Measure How Much Energy a Charge Uses with a Plug-In Energy Meter

Why this happens

Vehicle displays show a percentage, not energy, and they rarely tell you how much you actually drew from the mains. A plug-in energy meter measures voltage, current, power and cumulative kWh at the wall without any wiring work. Repeated readings reveal your charging pattern, standby draw and gradual changes in the charger or battery.

What you need

Step by step

1. 1

   Choose a suitable meter

   Pick a plug-in energy meter rated for at least your charger's input current, with a socket that accepts your charger plug including its earth pin. It should display kWh, watts and elapsed time. Check for a recognised safety certification. Avoid meters that need an adapter for your charger plug.

2. 2

   Set it up at the wall

   Plug the meter straight into a sound, earthed wall socket, never into an extension board or multi-plug. Plug the charger into the meter. Reset the kWh and time readings according to the meter's instructions. Check the display shows mains voltage, which confirms it is working.

3. 3

   Check standby draw

   With the charger plugged in but not connected to the vehicle, note the watts reading. Many chargers draw a small amount even when idle. This is a good reason to unplug the charger from the wall when you are not charging.

4. 4

   Measure a full charge

   Note the vehicle's charge level, connect the charger to the vehicle as the manual describes and start charging. A few minutes in, note the watts reading; this is the charging power. Let the charge finish, then note total kWh, elapsed time and the final charge level. Photograph the meter display so you do not mistype the numbers.

   Tip: Glance at the watts reading near the end of the charge; it usually falls as the charger enters its finishing stage.

5. 5

   Work out what the numbers mean

   Divide the kWh used by the percentage of gauge gained to estimate the wall energy for a full charge. Compare charging power with the charger label: power drawn should be roughly the charger's output voltage multiplied by output current plus some loss. A charge that draws much less power than usual may be limited by temperature or a fault.

6. 6

   Log over several charges

   Record each charge in your battery log: date, start and end level, kWh, average charging watts and time. After several charges you will see what is normal for your vehicle, including how weather affects charging speed. Consistent numbers mean a healthy charger and battery.

7. 7

   Watch for changes

   Over months, watch for charges that take much longer for the same gain, a charging power that drops, or kWh per full charge falling noticeably, which can indicate lower usable capacity. Any sudden change is worth a check by the service centre. Bring your log; it helps them diagnose.

How to tell it worked

You have accurate kWh, power and time figures for each charge, a clear picture of what is normal for your vehicle and an early signal if charging behaviour changes.

Questions