Speak EV - Electric Car Forums banner
1 - 17 of 17 Posts

·
Registered
Joined
·
161 Posts
Discussion Starter · #1 ·
Can anyone tell me how I can calculate my home charging costs based on knowing my electricity unit cost for the actual time my car is plugged in and charging?
I've asked this before and someone told me a way of doing it, but it gave me a very strange result.
 

·
Registered
Kia E Niro 4
Joined
·
318 Posts
If you have a charger @2.4Kw ie a granny charger for each hour it will take 2.4 Kw so if electricity rate is say 15p a Kw then 2.4X 15p for each hour you're connected, adjust for other chargers rates or time. The car will only receive 2.2Kw at least my E-Niro does.
 

·
Registered
Hyundai Ioniq 28
Joined
·
8,037 Posts
It's just the pence per kWh that you are tariff charges multiplied by the number of kWhs delivered to the car.

If your wall unit is a 32 amp model it will deliver 6.6 kWs per hour. ( If it's a 16 amp charger that will deliver 3.3 kWs in every hour)

If your system reports how many kWhs are delivered that is easy. If not, you can still calculate approximately how many units have been delivered by timing the session.

For instance - a 32 amp wall charger that is plugged into the car for 2.5 hours will have delivered 2.5 x 6.6 = 16.5 kWhs. Then, if your price per unit is 15p that will have cost 16.5 x 15 = £2.48.

Obviously, this only works if the wall unit is actually delivering for the time recorded. If the car is plugged in overnight it will have stopped delivering in the early hours and taking that 8 or 9 hours time is meaningless.

All very approximate when using this method but near enough to make no real difference. In any case, it is also easy to multiply the size of your car battery by the price per unit to work out what a full zero to full charge would cost in theory. And then just divide that by the % increase in charge shown by the car.

Example :- A car with a 50kWh battery plugs in at a reported charge of 25% and unplugs at 75%. That is an increase of 50%, which is half of the cars battery - or 25 kWhs. Thus 25 x 15p = £3.75 for a half charge session. Simple maths required in other situations.
 

·
Registered
Kia E Niro 4
Joined
·
318 Posts
Or read your electric meter before and after charging to see how much has been used, may be affected by what else you have running in the house. I know I use around 7Kw a day so anything above that is down to charging the car.
 

·
Registered
Joined
·
1,463 Posts
For instance - a 32 amp wall charger that is plugged into the car for 2.5 hours will have delivered 2.5 x 6.6 = 16.5 kWhs. Then, if your price per unit is 15p that will have cost 16.5 x 15 = £2.48.

Obviously, this only works if the wall unit is actually delivering for the time recorded. If the car is plugged in overnight it will have stopped delivering in the early hours and taking that 8 or 9 hours time is meaningless.
It's not quite that simple !

The wall box would be capable of delivering up to (your household voltage ) x (the fuse rating of its supply). Although the nominal voltage of U.K. plugs is 230v, most people still find around 240v (the old standard value) but individual houses might be as low as 218V (5% below nominal 230) or as high as 253 (10% above nominal). So a 32A fuse might permit as much as 8kW or as little as 7kW.

However, the important thing is what the charger in your car will accept. e.g. an early Leaf would be capped at 3.3kW, most modern ones at 6.6kW, several other models might well accept even more. Then of course you also need to allow for the present state of charge of your battery : if it's very low, car will ask for the max rating of its onboard charger, if it's almost full charging rate drops to near zero whilst cells are balanced then stops.

Thereafter it gets a little easier. Cost will be :-
(actual charging rate in kW) x (length of time in hours) x (cost per unit - which you may find quoted with or without vat)
although it could get more complicated if you're on any sort of time dependent tariff.

The 'good news' is that whatever it costs to charge a car at home will be a lot less than driving your smelly ICE powered car into a petrol station !
 

·
Registered
Joined
·
367 Posts
Plus charging losses :) so measure at the meter not the car.

but for rough calculation and to check your ‘Strange’ results, a simple kWh added x unit price should give you a close enough number

eg adding 32kwh (50% of an e-Niro) at 15p per unit is roughly £4.80

adding 28kwh (to take best advantage of 4 hours octopus go) -£1.40
 

·
Registered
Hyundai Ioniq 28
Joined
·
8,037 Posts
It's not quite that simple
I know.

Which is why I said " All very approximate when using this method but near enough to make no real difference ".

I got the impression that the OP just wanted a basic guide on how to calculate the cost. Not to micromanage it to the nearest five decimal places like an obsessive anorak.
 

·
Registered
Joined
·
1,463 Posts
I got the impression that the OP just wanted a basic guide on how to calculate the cost. Not to micromanage it to the nearest five decimal places like an obsessive anorak.
Which of course is why I added my final sentence :devilish:
 

·
Registered
2021 Citroën ë-C4 Shine Plus
Joined
·
179 Posts
My home charge point was installed with a government grant a fair few years ago, when part of the grant conditions was that the supply to the charge point was metered. This means there was a dedicated meter installed between the house supply and the charge point itself.

Do you have anything similar?
 

·
Registered
Nissan LEAF30
Joined
·
7,111 Posts
My home charge point was installed with a government grant a fair few years ago, when part of the grant conditions was that the supply to the charge point was metered
The later grant requires that the charge point is "smart" and records that data for the owner to access, so either way you should have it.
Plus charging losses :) so measure at the meter not the car.
Which improves the accuracy but makes it hard to apportion the use to the car rather than other loads.
 

·
Registered
'18 Zoe ZE 40 R110 + '21 VW ID.4 1st
Joined
·
364 Posts
If your wall unit is a 32 amp model it will deliver 6.6 kWs per hour. ( If it's a 16 amp charger that will deliver 3.3 kWs in every hour)
That depends on the install and the location, I average 7.4-7.5 kW on my 32a Zappi, so I generally plan on 15 kWh every two hours when estimating costs.
 

·
Registered
Hyundai Ioniq 28
Joined
·
8,037 Posts
That depends on the install and the location, I average 7.4-7.5 kW on my 32a Zappi, so I generally plan on 15 kWh every two hours when estimating costs.
Most wall boxes are just switches to release AC power. The charger is in the car and that is what determines the max number of kWhs that can be transferred. But the systems will always default to the lowest common denominator in any case. And most cars can only accept up to 6.6 kWhs.

But, again, the OP was just asking for info on how to price up a charge session. A simple back of ciggy packet calc will do that. But some in here seek to nail that down to fractions of a penny. In real life even if a quick estimate is out by as much as a couple of kWhs that is still only 25p difference and not exactly worth getting all bent out of shape over.
 

·
Registered
'18 Zoe ZE 40 R110 + '21 VW ID.4 1st
Joined
·
364 Posts
Most wall boxes are just switches to release AC power. The charger is in the car and that is what determines the max number of kWhs that can be transferred. But the systems will always default to the lowest common denominator in any case. And most cars can only accept up to 6.6 kWhs.

But, again, the OP was just asking for info on how to price up a charge session. A simple back of ciggy packet calc will do that. But some in here seek to nail that down to fractions of a penny. In real life even if a quick estimate is out by as much as a couple of kWhs that is still only 25p difference and not exactly worth getting all bent out of shape over.
I agree that the cost difference isn't huge (though a 13% difference is not exactly "fractions of a penny"), but I thought it was only Leafs that are limited to 6.6 kW? I was under the impression most non-Leafs can take more than 7 kW AC. (I could definitely be wrong, my two EV's can take 22 kW (Zoe) and 11 kW (ID.4))
 

·
Registered
Joined
·
1,463 Posts
. . . but I thought it was only Leafs that are limited to 6.6 kW? I was under the impression most non-Leafs can take more than 7 kW AC. (I could definitely be wrong, my two EV's can take 22 kW (Zoe) and 11 kW (ID.4))
Not so sure that it's "most EVs" that will take more than 7kW but agree that there are many that do.

For a Zoe to take 22kW, I believe you need a 3ph supply - otherwise it would be just over 7kW; no idea if the ID4 also needs 3ph.

N.B. @Simon Thornton didn't actually specify what sort of charger he had in mind.
 

·
Registered
Hyundai Ioniq 28
Joined
·
8,037 Posts
... but I thought it was only Leafs that are limited to 6.6 kW?
You may be right. But my point was that debating finer details such as how many kWhs are registered on your tariff meter against how many actually arrive in a car's battery is hardly measurable when costing a charge session individually. It may be that £3.45 is charged to you and that only £3.25's worth is available to run the car. But the bottom line is that it has still cost you £3.45 and understanding the value of losses in transfer and details such as how long it took due to battery temperature and % SOC are academic, and of little interest to the OP who simply wanted to know how to do the maths.

My Podpoint app tells me that. Without it I use the increase in % SOC over a session and a few button presses on a calculator. And if that is 10p out I neither know nor care. And I suspect that the OP is only looking for that level of accuracy too.
 

·
Registered
'18 Zoe ZE 40 R110 + '21 VW ID.4 1st
Joined
·
364 Posts
Not so sure that it's "most EVs" that will take more than 7kW but agree that there are many that do.

For a Zoe to take 22kW, I believe you need a 3ph supply - otherwise it would be just over 7kW; no idea if the ID4 also needs 3ph.

N.B. @Simon Thornton didn't actually specify what sort of charger he had in mind.
ID.4 can charge up to 7.2 kW on single-phase AC or 11 kW on three-phase AC:

143957


I do wonder if that 7.2 kW figure is an estimate, as I regularly see my Zappi putting 7.5 kW into the car. I've even seen it hit 7.8 kW a few times!
 

·
Registered
'18 Zoe ZE 40 R110 + '21 VW ID.4 1st
Joined
·
364 Posts
You may be right. But my point was that debating finer details such as how many kWhs are registered on your tariff meter against how may actually arrive in a car's battery is hardly measurable when costing a charge session individually. It may be that £3.45 is charged to you and that only £3.25's worth is available to run the car. But the bottom line is that it has still cost you £3.45 and understanding the value of losses in transfer and details such as how long it took due to battery temperature and % SOC are academic, and of little interest to the OP who simply wanted to know how to do the maths.

My Podpoint app tells me that. Without it I use the increase in % SOC over a session and a few button presses on a calculator. And if that is 10p out I neither know nor care. And I suspect that the OP is only looking for that level of accuracy too.
I 100% agree with everything you said. I personally don't worry about efficiency losses when it comes to charging my ID.4 at 7.5 kW, though I do when I consider charging my Zoe on a granny cable - but the Zoe is notoriously inefficient when it comes to slow charging: I've seen as low as 80% of the energy pull going into the batteries while granny charging. (I shudder to think how low that number goes when I use my Zappi on "Eco" mode, which only puts 1.4 kW in depending on solar generation!)
 
1 - 17 of 17 Posts
Top