Solar PV savings estimation, have I gone wrong?

the_amazing_raisin · 02-09-2022 9:42am #1

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? 😁

yankinlk · 02-09-2022 10:07am

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! :)

the_amazing_raisin · 02-09-2022 10:37am

https://www.boards.ie/discussion/comment/119558413#Comment_119558413

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

the_amazing_raisin · 02-09-2022 10:43am

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

bullit_dodger · 02-09-2022 10:54am

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.

the_amazing_raisin · 02-09-2022 11:35am

https://www.boards.ie/discussion/comment/119558690#Comment_119558690

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

bullit_dodger · 02-09-2022 12:40pm

https://www.boards.ie/discussion/comment/119558906#Comment_119558906

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.

slave1 · 02-09-2022 1:00pm

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

the_amazing_raisin · 02-09-2022 1:54pm

https://www.boards.ie/discussion/comment/119559410#Comment_119559410

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

the_amazing_raisin · 02-09-2022 2:00pm

https://www.boards.ie/discussion/comment/119559303#Comment_119559303

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

slave1 · 02-09-2022 2:07pm

https://www.boards.ie/discussion/comment/119559731#Comment_119559731

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

the_amazing_raisin · 02-09-2022 2:20pm

https://www.boards.ie/discussion/comment/119559837#Comment_119559837

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

DC999 · 02-09-2022 2:29pm

@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)

slave1 · 02-09-2022 2:35pm

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

the_amazing_raisin · 02-09-2022 2:47pm

https://www.boards.ie/discussion/comment/119560072#Comment_119560072

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

slave1 · 02-09-2022 4:27pm

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

the_amazing_raisin · 02-09-2022 7:15pm

https://www.boards.ie/discussion/comment/119560710#Comment_119560710

I need to find a good roofer then 😁

paulbok · 02-09-2022 7:44pm

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.

KCross · 02-09-2022 8:14pm

https://www.boards.ie/discussion/2058261503/solar-pv-savings-estimation-have-i-gone-wrong

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+! 😉

the_amazing_raisin · 02-09-2022 8:18pm

https://www.boards.ie/discussion/comment/119561726#Comment_119561726

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

the_amazing_raisin · 02-09-2022 8:26pm

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

https://www.seai.ie/grants/home-energy-grants/solar-electricity-grant/SPV-Contractor-Technical-Bulletin_2022_08.pdf

KCross · 02-09-2022 8:29pm

https://www.boards.ie/discussion/comment/119561742#Comment_119561742

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!

the_amazing_raisin · 02-09-2022 8:44pm

https://www.boards.ie/discussion/comment/119561786#Comment_119561786

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 😂

KCross · 02-09-2022 8:53pm

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.

poker--addict · 02-09-2022 8:59pm

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

the_amazing_raisin · 02-09-2022 9:05pm

https://www.boards.ie/discussion/comment/119561922#Comment_119561922

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

KCross · 02-09-2022 9:14pm

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.

yankinlk · 02-09-2022 9:17pm

https://www.boards.ie/discussion/comment/119561786#Comment_119561786

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).

KCross · 02-09-2022 9:46pm

https://www.boards.ie/discussion/comment/119562002#Comment_119562002

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!

KCross · 02-09-2022 10:02pm

https://www.boards.ie/discussion/2058261503/solar-pv-savings-estimation-have-i-gone-wrong

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.

yankinlk · 02-09-2022 11:28pm

https://www.boards.ie/discussion/comment/119562174#Comment_119562174

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

poker--addict · 02-09-2022 11:42pm

What sort of self consumption percentage are people assuming or realising without batteries?

In my own case, array will be larger circa 7kw, with household above average usage.

the_amazing_raisin · 03-09-2022 8:24am

https://www.boards.ie/discussion/comment/119562324#Comment_119562324

You're correct that I'm planning to charge the battery at night. I did rejig the calculations to account for the losses of charging and discharging the battery. Didn't seem to make a massive difference to the payback, maybe added a year worst case

KCross · 03-09-2022 8:29am

You're correct that I'm planning to charge the battery at night.

your battery won’t cover all your usage though. You will be importing.

DC999 · 03-09-2022 9:08am

https://www.boards.ie/discussion/comment/119562933#Comment_119562933

I'd be keen to see that too.

I'm only live 5 weeks but only self consuming 40% average and that's with an ev on driveway a lot. Have backwards spinning meter so not in a panic to get it higher.

Assume I have to go down the automation route if I don't buy a battery. But again that's a rabbit hole for me and will suck time. As is a DIY battery if I went there. So solar moves to being a hobby at thay point or at least taking the time of a hobby..

Have a look at the monthly stats thread. Some include their self consumption but most there would have a battery I expect

spose · 03-09-2022 9:25am

Coming up on 3 years with 4.7 e/w and no battery. High enough users. Have used 50% in the house, put 30% in the hot water and 20% exported

KCross · 03-09-2022 11:27am

https://www.boards.ie/discussion/comment/119563549#Comment_119563549

That’s very good. Battery unlikely to pay for itself with your 20% excess, particularly since you’ll get paid for that 20% now as well.

slave1 · 03-09-2022 12:00pm

20 months, 10.7MWh generated, 43% to Water, 28% to Home, 16% to EV, 13% Exported.

Of imported electricity, 84% night, 16% day

yankinlk · 03-09-2022 12:12pm

https://www.boards.ie/discussion/comment/119564006#Comment_119564006

Small array tho - so what are the import numbers? especially going in to winter. And if they have a day night meter than FIT wll be an estimate anyway. SO i get to fill battery and get paid for export same time! ;)

yankinlk · 03-09-2022 12:13pm

https://www.boards.ie/discussion/comment/119564164#Comment_119564164

84% night! thats amazing? done with a battery or are u just a night owl?

slave1 · 03-09-2022 12:17pm

No battery (yet), if you think about it I have little need for electricity during the day with PV in place so that will lower day time import, also from a maths perspective we have two EVs so a lot of Winter night rate usage (along with moving washing machine/dishwasher etc to night time, immersion too obviously)..when I get the battery up and going that 13% export should drop considerably and eat into the day/night basic house load usage.

PV is a long game, start, observe/learn from usage and expand through more generation or battery/FIT

poker--addict · 03-09-2022 1:25pm

13% export with no battery, on a large system, that is impressive, if one is an average or above average user of hot water an EDDI may be the simple middle ground while waiting to consider battery options? 2 small kids in our house so bath requiring hot water almost daily. Are you actually using that amount of hot water?I suspect some people are heating water with EDDI's because they can without considering the value of doing so.

the_amazing_raisin · 03-09-2022 1:28pm

I'd like to get an Eddi as well but my heat pump has the controller built into the cylinder, so I'm not sure I'd be able to hook up to the immersion

KCross · 03-09-2022 1:34pm

https://www.boards.ie/discussion/comment/119564551#Comment_119564551

Some heat pumps have the ability to fire up their immersions via an external signal on their AUX lines which you could then control. Might be worth pulling out the manual and see if you can directly control the immersion. If it can, it would be alot cheaper than an Eddi which is €500 I think to buy. Worth looking into anyway.

What you dont want to do anyway is start cycling your compressor. That's a bad idea for HP longevity.

slave1 · 03-09-2022 2:05pm

https://www.boards.ie/discussion/comment/119564529#Comment_119564529

4/5 of us, multiple showers so 100% yes to using the water, after house load it's the first I divert to. Maybe in a few years the battery will get more but that's us for the foreseeable...

spose · 04-09-2022 1:06am

https://www.boards.ie/discussion/comment/119564006#Comment_119564006

If I was staying with existing system I wouldn’t bother with a battery but adding another 3kw of panels so that should help feed the new battery

KCross · 04-09-2022 9:34am

https://www.boards.ie/discussion/comment/119567815#Comment_119567815

It will of course help feed it but will it be worth it relative to just pocketing the FiT payment?

Have you got quotes for the battery?

spose · 04-09-2022 10:40am

https://www.boards.ie/discussion/comment/119568266#Comment_119568266

Everything is DIY so overall price won’t be too bad. Higher than it could be since I’ll have 3 inverters in the end but like a good few here it’s turned in to a bit of a hobby I’ve enjoyed messing around with.

slave1 · 04-09-2022 10:45am

3 inverters FTW, hobby is one word for it

spose · 04-09-2022 11:04am

https://www.boards.ie/discussion/comment/119568538#Comment_119568538

just 2 for PV and an me3000

curioustony · 04-09-2022 11:55am

@the_amazing_raisin this is the same journey that I am on (retrospectively in my case). I did ballpark figures before installation, and reckoned I would save money. I did.

Then I started to get real data (from the inverter) and the time of use stuff threw me for a loop. So I wrote a python app to help me out. I stuck it on github (You can read more here https://www.boards.ie/discussion/2058252242/calculating-the-best-time-of-use-tariff).

You are the exact audience I was thinking of so quite curious if it matches your own figures...

Solar PV savings estimation, have I gone wrong?

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