Should we get more home battery capacity?

[T1 Terry]

My own thoughts about a prolonged power cut are that although I can't seem to get the power from the car into my home battery, I could run an extension lead from the car directly into the house for most usage, and save the solar - going into the home battery - to run the central heating boiler, which can't be run directly from the car. This might not work in very dull weather in the middle of winter, but most days I get enough solar for it to work.

How much power does the central heating draw? Is your house battery 48vdc?

T1 Terry

 

[GaryMG4]

How much power does the central heating draw? Is your house battery 48vdc?

T1 Terry

Our oil boiler draws 100-150w when running, I suspect it may be an earthing issue.

 

[Rolfe]

The boiler draws about the same as Gary's does, I reckon. However it's hard-wired into the circuit and can't be plugged in to the VtL. I have an emergency power supply setup where, if the power goes off, the boiler continues to run from the home battery, and there is also a double power point in the fuse cupboard which also draws from the battery. This should be fine short-term so long as the battery isn't too low when the power cut happens.

The 9.5 kwh battery will fill up during the day from the solar, so in the summer it should be capable of powering the house indefinitely. However in winter if it's very dull there isn't enough solar to do that. At that point the extension lead from the car would substitute for the power point in the house, and all the battery (solar) would go to keeping the heat on.

Trip-hazard central, but it would work. Run the wifi all the time, TV, radio, computer, a table lamp or two as necessary, kettle, toaster and microwave for cooking. (I also have a one-ring portable induction hob.) Overnight plug in the fridge and freezer. The solar should at least keep the boiler going, whatever more, and if the car was getting low I could drive to somewhere with power and charge it. If the entire country for 50 miles in each direction is out for a prolonged period, we have more problems than just me.

 

[Everest]

The solution that I and others (e.g. @johnb80) have detailed on other threads - assuming you have standard low-voltage (nominal 48V) system is to charge / top up your home storage batteries via an adjustable telecoms-type power supply from V2L output of the MG. e.g. as discussed in the thread below. This decouples the vehicle from the house supply saving you having to add any extra wiring, earthing, RCD's etc.

Post in thread 'Connecting a V2L car to a house as a battery.'

Dec 2, 2025

Hi John. This sounds exactly like the setup I'm after. I haven't been able to find anyone that's actually tried this yet, so this is quite exciting. When you say 'feeding onto the wires', how have you done the connection? Paul

Hi Paul,
I connected to my DIY battery directly onto the DC wires (big black and big red) that go from the inverter to the battery. At this point the 48v DC is being fed into the inverter via the BMS (Battery Management System all batteries have). It's very easy to do and you could just purchase an extra battery cable if it's a commercial battery you have or...

• johnb80

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[Rolfe]

Would that that were possible.

 

[T1 Terry]

The boiler draws about the same as Gary's does, I reckon. However it's hard-wired into the circuit and can't be plugged in to the VtL. I have an emergency power supply setup where, if the power goes off, the boiler continues to run from the home battery, and there is also a double power point in the fuse cupboard which also draws from the battery. This should be fine short-term so long as the battery isn't too low when the power cut happens.

The 9.5 kwh battery will fill up during the day from the solar, so in the summer it should be capable of powering the house indefinitely. However in winter if it's very dull there isn't enough solar to do that. At that point the extension lead from the car would substitute for the power point in the house, and all the battery (solar) would go to keeping the heat on.

Trip-hazard central, but it would work. Run the wifi all the time, TV, radio, computer, a table lamp or two as necessary, kettle, toaster and microwave for cooking. (I also have a one-ring portable induction hob.) Overnight plug in the fridge and freezer. The solar should at least keep the boiler going, whatever more, and if the car was getting low I could drive to somewhere with power and charge it. If the entire country for 50 miles in each direction is out for a prolonged period, we have more problems than just me.

Rather than more battery, you need a V2X system that is a two DC circuit that plugs into the EV's DC port and the house solar and battery.
These systems can move the DC current in either direction from the EV battery:

1) charging it when there is excess energy from either cheap/free mains power and/or excess solar.
2) drawing from the EV battery when the load exceeds to solar and house battery capability.
This blends in with the inverter that produces clean AC power at whatever frequency and voltage is required to supply the house, plus send any excess back to the grid.

Here in Australia, we have an electrical energy market that connects all the east coast, yet to come up with a method to move that energy back and forth from the west coast without the losses exceeding the benefits ..... it's a long way from one side to the other .....
The benefit of this system is it puts a $$ value on the kwh you can buy or sell to the grid in any 5 min period Electricity market.
With the battery storage, you can decide when it might be worthwhile selling some of what you have stored to make a profit, or keep it for your own use, or even buy it when the market goes negative ..... they actually pay you to draw power ......

I have no idea how the energy market works over there, so I have no idea if it would be worth the investment, but over here, places with big solar arrays, actually earn quite good income from doing the energy trading ......

T1 Terry

 

[T1 Terry]

Would that that were possible.

If you have a 48vdc nom. house battery, you could simplify the V2X system down to a mains powered 48v battery charger and simply charge your 48vdc house battery from the V2L you already have for the MG4.
A step further, you could use a Victron Quatro inverter/charger and connect the MG4 V2L as well as the mains to it, and your house battery, and it will automatically switch the which ever supply you prioritise, moving down the order as availability dictates, while powering your house 24/7.

That is the type of system I am setting up at the moment on our motorhome, solar/battery/mains/V2L ... although in our case, the mains is via a big extension cord, so it can either plug into mains or the V2L as available, or simply run off the solar/battery and even charge the MG if there is enough power available ......

T1 Terry

 

[Ferretusmaximus]

I calculated my maximum daily power need (31kWh mid-February below zero degrees all day) to run the heat source air pump and everything else. So I needed 3 Powerwalls which is the maximum for my site. Expensive, yes, but is gives you a nice warm feeling when the power goes off for 3 days due to storm damage to the grid. Working all the costs through it has taken about 2 years to pay back each Powerwall (cheap car charging, exports from solar panels). In the winter it gives you very cheap heating and in the summer you can charge the batteries at 7/8p per unit and export all the solar power at 15/12p per unit (hence offsetting the cost of the batteries). Octopus give me credit for all my exports, so if HMRC want to tax me on that will they tax me for the No Claims Bonus discount on my car insurance if I drive carefully?

 

[GaryMG4]

I think the argument about arbitrage is that the tax free status was to encourage green electricity micro generation, which gives the counter argument that Octopus only supply green electricity. IIRC they were only considering battery only installations where the net income was over £1000, so over 20MWH a year before tax starts to be due.
Re NCB, its not really the same thing, its a discount on something you pay not money from something you sell.

 

[Rolfe]

If you have a 48vdc nom. house battery, you could simplify the V2X system down to a mains powered 48v battery charger and simply charge your 48vdc house battery from the V2L you already have for the MG4.
A step further, you could use a Victron Quatro inverter/charger and connect the MG4 V2L as well as the mains to it, and your house battery, and it will automatically switch the which ever supply you prioritise, moving down the order as availability dictates, while powering your house 24/7.

That is the type of system I am setting up at the moment on our motorhome, solar/battery/mains/V2L ... although in our case, the mains is via a big extension cord, so it can either plug into mains or the V2L as available, or simply run off the solar/battery and even charge the MG if there is enough power available ......

T1 Terry

I am told that this isn't possible with GivEnergy batteries.

 

[T1 Terry]

I am told that this isn't possible with GivEnergy batteries.

Is there battery cables that come of the battery that go to the inverter, if so, connect the battery charger negative and positive there and set the battery charger to a max output voltage of 54vdc, below the point the BMS will attempt to do a cell balance on the whole battery .....

T1 Terry

 

[Bam Bam]

Is there battery cables that come of the battery that go to the inverter, if so, connect the battery charger negative and positive there and set the battery charger to a max output voltage of 54vdc, below the point the BMS will attempt to do a cell balance on the whole battery .....

T1 Terry

I think Givenergy use proprietary cables. So you'd have to bodge their cable and convince the system that it had another official Givenergy battery. My guess would be that Givenergy will only work with their own batteries and if you tried to plug anything else in it just wouldn't do anything. The system has to be set up by a verified installer.

I guess it is like a lot of things, for instance computer software. You can get a system which is designed to be easier to use but you pay more and you have less flexibility, or you do a lot of research and learning on how to use the more open source equipment.

 

[Bam Bam]

I calculated my maximum daily power need (31kWh mid-February below zero degrees all day) to run the heat source air pump and everything else. So I needed 3 Powerwalls which is the maximum for my site. Expensive, yes, but is gives you a nice warm feeling when the power goes off for 3 days due to storm damage to the grid.

Wow, that is a lot of storage. Nice to have, but presumably you've got three phase in order to charge them all simultaneously?

Working all the costs through it has taken about 2 years to pay back each Powerwall (cheap car charging, exports from solar panels). In the winter it gives you very cheap heating and in the summer you can charge the batteries at 7/8p per unit and export all the solar power at 15/12p per unit (hence offsetting the cost of the batteries).

Nice.

Octopus give me credit for all my exports, so if HMRC want to tax me on that will they tax me for the No Claims Bonus discount on my car insurance if I drive carefully?

Yeah, the general aim is that you make a bit of money (as credit) in the summer, and save money in the winter and the overall outcome is that you have no bills during the year.

Some people end up in credit and they can withdraw it. However, it would potentially be taxable, subject to the thresholds etc. that apply at the time.

 

[T1 Terry]

I think Givenergy use proprietary cables. So you'd have to bodge their cable and convince the system that it had another official Givenergy battery. My guess would be that Givenergy will only work with their own batteries and if you tried to plug anything else in it just wouldn't do anything. The system has to be set up by a verified installer.

I guess it is like a lot of things, for instance computer software. You can get a system which is designed to be easier to use but you pay more and you have less flexibility, or you do a lot of research and learning on how to use the more open source equipment.

The cables must connect between the battery and the inverter somehow, I doubt it's an all in one unit. If there was a battery or BMS problem, the whole entire set would have to be replaced if that was the case.

I think the problem is probably to do with the battery BMS having no control over the charger, so it can't turn it off if a cell goes seriously over voltage.
That was the reason for suggesting a very safe end of charging voltage, to allow the BMS to take care of the last bit under its full control.

T1 Terry

 

[Bam Bam]

The cables must connect between the battery and the inverter somehow, I doubt it's an all in one unit. If there was a battery or BMS problem, the whole entire set would have to be replaced if that was the case.

I think the problem is probably to do with the battery BMS having no control over the charger, so it can't turn it off if a cell goes seriously over voltage.
That was the reason for suggesting a very safe end of charging voltage, to allow the BMS to take care of the last bit under its full control.

I should be clear I haven't tried doing anything like this.

Here is the Givenergy cable.

GivEnergy Battery cable for Gen 1 to Gen 2 or Gen 3

Battery connection cable for GivEnergy products. It has an all-in-one connection at one end to connect to all Generation 2 GivEnergy batteries, and ring terminals at the other end to connect to Generation 1 and Generation 2 inverters, or any other battery. Data Sheet GIV-BAT-CAB-G1-G2: Please...

coolenergyshop.com

I guess you could get one of these and connect it up via various other devices and cables to the car DC output.

Someone confident could buy all this equipment and then they might or might not get it all to work.

In terms of the further practicality, our car is parked close to the inverter, but there is a garage door in the way so there would need to be a way to connect without leaving the garage door open (and hence bikes etc. vulnerable to theft).

All this might well be doable for someone who is knowledgeable about these things and can make up the required cables, like John who has set his system up to charge from the MG.

 

[Rolfe]

It's that last paragraph that's the kicker. I do not have anything close to the necessary expertise, and my installer, probably wisely, doesn't seem very interested in tackling it.

Using the VtL to power appliances in the house directly seems like a reasonable work-round.

 

[Ferretusmaximus]

I think the argument about arbitrage is that the tax free status was to encourage green electricity micro generation, which gives the counter argument that Octopus only supply green electricity. IIRC they were only considering battery only installations where the net income was over £1000, so over 20MWH a year before tax starts to be due.
Re NCB, its not really the same thing, its a discount on something you pay not money from something you sell.

Indeed, but I was being disingenuous about the solar export and NCB. How will HMRC be able to tell the difference between a solar export and an export from the batteries (which may have originated from an overnight charge from the grid)?

 

[Ferretusmaximus]

Wow, that is a lot of storage. Nice to have, but presumably you've got three phase in order to charge them all simultaneously?

Single phase copes with it okay using the 12:30-5:50 charging window on Octopus Go. For example last night was 2 hours at 12kWh (12:30-2:30). It rarely needs a full recharge as the capacity was judged on worst case. Usually can charge the Powerwalls plus 2 hours for the car during the night.