Help calculating energy usage to charge car

[Shengis]

Day two of MG4 ownership. So far so good, except for the double bulge!

I’ve only got a granny charger available meantime until I get a proper charger installed early Jan.

Yesterday I charged the car from 70% up to 100% through the night.

I also got a smart meter installed last night so that I can move to an EV tariff for cheaper overnight charging.

As a result with the new meter starting at zero, I’ve noticed that between 18:00 yesterday and now, we’ve used 37 units. This sounds a lot as think we average about 10 kWh a day based on previous months.

By my calculations if I charged the 51 kWh battery from 70 to 100 that should be about 15-16 kWh, yes? I just want to check if this is correct.

Also, the car stopped being charged at 100% obviously, however the plug was still switched on at the wall till I was up in the morning. Would this account for the extra high energy usage or would the granny charger stop using electric when the car was full?

 

[tsedge]

Day two of MG4 ownership. So far so good, except for the double bulge!

I’ve only got a granny charger available meantime until I get a proper charger installed early Jan.

Yesterday I charged the car from 70% up to 100% through the night.

I also got a smart meter installed last night so that I can move to an EV tariff for cheaper overnight charging.

As a result with the new meter starting at zero, I’ve noticed that between 18:00 yesterday and now, we’ve used 37 units. This sounds a lot as think we average about 10 kWh a day based on previous months.

By my calculations if I charged the 51 kWh battery from 70 to 100 that should be about 15-16 kWh, yes? I just want to check if this is correct.

Also, the car stopped being charged at 100% obviously, however the plug was still switched on at the wall till I was up in the morning. Would this account for the extra high energy usage or would the granny charger stop using electric when the car was full?

Don’t forget to account for losses, charging is only something like 90% efficient. So I’d expect something like 17-18kWh for that charge.

That suggests with your daily usage that you still used maybe 9kWh than you expected.

No more electricity will be drawn when the car is full, it has nowhere to go.

Were there other appliances on that were unusual? How confident are you in 10kWh per day? That’s not a lot for a house of a decent size or a family (there are 2 of us and 2 cats and we average 17kWh per day).

 

[siteguru]

I have a smart wall box with an app that tells me kWh usage for each charge session. Bear in mind that this is total supplied by the wall box; not the total stored in the car. (There are charging losses, and these are proportionately higher from a low power "granny" charger than from a 7kW wall box ... there are fixed losses and variable losses; fixed losses apply regardless of the power source, therefore these are a much higher percentage of total losses when using a "granny" charger).

Total efficiency from my 7kW wall box is 90%+ ... this means to store 50kWh in my car I'd actually use about 55.5kWh of electricity.
Total efficiency from a "granny" charger may only be, for example, 70% ... so 30% into the car (15.4kWh) may actually be 21.7kWh from your supply

 

[Shengis]

Don’t forget to account for losses, charging is only something like 90% efficient. So I’d expect something like 17-18kWh for that charge.

That suggests with your daily usage that you still used maybe 9kWh than you expected.

No more electricity will be drawn when the car is full, it has nowhere to go.

Were there other appliances on that were unusual? How confident are you in 10kWh per day? That’s not a lot for a house of a decent size or a family (there are 2 of us and 2 cats and we average 17kWh per day).

I’ll be honest, it’s not something I normally keep an eye on at all, however on the OVO Energy app it gives me my monthly usage which is typically between 290 and 330 kWh so with roughly 30 days in a month that’s where I’m getting the 10 a day from.

I hope I’m wrong because if this month’s bill is bigger my wife will murder me and bury me in the back garden. Since I promised her savings with this new car! ?

 

[tsedge]

I have a smart wall box with an app that tells me kWh usage for each charge session. Bear in mind that this is total supplied by the wall box; not the total stored in the car. (There are charging losses, and these are proportionately higher from a low power "granny" charger than from a 7kW wall box ... there are fixed losses and variable losses; fixed losses apply regardless of the power source, therefore these are a much higher percentage of total losses when using a "granny" charger).

Total efficiency from my 7kW wall box is 90%+ ... this means to store 50kWh in my car I'd actually use about 55.5kWh of electricity.
Total efficiency from a "granny" charger may only be, for example, 70% ... so 30% into the car (15.4kWh) may actually be 21.7kWh from your supply

Are granny chargers that inefficient? I did not know that.

 

[Shengis]

I have a smart wall box with an app that tells me kWh usage for each charge session. Bear in mind that this is total supplied by the wall box; not the total stored in the car. (There are charging losses, and these are proportionately higher from a low power "granny" charger than from a 7kW wall box ... there are fixed losses and variable losses; fixed losses apply regardless of the power source, therefore these are a much higher percentage of total losses when using a "granny" charger).

Total efficiency from my 7kW wall box is 90%+ ... this means to store 50kWh in my car I'd actually use about 55.5kWh of electricity.
Total efficiency from a "granny" charger may only be, for example, 70% ... so 30% into the car (15.4kWh) may actually be 21.7kWh from your supply

Yep, I see what you’re saying. The quicker I get on an EV tariff with off peak rates the better then!!

 

[siteguru]

Of course your leccy bill will go up ... but your 3rd party fuel bill (petrol/diesel) will come down. The key is that the leccy increase is (hopefully) much less than the fuel decrease.

 

[tsedge]

I’ll be honest, it’s not something I normally keep an eye on at all, however on the OVO Energy app it gives me my monthly usage which is typically between 290 and 330 kWh so with roughly 30 days in a month that’s where I’m getting the 10 a day from.

I hope I’m wrong because if this month’s bill is bigger my wife will murder me and bury me in the back garden. Since I promised her savings with this new car! ?

If she does, can I have your car? ?

 

[siteguru]

Are granny chargers that inefficient? I did not know that.

This thread suggested ~80% efficiency ... my 70% was just a stab in the dark, but granny leads are less efficient than wall boxes.

Charging efficiency

I am charging my SE SR car using granny cable at home. My electricity use shows 2.3kW being used. My iSmart app shows 1.87kW charging power. This is a loss of 19%. My question is if you charge using the granny chargers what does your iSmart show as charging power and also what is your efficiency...

www.mgevs.com

 

[meicd]

Remember it may have balanced the batteries as well, I’m not sure how much energy that would use but if it was needed then worth the effort.

 

[Shengis]

Of course your leccy bill will go up ... but your 3rd party fuel bill (petrol/diesel) will come down. The key is that the leccy increase is (hopefully) much less than the fuel decrease.

Yes I am aware of that. It was the main selling point when presenting my evidence it to the wife! ?

 

[Shengis]

If she does, can I have your car? ?

Haha no! If she gives me the murderous looks I’ll just replace her instead of the car!