Big battery for leccy and hot water. A2A for space heating.

[GarethC]

Apologies to start a thread which is a variation of a prior post, but would greatly appreciate if you guys could sense check. I'm wondering about the following:

- Install 60kWh home battery (as large as I realistically can find space for)
- Get 12kW inverter
- Use this to provide for all of my electricity usage via an off peak tarrif (thereby reducing my electricity bill and carbon emissions)
- Also use it to provide my hot water...
- Install (probably multi split) A2A heat pump for heating, probably just doing 'main' rooms (living room, kitchen and bedrooms).

Would it be doable to provide my DHW off a battery like this? I've checked my annual usage, and a 60kWh battery would easily cover my electricity and hot water needs.

Wondering if I could just replace my gas combi with an electric boiler which would just do hot water (and maaaaaybe the 'non main' room radiators - bathrooms etc - as ASHP would do rest).

The rationale is, if I want to move to a conventional air to water heat pump, I need to find space for an expensive hot water tank. Not sure if I could find space for a battery AND hot water cylinder/thermal store, so wondering if can do away with the latter.

Sorry if the above is a bit unclear. Partially just thinking 'out loud' as it were.

EDIT: Actually, checked my usage again, and the above battery would cover ALL of my energy needs, even in winter, if the ASHP manages a COP of 3+, which it should do easily.

[Krill]

How do you heat your water in this scenario and which tabs do you use?

[GarethC]

Well.... I know it's easy enough to get 12kW electric boilers, and the DHW flow rate from one of those should be reasonable.

It would be great if one of those could be set to run off the battery (and only off the battery), cutting out if the battery ran low. But no idea really if that's possible.

Running even a 12kW electric boiler off the mains would require upgrading the incoming electrics, which would be an expensive faff.

Dunno... As I say would just be great if could essentially do away with need for DHW cyclinder by using battery instead.

[Moxi]

Are you on a three phase supply ? How do you plan to recharge the battery ? lets say you ran it down to 10% SOC that's 56 KW you have to pump back in to it (ignoring efficiencies), the charge window will be based on wiring capacity and cheap rate period of your preferred tariff - do you have that factored in ?

Moxi

[GarethC]

No single phase. Assuming I'll be able to use a tarrif such as Octopus EV (I do have an EV), and could charge at 7kW for 6 hours = 42kWh, which should be enough unless battery completely depleted, in which (hopefully rare) case would need to do some additional charging at peak rate.

[Moxi]

Looking at this through the other end of the tube.

Using resistive heating means a cop of 1 so your battery is bigger and more costly but doesn’t give you access to high value electricity just expensive hot water.

Have you calculated your DHW demand and determined what size HWC that comes to ?

How many A2A heat pumps would you need to maintain the house heating demand and what size would they be.

Then you need to consider what the peak kW demand would be and how that would affect the size and cost of the hybrid inverter.

My gut makes me think that a smaller battery and a HWC with A2W heat pump might be cheaper to buy, operate and maintain. But it all really comes down to your lifestyle and occupancy regime as to which route you go.

Moxi

[Moxi]

What’s the heat latency like for your house, does it have large thermal mass, is the heat demand constant or intermittent etc these all go towards dictating if air to air or air to water heat pump would benefit you best.

At the end of the day you are trying to determine your optimum source for heat comfort so picking something because it “ fits “ isn’t necessarily the only consideration.

Moxi

[Stig]

No single phase. Assuming I'll be able to use a tarrif such as Octopus EV (I do have an EV), and could charge at 7kW for 6 hours = 42kWh, which should be enough unless battery completely depleted, in which (hopefully rare) case would need to do some additional charging at peak rate.

Bear in mind that you're also assuming that such a tariff will still be available for the years it takes to pay back the cost of the battery.

[GarethC]

Using resistive heating means a cop of 1 so your battery is bigger and more costly but doesn’t give you access to high value electricity just expensive hot water.

Well, off peak electricity isn't that much more expensive than gas, and a gas combi boiler is less efficient at DHW than space heating. Needs more analysis on round trip and electric boiler efficiencies, but don't think the DHW costs should be -much- more expensive than gas, and carbon intensity (we're in Scotland) should be lower.

Bigger battery is more expensive, but installing a DHW cylinder is a very expensive, disruptive faff, so avoiding that should be a net win.

Have you calculated your DHW demand and determined what size HWC that comes to ?

Yes. Or at least, I've used our summertime gas usage to estimate hot water energy usage. We've averaged 6kWh of gas per day for hot water as far as I can tell. We're not big DHW users. I'm assuming 12kWh in future to allow decent margin.

How many A2A heat pumps would you need to maintain the house heating demand and what size would they be.

Yes, good point, need to work that out. Have looked at before, and fairly sure a single multisplit would do it.

Then you need to consider what the peak kW demand would be and how that would affect the size and cost of the hybrid inverter.

Yes, not sure how/if this would work. If battery to supply both 12kW DHW boiler and leccy, hybrid inverter size might have to be circa 20kW. That even available? If so, would be expensive.

My gut makes me think that a smaller battery and a HWC with A2W heat pump might be cheaper to buy, operate and maintain. But it all really comes down to your lifestyle and occupancy regime as to which route you go.

Perhaps. Just interesting to speculate. HWC would be and a pain. A2A has a higher COP than A2W, so is cheaper to run (and I'm fairly sure cheaper to buy and install, given I have micro bore pipes and old radiators).

Appreciate the thoughts.

[GarethC]

Bear in mind that you're also assuming that such a tariff will still be available for the years it takes to pay back the cost of the battery.

Given the amount of renewable generation planned to be added, surely time of use tarrifs will become more prevalent? I know there aren't many currently, but I think the current time is probably anomalous?

Things is, A2W would be very expensive and deliver NO financial savings vs. my current gas combi set up. It would almost certainly be somewhat more expensive to run.

With the proposed system, I at least save a lot on electricity costs. Space heating costs should be lower than with A2W (as A2A has higher COPs). DHW will probably be moderately more expensive, but that's not a great cost anyway. I should do more in depth analysis to understand net impact.

And proposed system should deliver greater carbon savings, as A2W does nothing to reduce electricity emissions.

IF feasible, would be interesting to compare net costs. I know shifting to an A2W system would cost me circa £25k (although £7.5k grant available), so there's a fair amount of budget to play with...