Heat pumps with extra batteries

matt_drummer

I do pay bills in the winter, they get deducted from the credit balance that I build up over the rest of the year. I just never part with any actual money apart from my £1 per month direct debit to get the dd electricity tariff.

You don't really need to cover everything.

My heat pump heats the house at a COP of 4.50 when it is 0c outside.

I think a SCOP of 3.0 is poor these days and you should aim for 4.0 or better.

At my level, even on a standard tariff, my heat pump is comparable to a gas boiler.

At something like 23p for electricity my heating costs 5.10 per kWh, gas at 6p per kWh and 90% efficiency is 6.7p per kWh. I also have no gas so no standing charge.

A heat pump, even with what you have now, will be better than a gas boiler if installed and operated correctly. 

maf20

Ok. That’s all good to know, thanks. 

One issue I have at the moment is that octopus won’t install one as they’re obvs only doing really easy installs, still. I’m struggling a tad for wall space for one and they won’t install it up high on the side of the house. 

The last time I checked (c. 6 months ago) alternative installers were significantly more expensive. 

Anyone know if things have changed on this front, recently? 

maf20

Also, I presume if my heating/gas usage is c. 11,000 kWh over the year I need to aim for the same output from an ASHP? 

Reed_Richards

If I were getting more battery capacity, and perhaps I should, I would look at how much capacity would cover 362 days out of 365, 320 days or, say, 290 days.  I would calculate the cost saving in each case and the cost of the battery needed and the payback period assuming whatever I am paying at the moment for electricity.  Although at the moment you can get very cheap rates for EV charging and as more people get EVs I doubt this will last.

maf20

Sorry. One more. How do these calcs sound…

in 2023 (whole year) I paid for 3,000 kWh of electricity, plus I had a yield from the panels of 4,200 kWh. I used 10,600 kWh in gas. I exported 908 kWh (will be a lot more this year as I had tariffs set up differently)

So, all in the house would consume c. 18,000 kWh over the year. Minus the yield, that means I need to pull in c. 14,000 kWh over the year. 

If I put panels on the back of the house I’ll get 5/8 of what I would from the front, meaning panel yield per year would be c. 6,825 kWh. Add the export and I’d get an additional c. 2,680 kWh (which would include the extra yield from the new panels plus the changes I’ve made this year). My current battery loads up with around 2,110 kWh per year meaning I’m now at c. 11,620 kWh. If I added another battery in I’ll then be up to c. 13,730 kWh over the year

Hopefully, the extra yields/export in the summer months would balance out the extra import needed in winter months. 

Does the above logic sound somewhere near, do you think?

EDIT: Actually noticed I haven’t applied the same logic across both - I’ve referenced the yield differently in both. Other than that, though, sound about right? 

maf20

Think I’m making hard work of this! 😂. Thanks for bearing with me. 

I need 18,000 kWh a year 

if I invest in additional panels and batteries, it could go like this

Yield - 6,771
Export - 2,600

leaves me c. 8,700 to find, which I’d need to get from battery storage. I currently have one 5.8kW battery which can load up at 2,117 over a year, meaning I’d need around 4(ish) batteries in total to get to 18,000 ish? 

matt_drummer

maf20 said:

Think I’m making hard work of this! 😂. Thanks for bearing with me. 

I need 18,000 kWh a year 

if I invest in additional panels and batteries, it could go like this

Yield - 6,771
Export - 2,600

leaves me c. 8,700 to find, which I’d need to get from battery storage. I currently have one 5.8kW battery which can load up at 2,117 over a year, meaning I’d need around 4(ish) batteries in total to get to 18,000 ish? 

No, you need to look at what you need on the worst day to know how much battery storage you need to cover everything.

Then work backwards to assess what you are prepared to import if you don't have enough battery storage.

Averaging it out over weeks and months won't give you the answer you are looking for.

maf20

matt_drummer said:

maf20 said:

Think I’m making hard work of this! 😂. Thanks for bearing with me. 

I need 18,000 kWh a year 

if I invest in additional panels and batteries, it could go like this

Yield - 6,771
Export - 2,600

leaves me c. 8,700 to find, which I’d need to get from battery storage. I currently have one 5.8kW battery which can load up at 2,117 over a year, meaning I’d need around 4(ish) batteries in total to get to 18,000 ish? 

No, you need to look at what you need on the worst day to know how much battery storage you need to cover everything.

Then work backwards to assess what you are prepared to import if you don't have enough battery storage.

Averaging it out over weeks and months won't give you the answer you are looking for.

That’s fair enough. Not sure I’d be willing to outlay all that capital, though, if I’m honest. 

My worst day was 107kWh of gas, meaning I’d need something in the region of 20 5.8kW batteries, at a cost of c. £57k. 

Unless I’m missing something? I presume I am? 

matt_drummer

107kWh is the heat produced.

At a COP of 3 that would need 35.67kWh of electricity, at a COP of 4.0 it would only be 26.75kWh

Your heat pump doesn't run at 100% efficiency, it will run between 300% and 500% efficient

Don't forget though, other things in your house use electricity too, if you want to cover everything.

maf20

Ahhhhhhh! Right. Got it, now. Makes sense. 

Thought I must be missing something 

Yeah. All the kWh calcs I did also include general usage. 

Cheers for the help