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Solar PV savings estimation, have I gone wrong?

  • 02-09-2022 9:42am
    Registered Users Posts: 10,007 ✭✭✭✭

    So we're seriously considering installing Solar PV within the next 6 months and I've been trying to estimate the potential savings

    TLDR, we're currently consuming around 10,700kWh from the grid, around 43% at night rate. Currently that costs €185 per month, but if Energia raise prices by 30-40% in line with other suppliers, this could go to around €250 per month

    I've estimated with a 7.2kWp array we'll generate 7,000kWh per year. I'm confident that using a 9.8kWh battery I can shift almost all daytime consumtion to night rate.

    So when I broke it down, I estimated between self consumption, shifting import to night rate, and export payments our bill would fall to just under €50 per month

    So between €135 and €200 saved per month, depending on how the price rises go

    This seems VERY good. Like a bit too good...

    I know it's best case, so I'd be expecting to maybe fall short by around 10-20%

    I've got my figures below, if someone could double check them for me that would be really helpful:

    First, the situation today. I based these on my current plan and meter readings I took for the past year. I'm currently on a level pay of €182 per month so the estimate looks fairly close

    Then we've got at 40% increase, which seems like the worst case for now. Average cost goes up to over €260 per month

    Then I used PV-GIS to calculated my monthly generation. I used this and my consumtion data to estimate my import and export from the grid. I then calculated the cost/value of these using the current cheapest night rate (Energia EV plan) and Energia's export rate

    Finally I worked out the annual cost by subtracting the export value from import cost, added in the standing rate and averaged across 12 month

    The system cost is VERY much an estimate, I think if I buy the panels and inverter myself, do a DIY battery and get the SEAI grant to cover the installation then €13.5k is possible. It'll probably be a bit more, but I don't think it'll be double that

    The savings and payback time are based on today's rate and the rate with a 40% increase

    So...where did I screw up? 😁



  • Registered Users Posts: 2,682 ✭✭✭yankinlk

    To me it looks like a great attempt... if you dialed the direction/aspect in correctly - the PV estimate is pretty bang on.

    Your import/export figures seem low to me - it looks like you are assuming 100% usage of any sun generated... ,maybe someone with a 10Kwh battery can comment - this might be true - if the TIMES the battery are full is always BEFORE the day usage is drawn down.

    my summary: The price you are working off sounds a.m.a.z.i.n.g for a 7Kwh array and 10Kwh battery (is it diy) - i would bite the handoff that installer. go for it you will defintely save money! :)

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    Thanks, I actually need to refine the battery size to ensure it'll cover my day usage. I think what we would do it in winter would be to either charge the battery to full during the night rate and sell any Solar we don't use, or try to estimate the next days production using a forecasting tool and figure out how much juice we need in the battery

    We've got 2 EVs as well and WFH, so I'm pretty confident that we can consume everything we produce, up to our current consumtion at least. With some power management in winter I'm pretty sure we can stick within the battery's limits

    I'm fairly sure the actual system cost will be higher. I based that cost on buying in the panels and inverter myself and a DIY battery. So the installer would literally just be installing the gear

    I'm probably being very optimistic with 13.5k after the grant, but equally I don't think I'm a million miles away

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    Okay, I've found one flaw already, my battery is about half the size it would need to be in winter to cover my daytime usage. I'll need more like 20kWh as opposed to 10kWh

    I guess I should throw that estimate out the window

  • Registered Users Posts: 2,226 ✭✭✭bullit_dodger

    First of all - I like that! In case you think I'm poopoo'ing on it below. :-)

    A few things to comment on.

    • Ignore standing charges in your math. you pay the same amount with/without solar so it doesn't come into play.
    • The PVGIS are accurate, but only accurate on a per month basis. What I mean by that is that is (taking me as an example), some days in August I generated 40+ Kwhr. I couldn't use all that, so some got exported. Other days I generated <10Kwhr. The point being is that while the monthly averages are accurate, what happens on individual days to generate that monthly figure may not map to your consumption framework.
    • The battery math is "messy" to work out in summer. Is your battery full from the days solar? If so, can you depleate it before 12pm (considering that the sun goes down at 7pm so you only have 5 hrs at "daytime rates" etc.

    It's good to get a "ballpark", but I wouldn't get too tied down on 8 years verses 9 years. The general region is the point I think. Sub 10 years, or even 12 years is good - after that it's still earning it's keep! 15 years and beyond......yeah, ok it'll make you it's money back, and you've done alright by Greta, but there are probably better deals out there. With you in about 8-9 years, that's looking good.

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    Thanks for that, as I said I'm after a good estimate without getting bogged down in trying to figure out every day of the year before I've even got the system. If I estimate €130 savings and an 8 year payback, and in reality I save €110 and it's a 9 year payback then I'm still happy

    I think for the battery, my goal would be to size it that I can get through a days usage in winter on night rates + some meagre PV generation. The problem is that's a fairly hefty battery of around 20kWh, which adds expense

    If I redo the math with the smaller battery and account that I'll be pulling some energy from the grid during the day, my payback is still 8-10 years. So I might go with a 10kWh battery for now and look to add another 10kWh pack later if the opportunity arises

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  • Registered Users Posts: 2,226 ✭✭✭bullit_dodger

    Yeah, I think that's smart - backing both horses. Me? I'm investigating an additional 8.2Kwhr battery myself to bring me up to ~17kwhr. The math is questionable, but it's not all about the payback math for me. It's a factor sure, but i'd also like to be able to smooth my winter bills and having a 15-20Kwhr of storage with €0.08/kwhr unit rates from Energia is appealing.

  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    I've estimated with a 7.2kWp array we'll generate 7,000kWh per year.

    I'd be careful here, your generation will be maxed on your Inverter which is 6kW

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    True, but between the battery and EVs I think my consumption will be high enough that I won't hit that 6kW export limit too often.

    I'm getting an 8kVA inverter so it'll at least be able to handle all the PV output when it happens. I'll limit the export to 6kW to stay within the NC6 boundary

    EDIT: Also forgot to mention that I'll probably put the 2 EV chargers on the grid side of the inverter, so they'd only ever get the excess from the PV array. When they're charging at night they'll get power from the grid, so it means I don't need to worry about the EVs trying to pull 10kW from my inverter

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    The other side of it for us is that between the EVs, heat pump and electric cooking we're extremely dependant on the grid right now

    There's a lot of talk in the news about blackouts, and tbh I think 90% of it is BS, but I think some peak time brown outs are quite possible this winter

    Paying more for a bigger battery does give some peace of mind that you've got a buffer for if a brownout every hit

    So as you said, payback time isn't the only factor to consider

  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    Check is that inverter okay for the grant before you proceed, they are quite picky on inverter size etc

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  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    Will do, it's a Victron inverter. I know I need the extra anti islanding box to make it compliant to EN50549 since Victron don't support the Irish grid setting out of the box

  • Registered Users Posts: 1,133 ✭✭✭DC999

    @the_amazing_raisin, from one over thinker to another - go for it :) I spent too much time crunching in excel and looking at 'what ifs'.. when I could have been booking an install date to get free juice. As we know from the 'seniors' on this forum, what we start with may change in time (in a good way)

  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    I think if I buy the panels and inverter myself, do a DIY battery and get the SEAI grant to cover the installation then €13.5k is possible

    It's well possible, I just put in a 7.3kWp system and 20kWh DIY battery for way less than €10k and no grant

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    That's my thinking, now I just need to find an installer who will put the panels up 😁

  • Moderators, Sports Moderators Posts: 17,888 Mod ✭✭✭✭slave1

    Most installers now sub contract that to roofers who know what they are doing, follow SEAI guidelines, take photos for grant submission and feed cables into the attic. Makes and electrician's life easier as they are not roof trained professionals. When electrician comes on site they can then 100% concentrate on the electrical side

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

  • Registered Users Posts: 4,755 ✭✭✭paulbok

    As long as your were erring on the cautious side on your figures you should be fine with that.

    Easy to get over optimistic on the savings, though you will have a good handle on if they were right or wrong after 3 months.

  • Registered Users Posts: 11,848 ✭✭✭✭KCross

    Why have you not calculated the figures for no battery and FiT payments?

    And then compare that as that will be making you money from day 1, not day 3000+! 😉

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    Tbh, I think I'd get a battery regardless, the only question is what size

    I mentioned it a few posts back but my house is basically entirely dependent on electricity for transport, heating and cooking. We've no option of a stove or fireplace, so no backups if there's a power cut

    I know power outages are rare, and hopefully stay that way, but having some backup power is worth some serious peace of mind IMO

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    I was reading the grant info today and noticed something saying the string inverter has to be within 1.5m of the cable entry to the house

    Is that a recent addition? I thought that only applied to the fireman switch

    I was hoping to put the inverter in the utility room, which has a 6m conduit running to the outside already

    I can use the attic instead, I guess, that seems to be standard practice

    Also I noticed that they're saying a manual changeover switch to bypass the fireman switch is against the rules now

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  • Registered Users Posts: 11,848 ✭✭✭✭KCross

    Each to their own but there is an obsession with batteries and I think it stems from people wanting to stick it to the energy providers but you are just shifting the money to a chinese battery manufacturer.

    Is your motivation money, environment or the brownout thing or all of those?

    Brownout.. other options like your EV which has a massive battery

    money… batteries are questionable with FiT now in. You might make it work as DIY but the figures you’ve mentioned sound very high to me for DIY

    environment… lowest co2 is not to manufacture it in the first place and get paid by the grid to offset your bill

    As an exercise, if nothing else, you should run the numbers on it anyway.

    like I’ve said on other threads, drop the battery and put up more panels with the money or spend it on some other renewable item or on a water diverter or whatever.

    I mentioned it a few posts back but my house is basically entirely dependent on electricity for transport, heating and cooking.

    im the same. I even rely on a robot for cutting the grass! No oil, petrol, diesel in my house. All electric!

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    There is a bit of an obsession with batteries, I think it's from not having a FIT for so long and people got entrenched in their viewpoint

    I'll run the numbers again for no battery, but I don't think it'll make a massive impact. If I did a DIY battery it was working out around €3.5k for a 19.2kWh one

    Unfortunately my EV can't feed power back into the house. And getting a wallbox to do that would probably cost as much as a decent home battery 😬

    And I'll be honest, sticking it to the power company does have a certain appeal 😂

  • Registered Users Posts: 11,848 ✭✭✭✭KCross

    I'll run the numbers again for no battery, but I don't think it'll make a massive impact.

    Over 20yrs it will as you’ll be replacing the hybrid inverter and/or battery in that timeframe so you’ll need to account for that. If you don’t you are fooling yourself. You’ll never have to maintain or replace the grid… but tbf we don’t know what FiT will be like long term either… so you need to make up your own mind and go with it, but I think you’re overpaying

    Unfortunately my EV can't feed power back into the house.

    Most cars can’t but you can drive essentials off the 12V from any car and have that powered by the HV battery for the few times you get a power cut.

  • Registered Users Posts: 1,474 ✭✭✭poker--addict

    I think EVs will feed into the house in 6-7 years time, about the time I suspect FIT will start reducing.

    This used to be a referral link 😎

  • Registered Users Posts: 10,007 ✭✭✭✭the_amazing_raisin

    Honestly I suspect the FIT will reduce a lot quicker than that, as more people get solar the suppliers will lower the rate so it doesn't cost them more

    A lot of EVs are already getting V2L so could power appliances, and the Ford F150 Lightning can feed power into the house via a special wallbox

    I think EVs feeding power into houses is a lot closer than 6-7 years, I'm thinking 2-3 years we'll start seeing it

  • Registered Users Posts: 11,848 ✭✭✭✭KCross

    Id consider feeding the house to be already here tbh.

    Any car with V2L can feed your house without any expensive wallbox. Just a changeover switch when you have a power cut.

    The new Niro has it. Watching Jonny Smith on the MG4 review right now and that entry level car has it! It’s going to be the norm. They will deliver 3kW+ which is plenty to run a house during a power cut.

    On FiT

    Honestly I suspect the FIT will reduce a lot quicker than that, as more people get solar the suppliers will lower the rate so it doesn't cost them more

    Very hard to predict since the market will drive it. Renewable providers like Energia and Airtricity can’t really be seen to be offering a crap FiT when they are pandering to their green customers! But it’s impossible to even guess how it will evolve, but the initial rates are very good.

  • Registered Users Posts: 2,682 ✭✭✭yankinlk

    I dont agree with you - but i like that you make folks think. Buying an expensive battery without thinking of the cost is not a good idea.

    I went back and looked at my quote from a year ago... the battery was NOT listed as a specfic line item - so i have to kinda guess the cost in the quote. Based on a fairly conservative estimate - my battery payback - just counting LAST YEARS difference in night rate to day rate (not inclduing the rises since) is 3.8 years. SO yeah, the math is important, the cost is important. Im glad i got one, but i wont be adding one now until prce comes down (if even then).

  • Registered Users Posts: 11,848 ✭✭✭✭KCross

    just counting LAST YEARS difference in night rate to day rate

    Its not that simple though. We've had a fairly long debate about it on another thread with fully worked examples with losses etc. You need to be realistic on what its actually saving you (including replacement costs) and since its not listed in your quote its impossible for you to tell really what it cost you so I dont agree with you either! 🤣

    But that's healthy debate so I'm good with that. The forum is very one sided in terms of batteries and I think alot of people have been sucked in by the marketing and haven't really run the numbers.

    As I said, each to their own and I dont expect I'll change Mr Rasin's mind anyway.... he is going to stick it to the providers regardless and sure if he gets satisfaction from that maybe its worth it for that reason alone!

  • Registered Users Posts: 11,848 ✭✭✭✭KCross

    Just trying to decipher some of your figures here...

    In your 3rd graph you have "Import cost" which appears to be "Import tariff" * "Shortfall"... yes?

    However, "Import tariff" is all counted as night rate. So your assumption is you are going to be importing zero by day for 365 days!? I dont think that's realistic at all... not by a long shot.

    e.g. In January you use 600 units by day. Your Solar will give you 250. That 250 will be consumed at source and very little going to the battery so you will be pulling from the grid there even with your battery.

    Maybe you plan to charge the battery on night rate?... but in that case you need to add that to your calculations as a unit into the battery from night rate is not a unit out for day rate.... its not 100% efficient (more like 80%) and its not saving you the full day rate, its only saving you the difference... your calcs dont have that. Looks like you are saying everything will be on cheap night rate.... you wont achieve that in reality.

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  • Registered Users Posts: 2,682 ✭✭✭yankinlk

    I used my real costs, I had a grant plus I got a really good deal tbh. And it was a year ago as I said... it woukd not be that cheap now. I have told friends not to buy bigger more expensive batteries.

    For data I used 10 months of my battery numbers, and averaged the last two, so losses are in that. If difference in night rate versus day rate increases (it already has) then I think my calculations are good