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Anyone running Electric Rads off PV?

  • 12-01-2022 4:23pm
    Registered Users Posts: 658 ✭✭✭ bemak

    I'm trying to decide on a heating system for a refurb of an existing farmhouse. I had previously thought I would heat it with a boiler but I'm thinking now that electric rads which are supplemented with PV might be a better option, particularly in terms of future proofing the house.

    Electric rads would really simplify the install and the fact that the PV would provide a lot of the power means that I could have the heating ticking over when the house is unoccupied.

    Just curious to hear if anyone has done this themselves and how its working out for them?


  • Registered Users Posts: 5,426 ✭✭✭ Ubbquittious

    I know someone who has a setup like this. They are not "electric rads" but normal ones, heated up with water that's heated up by a resistive element. Not as efficient as an ashoop but the rads do get good and hot when they're on

  • Registered Users Posts: 2,149 ✭✭✭ Yellow_Fern

    If the house is efficient enough, it could work out ok and be a lot cheaper than a heat pump, but you need get a t's crossed and i's dotted if want to be sure it is inexpensive to run and comfortable, but you wont be getting much from the PV at all.

  • Moderators, Home & Garden Moderators Posts: 10,101 Mod ✭✭✭✭ BryanF

    Cheaper than heat pump? ?? According to the OP This is a farmhouse refurb

  • Registered Users Posts: 658 ✭✭✭ bemak

    The intention is to put an insulating lime render on the walls internally which will bring the u-value of the wall down to circa 0.5 w/m2k. I had hoped to take the sand and cement render off the outside as well and replace it with lime but the cost is soaring so I might leave that for another day.

    The question then for me really is, would electric rads supplemented with PV cost more to run than traditional OFCH?

  • Registered Users Posts: 6,349 ✭✭✭ correct horse battery staple

    Looking at my pv production (4.7ish kw) panels on roof)

    daily production for last week was between 1.5kwh and 8kwh (I think highest I seen in summer is bout 25-30)

    so I guess the question is can you heat your barn with 1kwh in one day in January? With your house not being heated by solar gain through windows either that day

    pv produces power precisely the least when you need heating the most

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  • Registered Users Posts: 658 ✭✭✭ bemak

    "pv produces power precisely the least when you need heating the most"

    this is very true. In the summer months though, when daily production is far greater than demand, you'll be contributing back to the grid. I wonder then what the overall cost of heating a house would be then when this offset is considered. I've asked a company to model this so I will get back with results when I have them.

    Even if it was on par with the cost of OFCH, you would imagine it makes more sense in the long term considering what way oil prices are going?

  • Registered Users Posts: 2,149 ✭✭✭ Yellow_Fern

    Unless I am missing something, electric heating is very cheap to install. Heat pumps are not. Electric heating plus PV wont be so cheap.

    Oil prices go down as well as up. See graph The big fear is carbon taxes.

  • Registered Users Posts: 658 ✭✭✭ bemak

    I got some preliminary figures back on the running cost. Based on a typical 4 person household, the estimated cost per annum is €1200 for domestic electrical loads and €800 for electric heating. Apparently the PV panels should offset €1000-1200 of that.

    What isn't considered in that calculation is that the house will probably only be used for 50% of the year so the idea behind having PV running electric rads is that the PV will probably generate far more than the example outlined above, and as the demand will be less than typical the annual running costs *should* be lower. In theory!!

  • Registered Users Posts: 37,372 ✭✭✭✭ Mellor

    Sounds like an awful plan tbh.

    • Electrical heating is cheap to install. But expensive to run.
    • PV provides energy in summer. You want heating in winter. The numbers above illustrate that even the best day is not getting close to needs. It's supplementary.
    • So you'll need another full heating system, so you'll be installing redundant electric radiators and the like. Unnecessary cost.
    • Heating the house while unoccupied is just burning money.
    • Proposed U-values are pretty poor. There will be a large heat load.

    Honestly, seems more sensible to use PV for electricity and just use the heating system (that you'll need anyway) for heating.

  • Registered Users Posts: 658 ✭✭✭ bemak

    Don't think the plan is awful. The house we're in at the moment is costing us €800 a year to heat with Kerosene and it would have similar u-values to the house that's being renovated.

    The heat loss doesn't really come into the equation as it will be the same regardless of whether we decide to go for OFCH or electric. My question is, what is cheaper to run over a year when you consider the PV will supplement the electrical demand.

    If the house is occupied for 50% of the year - surely the PV is reversing the meter on those days where there's no demand whatsoever? How is that an awful plan?

    There's a chap on Reddit Ireland that has a 3.6kw system and last year it generated 3000kwh of electricity. Our heating demand is calculated at 15kw per day (6 hours ground floor, 4 hours first floor bedrooms) which equates to 4500kw per annum (based on 10 months). The shortfall in this example is 1500kwh which costs €330 (based on a unit price of 0.22c). That's not too bad in my opinion especially as we were looking at the possibility of installing a 5.8kw system which would be located on a nearby agri shed.

    Post edited by bemak on

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  • Registered Users Posts: 37,372 ✭✭✭✭ Mellor

    WellI think it’s awful for the reasons I outlined. If you don’t, great, but you haven’t actually addressed any of the reasons. The house you are in now is irrelevant really.

    Of course heat loss comes into it. Mad to suggest otherwise. If the house was well built and well insulated. Heat load would be tiny, so small elec system might get you through winter. The house you are looking at is terrible for heat loss, borderline non-compliant with building regs.

    PV will supplement electrical demand. But the elec rads will increase the demand by more than PV provides. So either electricity bills increase, or you need a second system.

    The PV won’t be reversing the meter on unoccupied days. Because you said it would be used to “have the heating ticking over when the house is unoccupied”. You can’t count it twice. Its an awful plan to heat a house that’s unoccupied. I’m surprised I have to explain why. It’s literally burning money.

    3MWh from a 3.6kW system about right. The issue with your annual calc is that you’re counting your summer energy towards your winter heating. That guy on Reddit was generating 500w last week. Which is a 67% shortfall, twice what you estimate

    And, I’m interested in how you figure 15kWh per day, that’s very low for an uninsulated farmhouse. Your current €800 of kero is about 1000L right? Do you know how many kWh that is?

  • Registered Users Posts: 658 ✭✭✭ bemak

    Ok let's forget the idea of having heating on when unoccupied. In reality it might be turned on remotely for a few hours if there was a period of wet weather to help dry out the walls. The walls are 500mm thick stonework and the diathonite internally will give the u-value of 0.5 w/m2K. It's probably as good as I can do without EWI. I'm not proposing EWI as it will give other problems considering the wall buildup and how it needs to be able to breathe.

    It's very difficult to calculate the heating demand based on the performance of the walls after the diathonite is added. We can only assume and from being in the house the main issue has always been dampness. It's actually not too bad in terms of being cold despite having single glazing. Our intention is to upgrade the glazing to double glazed while we're carrying out the work.

    The 15kWh of heating per day is based on having the heating on for 6 hours a day down stairs and 4 hours a day upstairs. From experience of living in the house previously (with no heating) I know this will improve the comfort level of the house. In reality it might be more, it might be less. Even at 15kWh a day, the overall usage was based on a 300 day period. In reality it will be 50% of that, most of which will be during the summer months. So even if the heating was 30kwh a day. We're looking at 150 days which works out the same. For the purpose of the calculation.

    I was going to add stoves to sitting room and kitchen which would both burn timber.

    I'm not claiming the above is a brilliant idea, I'm questioning if it's a valid idea worth exploring.

    You clearly think it's mad and that's fair enough. You're one opinion. I want to find out more before I decide myself as I'm still on the fence

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,497 Mod ✭✭✭✭ Jonathan

    Have you considered a wet heating system powered by solar thermal? Solar thermal collectors are far more efficient at harvesting heat energy per m2 of collector area than PV.

    During the summer months, when no heating is required, you could still use system for DHW, although you'd need to go with a drainback system to avoid the gylcol stagnating (boiling) during the summer months when DHW cylinder is up to temperature.

    You might also get more interest in this topic over on the Renewable Energies forum:

  • Registered Users Posts: 37,372 ✭✭✭✭ Mellor

    Calculating the heat load should be straight forward is the area, volume and U-values are known. Pretty straight forward energy assessment. 4500 KwH seemed low to me, as you current similar house is using 10,000kWh.

    Summer only occupation will reduce you needs. But if I was building a house, that I might one day want to sell. I would want a heating system is that is capable to handling winter, otherwise its not very appealing to a buyer. Remember the BER cert will reflect it.

    And yes, this is all just my opinion. You are free to take it or leave it, and/or get others. I'm not against PV as a source of energy. But I'd prefer to keep it simply and an electricity system (PV) for my electricity and a heating system for my heating.

  • Registered Users Posts: 658 ✭✭✭ bemak

    you're probably right the more I think about it. I'm overestimating the offset as well - it's not like for like, it's more like 25% payback so the idea starts to lose validity quite quickly on that basis.

    I think what I'll do is go with PV for the electrical system and put in OFCH for the heating system for now. Going for an OFCH system doesn't really preclude me from renewable heating in the future but at least by then I'll have a much clearer idea of what it takes to heat the house. I'm reluctant to go with a heat pump system now because I don't think the house is well insulated enough - the walls will probably soak all the heat.