@jeff if its the middle of winter you are going to burn through your batteries in a couple of hours (if its for your ASHP) - IF they are fully charged. Realistically power cuts are highly unlikely and if they do happen over a prolonged period you've probably got bigger problems.

As you said, better to have some bigger batteries or spend the money on energy efficiency to reduce your grid reliance.

All valid points above. While the UK has an extremely robust electricity supply with limited power outages, the drive to electrify the country, coupled with the current state of affairs in Europe, does not guarantee that it will remain like this. Personally, I'd like to be as off-grid capable as possible, especially when it comes to electricity. 

Posted by: @jeff

Posted by: @editor

I can't get over how complicated the system of battery storage is with PV when there's a power outage. There are presently, from my conversation with installers, no out of the box solutions to have a system in place where the PV can recharge batteries when the grid's gone down without all kinds of modifications and additional equipment. I'm getting quotes for a system, but I suspect it'll be prohibitively expensive due to all the additional kit that needs to go alongside it. I think SolarEdge are coming out with an inverter this year that may do just that, but who knows.

An honest question, whatever the cost, wouldn't it be better to spend the money on more battery capacity or other efficiency work? 

I struggle to see the point in households spending any money on coping with power cuts? Is it really a priority, especially given your current high electricity use?

Generally yes.

I asked the original question because we'd just had a couple of power cuts courtesy of Eunice and I still had a full battery sitting doing nothing, yet couldn't get online to do my job. If there had been a simple answer, I wanted to know it and make use of it. Now I've found any answer isn't simple, so the most likely is that I'll get a couple of UPS boxes and sort out the router, the computers and the fridge. Job done.

@derek-m, SMA looks like a useful company. I've just contacted them and sent them an email. Good find. Thank you.

Posted by: @editor

All valid points above. While the UK has an extremely robust electricity supply with limited power outages, the drive to electrify the country, coupled with the current state of affairs in Europe, does not guarantee that it will remain like this. Personally, I'd like to be as off-grid capable as possible, especially when it comes to electricity. 

Interesting. I take the view that it is the responsibility of the network operators to harden the resilience of the grid to power outages, not home owners, and that the probability and impact of the risk personally to me is low. 

I get the use of home solar to reduce the draw from the grid and home batteries to reduce peak power draw. Although i do wonder about the network opperators themselves funding a huge increase in local battery storage rather than leaving it to the few consumers who can afford it and have the space to house them. 

Having spent much of my career in risk management i find it interesting how others identify, quantify and seek to minimise the probably and/or impact of risks.

Am not saying you are right or wrong, you may well be really worried about the future resilience of the grid and may be more impacted than me if the grid was down.

Be really interested to hear what the additional cost options are and how long they would run a house for, particularly in winter when there would be little solar. If the network went down when the batteries were empty towards the end of the day how will the system work for example? I assume it depends on how much notice you would get for a power cut and if you kept the batteries charged for a power cut rather than using the electricity. I wonder for example if a biofuel electric generator might be useful as part of a solution for resilience.

Fascinating topic.

@Jeff, it is very interesting, and I think we all have our own interpretations and reservations about the future of, well, everything I suppose.

I'm far from being a grid or electricity expert, so our reservations may have no basis. As a household and married couple, we are very risk averse, and we like to keep as many bases covered as possible – within financial constraints of course. Independence from the grid would be a major step forward. Our intention was always to be as off-grid as possible, so this would be a big step provided that we can get the system at a reasonable price. 

And yes, the biofuel generator will very much be part of the final solution to take on the load when there's no solar production.

I hope to have more clarity this week, and we will be interviewing a battery expert next week to get his views. Should be a fascinating interview.

@editor

Putting aside environmental questions for one moment. 

At current peak electricity rates unless you happen to be on a good fixed rate, it may weirdly be cheaper to run a diesel generator using HVO than purchase peak time electricity.

Have you done any calculations on that?

@editor how much battery storage are you planning? You're going to need a lot of batteries and a lot of capacity to actually use them to power an ASHP in the winter months.

I think you said 16kwh? You wont be able to use all that - assume 80% Depth of Discharge so you'll get 0.8 x 16kwh = 12.8kwh useable. Then assuming you can draw at 5kw you'll have a little 2hrs of run time a day if its very cold and you aren't using any other electricity.

I would suggest you have a circuit disconnected from the ASHP to run fridges etc

@batalto, in the event of a power outage, the ASHP will go off and the HVO-driven boiler will kick in - won't be automatic. I will switch over myself. With the whole house running, including distribution pumps, we'd be idling at around 400-450W per hour. Depending on which solution we opt for, I guess we'd get between 15-20kW of battery - enough to see us through to 24-30 hours. If there's solar, we'd hopefully get juiced up. If not, maybe a HVO generator could help out.

@Jeff, good thinking. I will go through the numbers. We are on a lousy rate, but I'm also thinking long-term now. There's a lot to contemplate and I never envisaged solar and back ups to be quite so complicated. 

Posted by: @editor

@batalto, in the event of a power outage, the ASHP will go off and the HVO-driven boiler will kick in - won't be automatic. I will switch over myself. With the whole house running, including distribution pumps, we'd be idling at around 400-450W per hour. Depending on which solution we opt for, I guess we'd get between 15-20kW of battery - enough to see us through to 24-30 hours. If there's solar, we'd hopefully get juiced up. If not, maybe a HVO generator could help out.

Hi Mars,

If you are just looking to keep systems working during a power cut, then a portable generator would be the much cheaper option than a full blown battery system.

I suspect that your primary intention is to reduce your energy costs, in which case a battery system would be required, to both fully utilise all available power from your solar panels, and also maybe take advantage of cheap overnight tariffs. This of course would not work during a power cut without the grid connection being disconnected, which could be done manually or automatically dependent upon how you would wish the system to operate.

@derek-m, spot on Derek. We still want batteries to recharge via PV during the year to run the house at nighttime and to avoid the pain of rising electricity tariffs. It would just be nice to have an all-encompassing solution. There's a lot more reading and research ahead.