Thanks
@b0rn2sk8. The essay is great, I like getting into the detail and numbers. The place I'm looking at is a 2-bed, detached bungalow but a lot smaller than your place so I'd guess the heat loss will be a little less to start with, although yours isn't bad at all.
Where you've got the months listed, that's just the heat pump use I'm assuming?
Yes, just the heat pump, I have a Shelly EM monitoring its consumption.
What happened in January, just down to the outdoor temperatures?
Yes, exactly this, lower outside temperature = more energy needed to keep the house at target.
How do you know your average unit cost, is it itemised on your bill how much is off peak and how much is on peak (I'm not on a smart or multi-rate tariff at the moment so don't know)?
Yes, Octopus break it down to every 30 mins on your bill if you really want to get into a day by day detail.
For example, the worst day in January I referenced the heat pump consuming 31kwh in total I consumed 38.15kwh from the grid. Of which 28.10kwh were was off peak, 10.05kwh was peak, the battery ran out just after 5pm, I can tell from when I started using peak time electricity.
On that day I also generated 6.9kwh of solar so my total energy consumption was around 45kwh (no car charging on this day). The benefit of cold winters days is that they also tend to be clear so there is more solar generation than the warmer but cloudy days.
With your battery could you charge it overnight and shift some of the heat pump load to that, therefore avoiding the heating up and 'coasting' that you're doing now?
I charge the battery to 100% every day regardless of whether it is summer or winter. On my EV tariff it is 7p overnight, I can export in the day for 15p.
What I meant by coasting was utilising the thermal mass off my house and everything in it to lower the energy need during the day. I'm using my heat pump to put in way more heat than is needed overnight, most of this is getting absorbed into the fabric of the building and everything in it and the actual air temperature isn't overshooting my day time target by much, perhaps 0.5C on most days. This 'stores' a surprising amount of energy overnight ready for 'use' later in the day.
Then the output of my heat pump is set ever so slightly below what is needed to maintain that during the day and the extra energy which is stored in the fabric of the building is released back into the air and by the end of the day I'm either at target or at most 0.5C below. The cheap period starts at 11:30pm and the process starts over again.
You don't notice the temperature change as 1C over the course of an entire day is nothing and the radiators are always warm so you get some of that direct radiant heat all the time.
On a gas system,, if your radiators are turning on and off all day like a yoyo, that's why you 'feel' cold during the times where your radiators are not up to temperature, and the air temperature is probably varying by 2C based on the over/undershoot of the thermostat.
I definitely wouldn't go too far on batteries but was thinking 24kWh-ish to cover a full day use and sell back any excess.
To be honest, this sounds quite high and you hit diminishing returns very quickly on batteries once you have 'enough' which is typically up to 80% of daily use without heating and a bit more with heating. Typical household use without heating is 10kwh a day and I am about bang on with this. As mentioned above, I've 'only' got 13.5kwh in total.
Don't forget when you are on an EV tariff or similar, you get 5/6/7 hours of cheap electric so you don't need battery storage to cover thus use and you run your heavy appliances (dish washer, washing machine etc) in that cheap period also so you also don't need to cover that. You start hitting diminishing returns from as little as 60% of daily use without heating. The more panels you have, the letter batteries you need.
Using batteries to cover heating loads is tricky because they will only be protecting you from peak rate electricity for a relatively small part of the year (Nov-Feb, solar can fully cover Sep-Oct and late Feb-Apr). If you are paying retail rates for batteries, the charge up at 7p and selling back at 15/16p doesn't really pay back, at best you are looking at just covering your cost of buying it in the first place over 10 years. A 10kwh battery will set you back £3000 plus installation, that is a lot of 7p's to make up (15p less 7p less AC > DC > AC conversion losses).
In my view, you'd be better off paying slightly more for your off peak energy on something like Octopus Cosy but you benefit from getting to charge a smaller battery 3 times a day for those colder ~3-4 months of the year. I didn't switch to cosy over winter, my average import cost when using some peak rate was still below the off peak rate of cosy.
