The joy of -2c, an ASHP and an externally installed battery!

So here's a pitfall I was not ready for but I will improve the systems as soon as I can.

Last night hit -2c outside, nice layer of ice on the car this morning. I have a low heat ASHP doing heating and hotwater. Pump draws around 3000w and my home has 4 heat zones. Entrance way, kept 19 - 20c. Kitchen (larger room) kept 20 - 21c, Living room (half of downstairs is my living room) all downstairs is underfloor heating. Upstairs single zone kept 18 - 19c with oversized rads.

I am on Octopus Go Faster 5 hour 21:30 til 2:30 5.5p per kwh, the battery charges during this time and and the house runs from the grid. Ok great. However when I got to work at 7:50am and checked my app the battery shows as 0%! Yes thats right from 2:30am my battery was empty by 7:42am. Not the 8% reserve not the 4% it is meant to be but 0%. Just over 5 hours of use.

I think I need more battery storage :-) But still had 10 hours at 5.5p instead, but if the sun didn't come out the current 24.25p per kwh would cause my wallet to be very sad today. Thankfully the sun is out and currently charging at 1.4kw. But yeah my home is 100% electric which means I am using 21 - 26kw hours per day for a 3 bed detached. I still do not have my EV so that will add on 13 - 18kwh per day.

Any suggestions for improving my usage? I thought I was doing things well but happy to take some advice/suggestions

I've noticed that the battery doesn't always stick to the reserve value. Mine is set to 4% but this morning it was 2%. Do you suspect that being outside in the cold temps that it is limiting it's ability to charge or are you simply thinking your usage is too much for the capacity of system you have and want to cut down demand?

chris_p I believe cold to be a factor, but I have no data as to how much. However the numbers say the batter charged and discharged correctly. But I am always looking for ideas to reduce my usage. Even tried the ECOEGG cleaning thing so you don't spin dry, no powder and no conditioner.

THALL Yeah me too. I think we are quite a low consumption household, 4 bed detached, three people in it and only 2,800kWh a year. We don't have tumble dryer, dish washer, washing machine always on quick wash at 30deg C, nothing on standby that doesn't need to be etc. But still would love to get the baseline usage down but I'm guessing that would need tweaking freezer/fridge temps which my other half will refuse I'm sure ha!

chris_p I do have a tumble, dish washer etc. I will max out my roof space for panels soon and am moving as much as possible to cheap 5.5p rate but i think my usage will only go up at the moment.

I have the most panels I can fit on my roof. 20x360watts gives theoretically 7.2kw. Installed in October 2021 with 10.4 kwh battery connected in November. A sunny day like today gives 2.5 to 3kw but I've had nearly 5kw on occasions. I set my battery to retain 25% for the EPS.

I too have an ASAP. When the panels stop generating, around 1600 to 1630 now, the batteries discharge to the reserve correctly. This lasts till about 2100 or 2130, depending on the temperature outside. I expected less than full perfomance at this time of year. Happy so far but waiting for my first energy bill since installation. This is in southern England.

THALL

Mine is now enclosed in a purpose built external cupboard. But today it dropped dramatically again. I have a 12v heat pad installed in my camper and i am tempted to stick one on the battery to see if it makes a difference 😆 .

I've gone from 20% use overnight (Octopus 2030-0030) to 33% with no noticeable increase in house baseload.

Don't worry about the EV side, I've yet to need to charge it outside of the Go tariff hours and the daily mileage varies from 25 to 50 miles in a day.

That said it's up from 1.5p a mile to 2.5p a mile at the moment mostly due to pre conditioning and heating loads.

I've also had the odd day where my battery hasn't supported household demand for the entire day. Whilst it's annoying an extra battery is a significant investment for an issue I'm only likely to encounter for 8-12 weeks of the year on odd days.

So given I'll have more PV production than I can use in summer and it'll probably be all free to charge and the car and battery I'm approaching it as performance over the whole year.

Do you have a buffer tank in the ASHP? Could you pump direct heat into it during the off-peek times? This would store more energy than your ASHP makes on a constant basis.

hamilton Yes, my understanding is the pump is heating the one tank that does my heating and hotwater. It keeps the water at 45c and the pump will turn off and on as required. This is lower than the 50c the manual suggests, and once per week the system will automatically ramp up to 60c to kill off an nasties. I did consider using the schedule tool to make the tank 50c during my 5 hours cheap rate which would reduce my bill a little bit during the day. But would result in more energy consumption. Also I am not sure if the schedule allows different temps at different times, it is very basic and might just be an on and off tool.

THALL
Although heat pumps are known to be efficient, its probably only 200% at those outside temperatures.
Possibly heating the buffer tank to 60 degrees (or higher) during the offpeak would give you more storage of energy instead of using it as you need.

Check your manuals etc though

I've been weighing this up recently while considering an ASHP, considering our house currently uses about 50kWh equivalent from gas on the colder days, I'd calculate we would be using about 15-20kWh depending on the system on these colder days. Only on these colder days though, of which there may not be many.
I've currently got an 8.2 battery.
To add another 8.2 battery is circa £2500 plus installer fees.
If I assume I'm running the high heating over the winter at 2/3 of our yearly consumption within 1/3 of the year.
This for us would be 7000kWh of heat in 120 days.
Assuming a SCOP of 3 over the whole winter we would require about 2300kWh of electricity to run purley the heating. based on my price p/kWh of 14.19p.
We would be spending roughly £211 running the system from our day rate more than our 5p night rate. (i've not factored in low cost heating overnight of a tank)
Based on this current cost we would be at the 25p rate this would double.
If we got 1 extra battery, it would cover HALF of our demand at best, the current one is already providing for the house with not much left to spare.

Battery would benefit us by £105 per year, per battery at our current tariff price.
Would cost justify in 25 years. 12 Years if (when) our tariff goes up in May.

Not really that economical, and we would only get the benefit of having a larger 'Solar dump' for maybe 1-2 months of the year to avoid any grid pull. For it to work best we would also be best to add a second inverter and try for G99 approval.
Back of the envelope calculations I know, but I think personally I'd be happier to take the hit and pay the bit higher cost rather than try to save long term, also no guarantee the 5p will last forever. That electricity cost is still slightly lower than my gas bill, and allows me to lower my footprint by stopping burning so much stuff.

@THALL Have you considered some solar thermal to fill up that extra space on the roof, it might work out more efficient to preheat the water using the solar directly rather than converting to electricity first. Don't know how/if it can integrate into an actual heating system. I do know it can help with heating tanks.

EDIT: a lightbulb moment for me about 5 mins after posting, will return after additional research, assuming G99 approval, V2G? ? ? . . . .

Salibaba I did not consider solar thermal. Though a couple of my neighbours had a unit installed recently I think I would benefit from more solar in general. I have my quotes to fully cover my remaining roof in solar but have not ordered it yet. Waiting for 1 more quote for a second battery before deciding which to do next.

If it helps on the few nights it has hit -1c or -2c a combination of Octopus Go 5h 2130 charging the battery and then the battery taking over from 0230 has gotten me to just over 8am @ 5.5p/kwh. When the temp stays over 2c outside the battery lasts a lot longer. In terms of actual energy use the difference of a -2c compared to a 2c overnight appears to be 4 - 5kwh. Obviously that data is just for my home and others will vary but I do find it interesting looking at the energy chart. 14 - 16kwh used on days the temp stayed 2 - 4c over night, 21 - 23kwh used on days temps dropped below 0c. Will be interesting to see what happens to my usage when/if we get some -5c etc.