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Average MPH on 13amp UK plug

arg

Supporting Member
Aug 22, 2012
1,793
1,749
Cambridge, UK
6-7mph is about right for Model S in the UK. Model X should be at least 5. If you were hearing 2mph, that may have been from the USA with 120V power (and even then would need other unfavourable conditions to get it that slow). Alternatively, it could have been someone who plugged in to a 13A socket and looked at the MPH on the display straight away, without leaving enough time for it to settle on the final figure. In general, the charging mph on the display is highly misleading and you are better to look at the power figure and make your own estimate of how that turns into miles.
 
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cizUK

Member
Mar 13, 2017
316
272
UK
13A * 230V = 2990W = 3kW
259 (NEDC) miles / 75kWh = 3.5 miles / kW
3kW * 3.5 miles/kW = 10.5 mph

237 (EPA) miles / 75kWh = 3.2 miles / kW
3*3.2=9.5 mph

213 (realistic) miles / 75kWh = 2.8 miles / kW
3*2.8=8.5 mph

Of course, you may have to account for losses, I don't know how significant it is.
Have I got the calculations right?

But Tesla's plug must only have a 10A fuse to get 2.3 kW
 
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arg

Supporting Member
Aug 22, 2012
1,793
1,749
Cambridge, UK
Have I got the calculations right?

The calculations are correct, but some of your assumptions are optimistic.

Although a "13A" plug, experience dictates that they aren't suited to continuous operation at that current, and manufacturers of EVSE (including Tesla with the UMC) have adopted 10A as the safe limit.

The NEDC range is hopelessly optimistic and no use for practical purposes. On the other hand, you are assuming that a "75" has 75kWh of usable battery, when in fact a portion of that capacity isn't available for use, so your "realistic" calculation is actually pessimistic. I use 333Wh/mile (or 3 miles per kWh) as a rule of thumb - it's an easy figure to work with, and actually matches very closely to my lifetime average shown in the car. I have the displays in the car set to "typical" rather than "Rated"(=NEDC), and I believe the "typical" calculation uses a slightly lower figure, about 300.

There is an overhead on charging - a (quite small) percentage overhead on the power conversion, but also a fixed overhead for running all the computers/cooling systems/etc that have to be powered up to enable any kind of charging. As a round number, deducting 1A for this (230W) is about right.

So:

Actual current drawn from 13A socket: 10A
Deduct 1A for fixed overhead: 9A
Convert to watts at 230V: 9 * 230 = 2070W
Convert to MPH at 333Wh/mile = 2070/333 = 6.2MPH
Convert to MPH at 300Wh/mile = 6.9MPH
 
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cizUK

Member
Mar 13, 2017
316
272
UK
The calculations are correct, but some of your assumptions are optimistic.

Although a "13A" plug, experience dictates that they aren't suited to continuous operation at that current, and manufacturers of EVSE (including Tesla with the UMC) have adopted 10A as the safe limit.

The NEDC range is hopelessly optimistic and no use for practical purposes. On the other hand, you are assuming that a "75" has 75kWh of usable battery, when in fact a portion of that capacity isn't available for use, so your "realistic" calculation is actually pessimistic. I use 333Wh/mile (or 3 miles per kWh) as a rule of thumb - it's an easy figure to work with, and actually matches very closely to my lifetime average shown in the car. I have the displays in the car set to "typical" rather than "Rated"(=NEDC), and I believe the "typical" calculation uses a slightly lower figure, about 300.

There is an overhead on charging - a (quite small) percentage overhead on the power conversion, but also a fixed overhead for running all the computers/cooling systems/etc that have to be powered up to enable any kind of charging. As a round number, deducting 1A for this (230W) is about right.

So:

Actual current drawn from 13A socket: 10A
Deduct 1A for fixed overhead: 9A
Convert to watts at 230V: 9 * 230 = 2070W
Convert to MPH at 333Wh/mile = 2070/333 = 6.2MPH
Convert to MPH at 300Wh/mile = 6.9MPH
Yes, that makes sense, although Tesla still says 2.3kW when you are saying 2.07kW
Anyway, it all gives very similar figures
 

arg

Supporting Member
Aug 22, 2012
1,793
1,749
Cambridge, UK
Tesla still says 2.3kW when you are saying 2.07kW

They are quoting the input power (as will be displayed in the car) without deducting for overheads - so 2.3kW will be drawn from the mains, but only about 2.07kW of it ends up in the battery (and those figures have too many digits of precision!).
 
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