How do you compute power?
It’s a simple formula:
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Amperage * Voltage = Watts
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Honorable mention: There is some minor loss of voltage over distance based on cable gauge but that goes beyond this post. Tables below will be simple math assuming zero resistance and zero voltage drop.
What about power storage?
This is pretty straight forward.
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the total energy in kilowatt-hours is the power in kilowatts multiplied by the time in hours.
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Go ahead and read the WikiPedia article.
And the formula to fill a battery is straight forward:
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kW * hours = kWh
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Example: If you have a 30kWh battery pack and you can charge at 6.6kW then it would take about 4.6 hours to charge from 0 to 100%.
Unfortunately this isn’t completely accurate. Electric vehicle batteries have a state of charge (or SoC). Once you plug it in it charges very fast from 0-80% and then the charge rate drops as it gets closer to 100%. Just like your cell phone.
What are normal usage rates?
| Car | wH/m | m/kWh |
|---|---|---|
| Model S | 275 | 3.64 |
| Model X | 316 | 3.16 |
| Model 3 | 242 | 4.13 |
| Bolt EV | 252 | 3.97 |
| 18 Volt | 347 | 2.88 |
| 17 Leaf | 280 | 3.57 |
| 18 Leaf | 267 | 3.75 |
From there you can calculate your time to charge based on your battery size. The table above would get unruly if I listed all the battery combinations.
Power to estimated miles per hour of charge
So here is a simple table of power outputs and I have included some samples of usage to miles per hour of charge. These are only estimates. Every vehicle uses different sized inverters and accept power at different rates and loss.
The rates below use summer time watt-hour (Wh) of usage per mile (or the reverse math if vehicle reports things in miles/kWh).
See table above for estimated usage rates used in the calculations below.
| Amps | Volts | Power | Model S [1] | Model X [1] | Model 3 [2] | Bolt EV | 18 Volt [3] | 17 Leaf [4] | 18 Leaf [4] |
|---|---|---|---|---|---|---|---|---|---|
| 24 | 208 | 5kW | 18 | 16 | 21 | 20 | 10 | 18 | 18 |
| 24 | 240 | 5.8kW | 21 | 18 | 24 | 23 | 10 | 21 | 21 |
| 32 | 208 | 6.7kW | 24 | 21 | 27 | 26 | 10 | 24 | 24 |
| 32 | 240 | 7.7kW | 28 | 26 | 32 | 31 | 10 | 24 | 24 |
| 40 | 208 | 8.3kW | 30 | 26 | 34 | ||||
| 40 | 240 | 9.6kW | 34 | 30 | 40 | ||||
| 48 | 208 | 10kW | 36 | 32 | |||||
| 48 | 240 | 11.5kW | 41 | 36 | |||||
| 72 | 208 | 15kW | 54 | 47 | |||||
| 72 | 240 | 17.3kW | 62 | 55 |
[1] 10kW onboard charger standard, 17.3kW charger in dual charger (2016 and later) and earlier models of the Model S supported up to 20kW (80Ax240v)
[2] Limited to 7.7kW on standard battery and 9.6kW on long range option
[3] 3.6kW onboard charger Link
[4] 6.6kW onboard charger upgrade Link
Conclusion
I hope this helps fill in some gaps or at least gives you an idea of what to expect.
The difference in 208v vs 240v doesn’t make a huge difference in the kW of power calculation.
The above are estimates and some loss will occur.
For example, at my home, I have a HPWC fed with 100A service (which is 80A max usable) for 19.2kW. I only know a few people with a vehicle that can support this (dual charger older Model S). My Model S is from 2016 and supports 48A service but it’s only a 10kW charger inside so while my delivery is 48A@240V (11.5kW) my vehicle charges at the slightly lower 10kW rate (or ~36m/h). More than enough for my overnight charging needs.
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Edit: Fixed Model X from 350Wh/m to 316Wh/m as bad initial math
Edit: Fixed power calculation early in the table because of transposing