Heat Pump - KwZ usage through the roof

matt_drummer

My water tank has just been heated using 700W at a COP of 4.46

It generated 3.05kWh of heat

I get 15p per kWh for solar export.

The cost of heating my tank with the heat pump was 10.5p (15p x .0.7, the solar export I gave up) and if I had used any form of direct water heating it would have cost me 45.75p

So I am 30.25p better off today by using the heat pump.

Assuming I wanted to shower outside daylight hours I would have to import to power an electric shower.

If the water tank is the correct size for the house needs then the heat pump will always be cheaper despite some small losses from the tank over 24 hours.

Any heat pump will come with a modern well insulated tank with minimal losses.

My maximum power draw was 1.73kW so less chance of swamping the solar generation on smaller arrays.

It also doesn't exceed what my inverter can supply from the batteries at the same time as powering the rest of the house.

Heat pumps are very efficient, you won't beat it with any form of direct water heating.

NedS

We typically use around 1kWh to reheat our DHW tank, today that cost us between 15p for the solar we gave up and the 17.7p import price at the time on Agile. Our ASHP is a little bigger than Matt's, so we can pull up to 4.7kW towards the end of the cycle meaning it's not completely covered by our solar, even on a sunny summer day.

We also have a 9kW electric shower - one person taking an electric shower uses ~2kWh in winter, or 1kWh in summer when it can be turned down to half power. So one person taking an electric shower uses the same amount of electricity as reheating the DHW cylinder in summer, or twice the electricity in winter.

We can schedule the DHW cycle to run at the cheapest time of the day, and then shower at our convenience. It would be far less flexible trying to limit showering using the electric shower to the cheaper Agile slots. I don't think the family would take well to showering at 3:30am or waiting until 2:30pm in the afternoon for a shower. For this reason, the amount paid for that 1kWh to heat the DHW tank may be significantly less than the amount paid to take that electric shower using the same 1kWh.

For us an ASHP is a more efficient and ultimately cheaper way to heat DHW.

FreeBear

matt_drummer said:

My water tank has just been heated using 700W at a COP of 4.46

It generated 3.05kWh of heat

For comparison, I used roughly 80l of water (at 44°C) over the weekend, and burned 3kWh of gas at a cost of 14.6p. A heat pump would have been cheaper (assuming 0.7kWh, 12.5p on my current tariff).

Reed_Richards

NedS said:

...We also have a 9kW electric shower - one person taking an electric shower uses ~2kWh in winter, or 1kWh in summer when it can be turned down to half power. So one person taking an electric shower uses the same amount of electricity as reheating the DHW cylinder in summer, or twice the electricity in winter.

Do you actually need this electric shower?  You should get all the water pressure you need from your pressurised DHW cylinder.  Unless it it is only plumbed with a cold pipe and getting hot water to it would be tricky, then I don't see any reason why you would want to keep it and waste money.

NedS

Reed_Richards said:

NedS said:

...We also have a 9kW electric shower - one person taking an electric shower uses ~2kWh in winter, or 1kWh in summer when it can be turned down to half power. So one person taking an electric shower uses the same amount of electricity as reheating the DHW cylinder in summer, or twice the electricity in winter.

Do you actually need this electric shower?  You should get all the water pressure you need from your pressurised DHW cylinder.  Unless it it is only plumbed with a cold pipe and getting hot water to it would be tricky, then I don't see any reason why you would want to keep it and waste money.

We will keep it as a backup, but the intention is not to use it (often). It's located in the en suite.

There will be a shower in the main family bathroom connected to the DHW tank, but that is a "work in progress" at the moment. The bathroom was due for renovation and got ripped out during the re-plumb for the ASHP.

john-306

matt_drummer said:

My water tank has just been heated using 700W at a COP of 4.46

It generated 3.05kWh of heat

I get 15p per kWh for solar export.

The cost of heating my tank with the heat pump was 10.5p (15p x .0.7, the solar export I gave up) and if I had used any form of direct water heating it would have cost me 45.75p

So I am 30.25p better off today by using the heat pump.

That's excellent COP for HW

I'm due to have same as your heat pump fitted in 2 weeks, but nowhere near your sized radiators. 
I've only got solar with 6.4kwh battery at present.

I'm assuming I'd struggle with heat pump in winter to only use the battery through the higher pricing hours if I go to the Cosy tariff?

Thanks.

matt_drummer

john-306 said:

matt_drummer said:

My water tank has just been heated using 700W at a COP of 4.46

It generated 3.05kWh of heat

I get 15p per kWh for solar export.

The cost of heating my tank with the heat pump was 10.5p (15p x .0.7, the solar export I gave up) and if I had used any form of direct water heating it would have cost me 45.75p

So I am 30.25p better off today by using the heat pump.

That's excellent COP for HW

I'm due to have same as your heat pump fitted in 2 weeks, but nowhere near your sized radiators. 
I've only got solar with 6.4kwh battery at present.

I'm assuming I'd struggle with heat pump in winter to only use the battery through the higher pricing hours if I go to the Cosy tariff?

Thanks.

Yes, I think it's good too.

Cosy is quite good, eight hours at the cheapest rate.

So you only need to find energy from elsewhere for 16 hours a day.

Maybe do DHW in a cheap slot, 13.00 to 16.00.

The rest of the time, if you leave it running, you'll use something like 700W on average at what I am guessing your flow temperature might be.

You will get some solar, I'll guess at 3kWh average during December.

You can charge your battery during the eight hours too.

6kWh of battery will last you eight hours at 700W.

It will work fine.

At the coldest times you will have to import at higher rates but if you can move some use to the cheap periods I am sure you can average well under 20p per kWh for everything.

If your heat pump averages at something like 16p per kWh at a SCOP of 4 then you'll be better than gas.

john-306

Thanks Matt, very informative and good to know. 
I might just about make it work.
6.1kw panels only produced 4.5kwh today, but it has been dark and miserable all day. It can't be much worse through winter I'd hope 🤞

matt_drummer

john-306 said:

Thanks Matt, very informative and good to know. 
I might just about make it work.
6.1kw panels only produced 4.5kwh today, but it has been dark and miserable all day. It can't be much worse through winter I'd hope 🤞

It will be worse.

I'm an accountant, so in theory, quite good with numbers.

Just focus on the average and don't let the odd bad day alarm you.

Some days will just be bad, cold and damp with little sun.

But generally, it just averages out, if you can get through most of the 7c days, which I think you can, then you'll be no worse than gas and probably better.

Reed_Richards

NedS said:

We will keep it as a backup, but the intention is not to use it (often). 

My backup is that I can heat the DHW cylinder with the immersion heater should the heat pump be out of action for some reason.