Efficient Charging

JuiceBox Pro 40 Installed

Earlier this year we talked about the advantages that Porsche get from doubling the voltage of their EV platform from the typical 400V used in most EVs today, to a whopping 800V. This is similar to reasons that electricity companies use higher voltages (often into hundreds of kilovolts) for their transmission lines, and then use transformers near your house to step it down. The reason is simple: the energy lost in the cables is proportional to the square of the current. Doubling the voltage, halves the current, which quarters the loss due to the resistance in the cables.

That got me thinking about how I charge my EV at home. I can’t double the voltage obviously, but I do have some options on the current our JuiceBox Pro 40 EVSE will deliver to the car. Since most of our charging is overnight anyway, slowing down the charge is not a big deal. If it takes 4 hours instead of 2, it will still be finished long before I need it the next morning.

But does it make a difference?

My Approach

What I know about my setup, and what I don’t dictated the approach I took a little:

  • I do not know exactly how long the cable is, though I do know that it is AWG 8 aluminium cable (which I discovered has a resistance of ~1.03Ω per 1000 feet). This was installed by the builder to power an air conditioning compressor, that we don’t have.
  • I do know that the steady state voltage (when not charging) is around 244V – the JuiceBox reports this in its dashboard
  • I can set the charging current up as high as 32A (the max allowable continuous current on AWG-8 aluminium wiring).
  • The JuiceBox reports the line voltage and current while charging, allowing me to calculate the effective resistance of the wire, and therefore the power lost in the cabling
State Current Voltage Voltage Drop
Idle 0 A 244 V
Charging 16 A 237 V 7 V
Charging 32 A 231 V 13 V

Now for Some Math!

We’re going to simplify this a bit and just use resistance (we should really use impedance). Given a voltage drop of 7V at a current of 16A, we can use V = I ⋅ R to calculate the resistance of the wires. That gives us 0.44Ω. Looking at the 32 A session, we get a resistance of 0.41Ω. Those numbers seem reasonably consistent given the possibility of temperature differences (ambient as well as from the heating effect of the power loss itself). We’ll use the average of these for our calculations, 0.425Ω.

To calculate the power loss in the cabling, now we know its resistance, we use P = I² ⋅ R. For the 16A charging session, the power lost is 109W. For the 32A session, it is 435W. So, for the 32A setting, the power loss is four times the loss at 16A (as expected), but the 16A session takes twice as long to charge the car. Let’s use a typical charge time for our EV, 2 hours at 32 A and 4 hours at 16 A.

Factoring in the time, the energy lost was 0.87 kWh at 32 A and 0.436 kWh in the 16 A session.

Just How Long Is That Cable?

Our electrician, when he came to fit the NEMA outlet for our EVSE, noted that the main panel and the garage are in diagonally opposite corners of the house, which makes the cable lengths longer than ideal! But just how long is it?

Above, we noted that resistance of aluminium wire that the builder fitted was 1.03Ω per 1000 feet. Our cable resistance is 0.425Ω, so that is just over 400 feet of aluminium wire. There are two wires we need to account for, so that means the cable length between the panel and our EVSE is around 200 feet long.

Using copper wiring would have made a big difference too. The resistance of 1000 feet of AWG 8 copper wire is just 0.628Ω according to this handy cable resistance calculator site. That is 40% less resistance.


It looks like the best recommendations are:

  • Make sure you keep the cabling between your main panel & EVSE as short as possible.
  • Try to use copper not aluminium (there is a cost difference there).
  • Charge at the lowest current setting that still meets your time requirements

Finally, bear in mind that the cost of the loss, even in our long, aluminium cable, is under one kWh per session, so less than 20¢ (based on our cost per kWh here in Alameda).

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