Battery with heat pump but no solar - your thoughts

HorseWhisperer

Hi all,

We're just starting our retrofit journey and have had a home energy assessment which gives rough guide costs for each of the project elements. Obviously we are going for a fabric first approach, adding more insulation above the ceilings, reducing drafts where possible & replacing our ancient double glazing (at a cost of around £35K).

Initially we were looking at a two year project, installing solar with battery this year along with insulation and windows, and heat pump next year. However, the projected costs for doing everything over the two years is likely to smash our budget out of the window. We are semi-retired and have no intention of taking out loans to extend the payback period - we'll be paying for everything from savings (inheritance).

So, I'm now thinking that we might need to refocus the priorities, as our biggest issue is our house is too expensive and drafty to continue heat with our current oil boiler (using 1600 litres p/a; heating on only 5 hours per day) & wood burning stove (approx £300 wood p/a). I am not doing more than another winter sitting in a sleeping bag with hot water bottles and loads of jumpers on while working in my office!

If we switch priorities and go for a "comfort first" over "lower annual consumption cost" approach, then my thinking leads me to:

This year:

1. Insulation & windows

2. Battery (on a charge from the grid overnight cheap tarrif)

Next year:

3. Heat pump (or this year if we can cope with that amount of disruption in one go and we can get it done before October, which is unlikely given demand) 

Following year:

4. Solar if we can afford it (we have room for 16+ panels on SW facing garage roof; meter is on garage wall)

The reason we initially wanted to get the solar done this summer is that our electricity comsumption is very high at 6100KwH per annum (we are a small farm and have loads of freezers, fridges and electric fencing!). We are at home all day most of the time. BUT I'm thinking that a warm house in winter trumps the nearly free electricity in summer, and solar panels won't help reduce costs in winter, plus there will be the additional cost of heat pump electricity.

So I guess my question is, if we couldn't afford to install the solar after the heat pump, would a large battery, such as a Tesla Powerwall, give us enough cheaper electricty when charged up overnight to not increase our electricity bill any further if we had additional consumption with a heat pump?

Our current electricity consumption averages around 15KwH/day in winter/spring; the most was 20KwH on the coldest days in February when we had the heating on for about 8 hours a day in the cold snap. When we were on holiday first week of April we were still consuming 9KwH a day!

FYI - It's a 6 bed, 260sqm, L-shaped house originally built 1930s, with two extensions in 1999 (old, wooden double glazing) and 2010 (newer double glazing and replacement external doors). Loft currently has 200mm rockwool which we will increase to 350mm. Cavity walls have Celotex. We haven't had a heat loss calculation done yet as we need to fix areas of loss first.

I understand that there is a debate going on about fitting batteries into new houses rather than solar panels ATM, so it would be good to get your thoughts on this. Many thanks!

QrizB

The financitcase for batteries without solar is a bit tenuous, in most cases.

Imagine you spend £7k on a Powerwall 3 and get 13.5kWh of storage. That's just over £500 for each kWh of capacity.

Imagine you cycle it fully every day, and you find an electricity tariff where you can buy off-peak electricity for 10p/kWh instead of the standard rate of 25p/kWh.

Each day, that kWh of capacity that cost £500 will save you 15p.

15p a day is £55 a year. That means it'll take nine years for your Powerwall to pay for itself.

That's ignoring the inefficiencies in the system that lose some of the energy as heat, and assuming that electricity prices (which are still inflated above the norm) don't fall for a decade.

All the above is only looking at your current situation, without the extra energy use of a heat pump. If you currently use 1600 litres of oil a year, that's 16000 kWh. You say you want more heat in future years, so let's say that increases to 20000 kWh.

Your heat pump might use 6000 kWh of electricity to deliver 20000 kWh of heat, which (split evenly across six winter months) is 33kWh a day. (In practice you might need double, 66kWh, that in the depths of winter.) so that means adding another two Powerwalls to meet the average demand, or four for the peak. Those Powerwalls will only be necessary for half the year (or less) so the time to pay back will double (or more). The Powerwall comes with a ten year warranty so you're relying on it lasting twice as long as the warranty if it's to break even.

HorseWhisperer

QrizB said:

The financitcase for batteries without solar is a bit tenuous, in most cases.

Imagine you spend £7k on a Powerwall 3 and get 13.5kWh of storage. That's just over £500 for each kWh of capacity.

Imagine you cycle it fully every day, and you find an electricity tariff where you can buy off-peak electricity for 10p/kWh instead of the standard rate of 25p/kWh.

Each day, that kWh of capacity that cost £500 will save you 15p.

15p a day is £55 a year. That means it'll take nine years for your Powerwall to pay for itself.

That's ignoring the inefficiencies in the system that lose some of the energy as heat, and assuming that electricity prices (which are still inflated above the norm) don't fall for a decade.

All the above is only looking at your current situation, without the extra energy use of a heat pump. If you currently use 1600 litres of oil a year, that's 16000 kWh. You say you want more heat in future years, so let's say that increases to 20000 kWh.

Your heat pump might use 6000 kWh of electricity to deliver 20000 kWh of heat, which (split evenly across six winter months) is 33kWh a day. (In practice you might need double, 66kWh, that in the depths of winter.) so that means adding another two Powerwalls to meet the average demand, or four for the peak. Those Powerwalls will only be necessary for half the year (or less) so the time to pay back will double (or more). The Powerwall comes with a ten year warranty so you're relying on it lasting twice as long as the warranty if it's to break even.

Interesting calculations and thank you for your time spent doing them! 

But if we added solar at say approx £12-14,000 installation costs, wouldn't we be getting even further away from any breakeven point? I'm not expecting you to do detailed caluclations, but surely the amount of electricity generated/stored over 6 months by solar (which would reduce our paid for consumption dramatically in those months), would be out-weighed by the extra consumption of a heat pump, even if we take away the consumption of oil from the equation?

I am struggling to get my head around the cost/benefit of each upgrade to home as my maths is rubbish. Maybe I'm just overthinging it, but we are likely to have to make a decision about which two of the three upgrades we can afford, and it is really difficult to work it out.

FreeBear

HorseWhisperer said: FYI - It's a 6 bed, 260sqm, L-shaped house originally built 1930s, with two extensions in 1999 (old, wooden double glazing) and 2010 (newer double glazing and replacement external doors). Loft currently has 200mm rockwool which we will increase to 350mm. Cavity walls have Celotex. We haven't had a heat loss calculation done yet as we need to fix areas of loss first.

Late 1920s build 3 bed semi here. Gas consumption had been running at around 8000kWh p.a. before I started trying to improve things. Originally had aluminium framed DG all round (14mm thick sealed units) and ~200mm of loft insulation. Lower half of the building had cavity wall insulation (badly installed), and solid brick walls on the upper half.

Fitted a smart programmable thermostat to the heating system and saw a 10% drop in gas consumption. Increased loft insulation to 300-400mm, and had new DG windows & door fitted. Sealed gaps between floor & skirting on the ground floor where I could. The lounge had a 7kW multifuel stove fitted (way too big for the room size). Gas consumption dropped to around 5000kWh.

2023 saw the heating system replumbed along with bigger radiators and a new combi boiler installed and I'm on track for just 3000kWh p.a. this year. Did all the plumbing myself to keep costs down. A heat pump is on the cards as the new heating system was designed with that in mind. But I've run out of money for now, so that will have to wait for another day.

The new windows & door made a huge difference, mainly down to sealing off the cold draughts that were coming in around the frames. Extra loft insulation is helping to keep the bedrooms warm. The rooms downstairs that have had the floor/skirting gaps plugged are a lot easier to heat. Insulation & draught proofing have both been low cost and delivered worthwhile results. Windows & door, whilst a good chunk of money, has improved quality of life and reduced heating costs.

A couple of comments based on my journey.

* Use a foam gun to seal gaps. You have greater control over how much expanding foam is applied.

* Check to see if you have lintels supporting the outer leaf of brickwork in the 1930s section. It was quite common for the old wooden frames to provide structural support. If no lintels are evident, budget for some to be fitted.

Solar, during the summer, you can sell back to the grid. This will offset usage during the winter months. You may generate a small saving, but at 15-20kWh consumption per day, I fear you may struggle to make the system pay for its self. Same for batteries.

QrizB

HorseWhisperer said:

! But if we added solar at say approx £12-14,000 installation costs, wouldn't we be getting even further away from any breakeven point?

£14000 should buy you something like 22kWp of solar PV system, or 11kWp including a Powerwall. I've written enough about batteries for now so let's run some numbers for the PV-only system.

If you install 22kWp of PV on a south-facing roof in the lower half of the UK, you can expect to generate about 20,000 kWh of electricity per year. Without a battery you'll probably only be able to use 2000kWh of that directly; at the current supply price of 25p/kWh that's a bill saving of £500 a year.

The remaining 18000 kWh you'll be able to sell back to the grid. Right now you can get 15p/kWh for it, so that's £2700 a year of export payments. A total benefit of £3200 a year.

You've paid £14k for the system so it'll pay for itself in four and a bit years.

Hopefully electricity prices will fall, but even at 20p import / 10p export you'll still be making £2200 a year with payback in a bit over six years.

HorseWhisperer

FreeBear said:

HorseWhisperer said: FYI - It's a 6 bed, 260sqm, L-shaped house originally built 1930s, with two extensions in 1999 (old, wooden double glazing) and 2010 (newer double glazing and replacement external doors). Loft currently has 200mm rockwool which we will increase to 350mm. Cavity walls have Celotex. We haven't had a heat loss calculation done yet as we need to fix areas of loss first.

Late 1920s build 3 bed semi here. Gas consumption had been running at around 8000kWh p.a. before I started trying to improve things. Originally had aluminium framed DG all round (14mm thick sealed units) and ~200mm of loft insulation. Lower half of the building had cavity wall insulation (badly installed), and solid brick walls on the upper half.

Fitted a smart programmable thermostat to the heating system and saw a 10% drop in gas consumption. Increased loft insulation to 300-400mm, and had new DG windows & door fitted. Sealed gaps between floor & skirting on the ground floor where I could. The lounge had a 7kW multifuel stove fitted (way too big for the room size). Gas consumption dropped to around 5000kWh.

2023 saw the heating system replumbed along with bigger radiators and a new combi boiler installed and I'm on track for just 3000kWh p.a. this year. Did all the plumbing myself to keep costs down. A heat pump is on the cards as the new heating system was designed with that in mind. But I've run out of money for now, so that will have to wait for another day.

The new windows & door made a huge difference, mainly down to sealing off the cold draughts that were coming in around the frames. Extra loft insulation is helping to keep the bedrooms warm. The rooms downstairs that have had the floor/skirting gaps plugged are a lot easier to heat. Insulation & draught proofing have both been low cost and delivered worthwhile results. Windows & door, whilst a good chunk of money, has improved quality of life and reduced heating costs.

A couple of comments based on my journey.

* Use a foam gun to seal gaps. You have greater control over how much expanding foam is applied.

* Check to see if you have lintels supporting the outer leaf of brickwork in the 1930s section. It was quite common for the old wooden frames to provide structural support. If no lintels are evident, budget for some to be fitted.

Solar, during the summer, you can sell back to the grid. This will offset usage during the winter months. You may generate a small saving, but at 15-20kWh consumption per day, I fear you may struggle to make the system pay for its self. Same for batteries.

Thank you so much for sharing your experience. Very informtive and food for thought. It's just so difficult to work out the cost/benefit of each of the parts of the project, though insulation and new windows are a no-brainer. We thought we had budget for heat pump, solar and battery, but I think one of them will have to be dropped or we may run out of cash to live on into our old age!

HorseWhisperer

QrizB said:

HorseWhisperer said:

! But if we added solar at say approx £12-14,000 installation costs, wouldn't we be getting even further away from any breakeven point?

£14000 should buy you something like 22kWp of solar PV system, or 11kWp including a Powerwall. I've written enough about batteries for now so let's run some numbers for the PV-only system.

If you install 22kWp of PV on a south-facing roof in the lower half of the UK, you can expect to generate about 20,000 kWh of electricity per year. Without a battery you'll probably only be able to use 2000kWh of that directly; at the current supply price of 25p/kWh that's a bill saving of £500 a year.

The remaining 18000 kWh you'll be able to sell back to the grid. Right now you can get 15p/kWh for it, so that's £2700 a year of export payments. A total benefit of £3200 a year.

You've paid £14k for the system so it'll pay for itself in four and a bit years.

Hopefully electricity prices will fall, but even at 20p import / 10p export you'll still be making £2200 a year with payback in a bit over six years.

Amazing news! And thank you for doing the calculations as it all befuddles me. I'll check out your other posts for further info. Much appreciated

QrizB

Just to add: all the solar geeks hang out on the "green and ethical" sub-forum:

https://forums.moneysavingexpert.com/categories/green-ethical-moneysaving

michaels

This was my thoughts on the matter

https://forums.moneysavingexpert.com/discussion/6588147/is-it-a-financial-no-brainer-to-get-a-battery-system-if-you-are-getting-a-heat-pump#latest

Reed_Richards

I replaced an ageing oil boiler with a heat pump.  I really like the heat pump but it's not saving me a lot of money in heating costs because oil is so much cheaper per kWh than electricity.  So even though the heat pump will, on average, give me 3+ kW of heat per kW of electricity, if I buy electricity at the standard rate that still works out at much the same running cost as an oil boiler.  This is exactly what I did for the first few years but now I have a cheap overnight rate which works out better.    

HorseWhisperer

Reed_Richards said:

I replaced an ageing oil boiler with a heat pump.  I really like the heat pump but it's not saving me a lot of money in heating costs because oil is so much cheaper per kWh than electricity.  So even though the heat pump will, on average, give me 3+ kW of heat per kW of electricity, if I buy electricity at the standard rate that still works out at much the same running cost as an oil boiler.  This is exactly what I did for the first few years but now I have a cheap overnight rate which works out better.    

I'm not expecting a heat pump to save us much, if anything. I just want a house that's warm all the time and I don't have to sit in a sleeping bag in deep mid-winter! So long as it just about breaks even, that's fine by me. There are really good tariffs out there now, especially if you want to charge up a battery in winter overnight. Thanks for sharing your experience :-)