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Optimal Energy Tariff for ASHP + Solar PV Combo

  • 02-11-2022 6:14pm
    Registered Users Posts: 18

    Hi folks,

    Just wondering if anyone has any thought on this.

    We're getting solar PV installed soon. Probably a 6 KW – 8 KW system, SW facing roof, no obstructions etc.. Not planning on getting an EDDI. We have an air source heat pump for DHW and central heating through those outsized radiators.

    We're due to switch our energy supplier soon. Have a smart meter. I was thinking about going for Electric Ireland's plan with €41.61 day rate units and €20.52 cent night rate.

    During the Summer months, I was thinking we could set the ASHP to come on during daylight hours to make the most of solar. Then in winter months I was thinking we could switch the ASHP to come on at night. This would be half price, but I'm wondering would the cold temperature hurt the efficiency.

    Any thoughts welcome.


  • Registered Users Posts: 5,988 ✭✭✭Ubbquittious

    Do you live in one of those fancy new Leo Varadkar approved A-rated houses? If you don't you'll probably need to run the ashoop during the day on days you don't get a load of solar. Which means paying the day rate for electricity. You might be able to minimise this by running it for the last few hours of the night tariff but it might not be enough. If your house is too lossy and you don't have a massive battery you might be better off on the day rate

  • Registered Users Posts: 18 Livegood117

    Yeah the house is A2 rated and has one of those heat extractors so it keeps it in well.

  • Registered Users Posts: 40 Duine_Eigin

    I'm in a similar situation; A1 rated timber frame build, ASHP, 4kWp solar, 7.2kWh battery, Eddi and Zappi.

    Up to now, I have been running my ASHP at night for both heating and DHW. Usually, this is enough to keep the house at a minimum 21deg. I actually heat to 22deg at night and this is enough to maintain a minimum temp of 21deg the following day. On a handful of occasions, the heat pump came on in the evening to bring it back up to 21deg but this was only during very cold days when there wasn't much solar gain.

    I have UFH on the ground and first floor so the thermal mass of the screed allows the energy from the ASHP to be retained for a longer period. Your experience will be different with the rads however and you may find that the house doesn't hold the heat as much.

    Yes, operating the AHSP at night won't be as efficient but if your night rate is cheap, it makes sense to do it. Those rates you quoted aren't the best. You should try to get a day/night meter and go with Energia's EV rate. It has the cheapest night rate (13.75c/kWh) at the moment as far as I know.

    Did you consider getting a battery for your PV system? I only got my battery recently and it has enabled me to avoid the expensive day rates by charging it at night. Also if the ASHP needs to come on in the evening this winter, the battery will cover the load rather than using the day rate. If the cost of a battery is prohibitive at the moment, you should at least get a hybrid inverter so you can easily add a battery at a later date.

  • Registered Users Posts: 1,204 ✭✭✭DC999

    Wow, that shows the benefit of a high A rated house. To have 1 degree drop overnight is unreal.

    We’re in a very leaky house with a poor BER rating. Temp rockets down once the heat is off – as you’d expect. We don't have a heat pump. In a parallel universe I’d live in the most energy efficient house we could. Fair play to you. You’re flying it now!

  • Registered Users Posts: 2,410 ✭✭✭ECO_Mental

    Jeeze you are keeping your house nice and toasty!!!😁

    Like yourself I am in a A1 rated house but I have most of my stats set to 20 and 18-19 degs in the bedrooms. I find if I set the stats to 21 (or even 22) my HP would be on A LOT! My house will hold 19-20 without too much effort, those extra few degs make a big difference.

    Only in the house 1 year this week and last winter I was only learning and not really managing it but only have had to have the heating on a day or two last week when it was really windy and cold, heating not on since March otherwise.

    The only time I ever get to use the Solar PV with the ASAP was is if it was a sunny day and the night was forecast cold, I would preheat some of the rooms to get the heat into the slabs like you say. But that is rare enough and I find that if its sunny anyway the solar gain already preheats the rooms.

    I dont have any timers set on the stats, but I find the house will only call for heat at night anyway so there is no need.

    6.1kWp south facing, South of Cork City

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  • Registered Users Posts: 40 Duine_Eigin

    Sorry, I should have mentioned that the 21deg is for the ground floor only. Like you, I have the bedroom stats set to 18-19 deg.

    I've been in the house four years and I find that running the heat pump at night works for me. I have an energy monitor on the heat pump I calculated my heating and DHW cost for last year at €112 for the whole year! That was thanks to the ridiculous night rate offered by Energia at the time (4.7c/kWh). Not sure if we'll ever see the likes of it again unfortunately. The Eddi also helps in keeping this cost down as the PV covers most of my DHW needs from April/May until October. Plus it has the added benefit of extending the life of the heat pump as it is only operating for half of the year.