measuring equipment.
Step by step
- 1
Find your real cost per unit
On your electricity bill, find the tariff slabs and which slab your extra EV units will fall into, since additional consumption is usually charged at the highest slab you reach. Add the per-unit taxes or duties shown on the bill. If that is complicated, divide the total energy charges by the units consumed to get an average cost per unit, but note it will slightly underestimate the marginal cost.
- 2
Measure the energy for a few charges
Plug the energy meter into the wall socket, the charger into the meter, and reset the meter. Charge as normal and note the kWh at the end. Do this for at least three or four charges to smooth out variation. One unit on your bill is one kWh.
- 3
Record the distance for the same period
Note the odometer reading at the first measured charge and again at the last. The distance between them is the distance those charges powered. Make sure every charge in that period went through the energy meter; a missed charge will make your figure look better than it really is.
Tip: Measure over a typical week of riding rather than one unusual trip.
- 4
Calculate Wh per km
Add up the kWh from all measured charges, multiply by 1,000 to get Wh, and divide by the distance in km. This is your wall-to-wheel energy use, which includes charger losses. Write it down; it is useful for range planning as well as cost.
- 5
Calculate cost per km
Multiply your total kWh by your cost per unit to get the money spent, then divide by the distance. Or multiply your Wh per km by the cost per unit and divide by 1,000. For example, at an illustrative 30 Wh per km, every 100 km uses 3 kWh, so 100 km costs three times your per-unit rate. Use your own measured figure and tariff.
- 6
Compare fairly
To compare with petrol, use your old vehicle's real kilometres per litre and the current fuel price to get petrol cost per km. For public charging, use the station's per-kWh rate plus any session or parking fee. Remember that tyres, servicing and eventual battery replacement are separate costs that a per-km energy figure does not include.
- 7
Recalculate when things change
Repeat the calculation when your tariff changes, when seasons change your riding or range, or after you change tyres or riding habits. Keep the figures in your battery log so you can see how your running cost changes over time.
How to tell it worked
You have a measured Wh per km figure and a cost per km based on your own tariff, which you can compare with petrol or public charging and track over time.
Stop and call a technician if
- The energy meter, plug or socket gets warm during charging.
- Energy per charge suddenly rises a lot for the same distance, which may indicate a charger or battery problem.
Questions
Why is my measured energy higher than the battery size suggests?
The charger and the charging process lose some energy as heat, so the wall energy is higher than what the battery stores. That is normal and is part of your real running cost.
Can I just use my electricity bill's total rise?
Bill totals include other household changes such as fans and AC in summer, so they are less accurate. An energy meter on the charger gives a direct figure.