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Calculating Solar PV Savings

  • 21-09-2022 12:14pm
    Administrators Posts: 217 Admin ✭✭✭✭✭ System

    This discussion was created from comments split from: Daily pv production.
    Post edited by Jonathan on


  • Registered Users Posts: 2,291 ✭✭✭ ECO_Mental

    Decent enough week 170 kWh.. but the good part is that I used 70% of it which is very good for me. Up until I got the EV I was only averaging about 40-45% self consumption. Worked out the EV is saving me about €270 per month in fuel costs based on the 9 weeks I have it.

    Must put together a spreadsheet to work out my savings every month with the PV and the EV.....I love a good spreadsheet!

  • Registered Users Posts: 5,419 ✭✭✭ championc

    Just be aware that savings would only be calculated against your cheapest rate period (likely night rate) since you could charge the car then anyway, rather than taking the savings as being against pricy day rate units when the car was actually charged by solar

  • Registered Users Posts: 1,753 ✭✭✭ deravarra

    Why so? Perhaps he might need to charge during the day. Surely you must factor savings during the day usage as well. Otherwise you might as well tell him he can only factor the cooking when done at night, the washing when done at night, etc.

  • Registered Users Posts: 3,531 ✭✭✭ graememk

    Generally charging the car is done on night rate, very few people have to charge in day rate.

    It's a very easy thing to count, otherwise your fooling yourself with how much you've actually saved.

    Other things are much harder to justify dishwasher etc but car and the likes of an eddi are a bit easier.

    Although eddi has got a bit more complicated. Oil/gas used to be cheaper than night rate.. not so much now.

    I'm even counting the solar used when on night rate, (with north east panels, mine get going early!)

  • Moderators, Sports Moderators Posts: 17,403 Mod ✭✭✭✭ slave1

    What's pointed out is correct, the correct financial approach is to use as much electricity during the cheapest rates, for most of us we will have a D/N meter so night time for the dishwasher. If you are using the dishwasher during the day to consume PV then the saving is versus your night rate, not expensive day rate.

    Dishwasher is an example, others are immersion, dryer, washing machine etc. Hob cooking is not, same as boiling a kettle for tea, those have to be there and then

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  • Registered Users Posts: 1,753 ✭✭✭ deravarra

    Generally. And few people charge during the day.

    But if he does have to charge during the day, then that's what he's saving, isn't it?

    I won't be getting up in the middle of the night to be boiling my kettle for my breakfast cuppa. So why should I or anyone only consider the savings at the cheapest point?

  • Registered Users Posts: 5,419 ✭✭✭ championc

    The point is, savings are based on night rate if a device or car can EASILY be used or charged at night.

    I think it's fairly safe to assume that 95% of cars can and will be charged at night. Timers can be set in car, or on many chargers themselves.

    Charging a car during the day, for most part, should be seen as a nice bonus, but in essence, that power is better served against getting you to midnight to avoid importing expensive units.

  • Moderators, Sports Moderators Posts: 17,403 Mod ✭✭✭✭ slave1

    Both EVs and Smart Charge points negate having to leave your couch to charge your EV, we've been driving EVs for over 5.5years now, the majority as a two EV household, once we had to charge during the day, other than that always at night rate.

    EV's are hungry, I put in 102kWh into our EVs one day last week, 50/50 Grid (night rate) and PV

  • Registered Users Posts: 3,531 ✭✭✭ graememk

    Ev charging should be done at night as daytime charging could be 2-3 x the price of night.

    Nobody is suggesting what your saying about your breakfast cuppa.

    If someone only charges on day rate, yes cost it at that.. but if they do they might as well bin the day night meter.

  • Registered Users Posts: 1,753 ✭✭✭ deravarra

    Should - but not always possible. Savings should only be calculated vs the time of use. Surely you would agree with that?

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  • Registered Users Posts: 1,822 ✭✭✭ bullit_dodger

    Technically your correct deravarra, but to look at it another way......would you charge your car every day during the day if you DIDN'T have solar and a day/night meter. No, you'd plug it in when you go to bed and use night rate. It's basically whatever time was the "normal" use for that component when you didn't have solar.

    Washing machines/dish washers the same. If you have a timer and used them at night before getting in solar, then you should count those as night rate. Some people don't like running appliances at night, others don't have a delay timer facility and have to run them at day time. Then sure, that would be day rate as it's what you normally would have done without solar.

    Ultimately, it's your math, if you want to fool yourself into thinking that your savings €3000/year, that's up to you. :-)

  • Registered Users Posts: 1,753 ✭✭✭ deravarra

    Not fooling myself into anything.

    So, taking your angle on things - what if I had an EV and used a combination of solar PV charging and night meter. And let's say my previous car was a diesel. How would I calculate the savings using an EV vs using a diesel? would I use the highest diesel price or the lowest diesel price to calculate my saving?

  • Registered Users Posts: 1,822 ✭✭✭ bullit_dodger

    So, taking your angle on things - what if I had an EV and used a combination of solar PV charging and night meter. And let's say my previous car was a diesel. How would I calculate the savings using an EV vs using a diesel? would I use the highest diesel price or the lowest diesel price to calculate my saving?

    Again ultimately it's up to you - but the normal way that people work savings is directly applicable to the solar installation. If I have a washing machine and I run it during the day, and then I get a D/N meter in and then run it at night to generate it fair to attribute that to a solar installation that I didn't purchase? it's nothing to do with solar - I just moved it from daytime to night time.

    Basically the kind of standard way that most people here look at solar savings it, is what savings have I generated directly related to the solar. To go back to the washing machine, if i ran it at night before I got in solar and then run it during the day to take advantage of the sun, really I'm only saving the night time rate. If I didn't buy the solar, I would have run it at night time, right?

    For your example, if before you had a diesel and then got in an EV, I think it's fair to give an average diesel price/liter. UNless you are the type of person who goes to the local station cause it's handy even though it might not be the cheapest (I do that myself). Whatever the norm was "before" you got in solar.

  • Registered Users Posts: 5,419 ✭✭✭ championc

    @deravarra The only person who can really truely claim a day rate saving for EV charging is a nighttime taxi driver, where there car is in use during the hours when if could be charged at low cost.

  • Registered Users Posts: 3,531 ✭✭✭ graememk

    We had an EV before solar and it was charged on night rate. (I think we charged once on day rate when I was away in the morning and had to re top up to 100 for a long run that evening.)

    Id be of the opinion of going with the cheapest within reason for savings.

    But charging the ev from solar would be reducing night rate units, and cost it that way.

  • Registered Users Posts: 17 viewofthehill

    I am enjoying this discussion around savings from PV and how people view them and as a result how optimistic/pessimistic the aback is. From my viewpoint I have come around to the view that only the house base load (during day rates) should be considered at the higher rate. All other use could potentially, while not ideal, be moved to the lower night rate tariff. This way the savings are realistic and the additional savings are the freedom and flexibility in using the excess (over base load).

    Im also interested in calculation of savings from diesel. A post earlier said how PV had saved €270 in diesel costs. Surely the majority of that saving was in moving to an EV (not a cheap move) and this would have been realised without PV?

  • Registered Users Posts: 1,822 ✭✭✭ bullit_dodger

    True the move to EV wouldn't be cheap, but it depends on what forced it. For example, if your existing car just went BANG(TM) and you needed to replace it or spend a fortune, it might not be too bad.

  • Registered Users Posts: 757 ✭✭✭ DC999

    You're right, they are different things. We got an EV and solar this year (but planned both to work together). We’ll save €1200 in petrol costs from last year (based on the night rate to charge). Easy calc. And likely it’s more as I haven’t bothered to look at the costs in petrol now – I just checked what we spent from mid 2021-mid 2022 (before prices went nuts) which was €1600. Simples.

    Where is gets a little muddier, is the % of free power your solar panels produce to charge the car. So for someone with a large setup (and that OP does afaik), their excess solar charges the car during the day. So night rate may not be needed. And for a fair chunk of the year, there would be excess to ‘dump’ into the car (assuming it’s in the driveway when sun shines). Make sense? We’ve done similar and have spent very little charging the car, but that will end soon season wise. So shows how solar and the EV go 'hand in hand'.

  • Registered Users Posts: 757 ✭✭✭ DC999

    Depends, we got a 2nd hand Leaf and save €1200 a year on juice. And €500 in tax. Ok, I could have bought an old diesel and reduced both, as in I didn't need an EV for it. But can spend as little or as much on an EV as you want. Ours is largely a Leinster Car(TM), and once we accepted the limitations of that we were grand. Of course we can go further but only does 150k before a charge.

  • Registered Users Posts: 3,531 ✭✭✭ graememk

    Got our EV in 2020, Just before lockdown, Still was doing 400km/week.. Savings were still there but not as impressive as when we ordered it.

    Had been on a lookout for a good long range ev (needed to be able to do a 300km run in winter.. Through Tyrone + Fermanagh so no chance of charging!)

    Had a feeling diesel has it days numbered and decided to move before we were pushed. Electric is costing us about 20/m now vs a current diesel cost of about 170ish/m vs our old car, an auto AWD sportage.. 45mpg on a good day. Not a huge amount of difference in the cars, a little shorter and a bit lower. Boot isnt great in either one!

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  • Registered Users Posts: 2,291 ✭✭✭ ECO_Mental

    **(After writing this I am thinking this whole conversation might be better in a new thread as we are going off topic of daily production but there we go anyway)

    Here are my calcs for how much I have saved by switching to an EV over the last 10 weeks. Before we start my previous car was a 2020 (201 Reg) 1.6l petrol Tiguan R-Line that was on PCP with VW. The Tiguan was my 3rd PCP deal with VW and I was paying about €610 pm for this, I went into VW and did the deal and they gave me an ID4 GTX Business for €450 per month with no additional deposit. So straight up I am saving €150pm in PCP loan...for that alone it was worth it. I am not including the PCP finance in my calcs.

    The basis of my calculations are below and are simple enough really

    Totaled up how much electricity I put into the car (only at night rate) including all external chargers ( a few of these are estimates French chargers so hard to get the data too lazy!) but within a few euro then divided by my total KM. Giving me €0.034 cent per KM.

    I then used 7.7l/100km for the fuel efficiency of the Tiguan, this off the internet as I don't know what it actually was but it should be around this or worse.. Then found out how many liters of petrol that would have cost me (at €2.00/litre, its less now but over the last 10 weeks it was around that price) and one minus the other gives me a saving of €717 over 9.7 weeks, which is about €295/pm saving.

    I need to do this every month to be accurate going forward as electricity prices are going up and petrol is volatile as well.

    Am I right!!!

  • Registered Users Posts: 757 ✭✭✭ DC999

    @ECO_Mental does it matter? And I don’t mean that to sound like a you know what. Why spend your own personal time each month checking petrol prices to add into excel when you don’t have one and don’t plan too?

    For me (as an over thinker anyway) it’s better to know I save at least a grand a year in juice on the EV (which I do). And if electricity prices rise (which they have) and petrol prices are all over the gaff (which they are), I’m still largely saving that grand. Close enough is good enough.

    Each to their own on how close they wanna calculate that, or if they even do at all. Fair play if people want to do that.

    When I got my last petrol car, I never compared the costs to run it against the last one. So why do that now? I think we do it with EVs as a form of justification to others. To settle their range anxiety worries, or whatever is their reason for not considering (again, each to their own). Like the way Irish people need to explain decisions like why they don't drink.

    I don’t have an ROI on the chair I’m sitting on now, or my TV. God know my car is only a depreciation suck, never an ROI on a car! Why do we get over so heated up about exact ROI on solar? Anyways, close enough is good enough for me personally. 

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,595 Mod ✭✭✭✭ Jonathan

    Split this out from the Daily Output thread.

  • Registered Users Posts: 3,531 ✭✭✭ graememk

    Ah! That's where the misunderstanding started.

    Yes you saved 295 from switching from diesel to ev.

    Your solar saved you about 25 from generating your own power.. then we got waaay off track 😂

  • Moderators, Sports Moderators Posts: 17,403 Mod ✭✭✭✭ slave1

    I don't do ICE versus EV savings anymore, just PV.

    Why don't you switch to the Energia EV plan, much cheaper night rate

  • Registered Users Posts: 2,291 ✭✭✭ ECO_Mental

    Wife works for ESBN so 😉 the EI rates work out good for us..... Planning to get a smart meter and use the 2 hour window every night they have to charge the car at the cheaper rate. Testing it over the past few weeks and 2 hours every night will have me covered no probs for all the driving I do.

    I do the ICE vs EV just a confirmation and peace of mind, probably get board of doing it later on during the year but its good to know.

  • Registered Users Posts: 2,291 ✭✭✭ ECO_Mental

    As far as I am concerned it is good to know and it matters....if you are going to make any significant capital investment you have to compare it to the counterfactual (what would be my cost if I did nothing and still use the ICE car) I did an initial rough calculation but out of curiosity I decided to do a more detailed calc. I get comfort knowing that yeah this month I saved this because I have an EV instead of an ICE. I will probably get board of doing this in a couple of months but I only have the EV 10 weeks now so its good to know.

    Also we are all involved here in an online discussion forum where we share info and there are people on here who don't have an EV or PV panels but are researching if they should get one. I always find it every useful when people share their numbers, experience and performance of their systems. Yes some of us can go over board with excel sheets and graphs and logging every bit of data but its like a hobby 😋 doing excel sheets and Home Assistant dashboards.

  • Registered Users Posts: 21 astaines

    Right now, we have a 24 hour meter. How do we apply for a D/N meter?

    We're with SSE Airtricity.



  • Registered Users Posts: 2,627 ✭✭✭ yankinlk

    A year ago when I did it, it was simple. Contact ESB schedule them to upgrade to day night. The provider was sse and didn't do anything in my case.

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