Solar PV microgeneration sweet spot

MicktheMan · 08-12-2023 9:44pm #1

I would welcome comments on the following question(s) about whether there is a general rule of thumb or sweet spot for PV. I've seen comments that you can't have too much but I'm wondering if this is really the case.

I have a 'T' shaped 240 sqm bungalow with a 30m due south facing and 15m east/west facing roof with no obstruction nor shading. Pitch is roughly 35 degrees.

Our annual electricity consumption is 2800 kWhr, space heating is 900 litres of kerosene and hot water a further 250 litres. Don't drive an EV currently but average 25k km a year in my 14 y/o diesel (still averaging 4.5 l/100km with 340k km on the clock).

There could be an air to water HP and/or EV in the future but nothing planned as yet.

Now that FIT is here, is there a sweet spot for how much of my roof I could or should cover with PV? (Assume funds available earning very little on deposit)

The questions I have are (in no particular order)

Are there planning restrictions on array size anymore?
Are there any house insurance implications?
Use the grid as a battery or install a battery (what size) or not?
What strategy would generate the strongest ROI, both now and in the future if an EV and HP were in the picture? (try to anticipate what I may need in the future and install all in one go or wait and do a piecemeal install).
Practical matters such as maintenance, service life, aesthetics etc
Is there a physical limitation of the electricity supply infrastructure into the house
Any thing else I haven't thought about?

Thanks in advance

graememk · 08-12-2023 10:41pm

Fill your roof!

on the NC6 the max inverter is 5.75kW (25 amps) but they dont come in that size, so its a 5kW

Would say to oversize (likely 8-9kWp), you'll clip on the really good days, but gain on the poorer days.

No planning implications for roof (but i think there is a limit on a ground mount)

Batteries: The saving with batteries is load shifting (charge on night rate discharge on day rate), Could be a factor if you get a HP, But you'd need a big one, and for it to be cost effective, Likely DIY.

Strongest ROI : Possibly a solar only, Export the excess. EV should be charged on night rate anyway, Even more so if Export is higher than night rate.

DC999 · 09-12-2023 12:22pm

Not for me re 'Are there any house insurance implications?'. Policy cost didn't change when I added 12k of solar.

I did renew policy this year and cost went up as tends to happen each year. I don't believe solar was a factor in that. Usual 'global insurance policy costs are climbing' spiel at renewal time.

Practical matters such as maintenance, service life, aesthetics etc: No maintenance year to year. Panels will outlive us. Inverter will die within a decade. Will cost under a grand to replace (if out of warranty) plus labour. Aesthetics are down to 'beauty is in the eye of the beholder' :)

Use the grid as a battery or install a battery (what size) or not? You are low usage energy house (for now) so ignore battery for now and max solar export on the 20C+ export rate (FIT). EV will charge on charge on night rate (if you get one). Relook at battery in future if move some heating to electric. Batteries tend to be devisive so you can park that decision for now.

allinthehead · 09-12-2023 10:39pm

As Graeme said, fill that roof. The future is electric and the best rates to be had are now.

Get a hybrid inverter and if you feel the need you can add a battery down the line.

The best return on investment is without battery at the moment due to the generous fit rates, which won't last forever imo.

I installed 12 panels thinking I wouldn't need a battery, I didn't, but I wanted one, and now I have 15 kwh of batteries and a booking for 15 more panels on my roof. Renewables are seriously addictive. Our usage before panels went in was about 4mwh per year. We added an Ev too in that time so probably about 6.2mwh now.

My plan is to remove the need for gas outside the winter by installing an Eddie and to charge the car from solar from March to October. Heat pump and MHRV in the future too.

zg3409 · 11-12-2023 3:29pm

Firstly the grant caps out at around 4kW of solar, so big discount on first part. Panel prices have dropped massively and vat now no longer applies. Grant reducing from Jan 2024 so this year was sweet spot.

No grant for batteries.

Personally I would get a few quotes from installers. Most can quote based on eircode and they use a satellite view to estimate solar gain and number of panels.

In terms of oil usage, solar wont heat your home in winter unless you crazy oversize it and have an A rated home. Values of 10% of peak power are common on cold days in winter. It's mainly a spring summer autumn technology and will reduce electricity usage by say 30%.

You will get free hot water in summer and ideally shower using water from hot tank.

A rough guide is maybe saving 400 a year off electricity bill.

If you are willing to use dishwasher only at lunch time in summer and same with washing machine and do main electric cooking at mid day you can keep electricity bill low. Heating home in winter is not realistic beyond A rated and then south facing windows would be better

That said extra panels are cheap and oversize means autumn and Spring savings increase. You get energy for more hours of the day but core hours are 10am-2pm. Get a few quotes by email based on eircode and see what advice they give you.

allinthehead · 11-12-2023 4:08pm

https://www.boards.ie/discussion/comment/121483044#Comment_121483044

The savings are much greater than €400 a year.

A 4.8kwp system costing about 6- 7k after grants will produce roughly 4000kwh annually.

Let's say your self use rate is 25% for easy maths, and you're on a plan with Energia giving day unit rate of .36cpkwh. That's €360 saving. Exporting the rest at .24cpkwh gives €720. So €1080 all in, 6-7 year payback.

Maybe I misunderstood you?

bullit_dodger · 11-12-2023 5:45pm

As many have said already fill that roof, but to give some context as to "why" that is sweet spot - let me clarify a little.

Let's say your getting a 5Kw inverter and it can handle 50% over spec. i.e. it can handle 7.5Kwp in panels although as you know ....it will only produce 5kw even if you have more panels and will in effect "clip". For the sake of some easy math the inverter costs €1000 and you are getting 500w panels.

So with 10x panels, your "inverter cost per panel" is €100/panel (€1000/10), with 15 panels which is your 7.5Kwp, your "inverter cost per panel" is €1000/15 = €67/panel. Basically it's the same inverter your using in both scenarios (and same cost), so you should maximise it's use. Following the logic and going the otherway, if you only got 1x500w panel, then your "inverter cost per panel" is €1000. (lets forget start up voltages etc)

Back in the day (pre-FIT) there was an argument since we didn't get paid for export, that you wanted to size your system so that you had enough for self-consumption most of the year, but you weren't OVERLY exporting in the summer months. Basically you wanted to be able to cover yourself in say March or October, but not be buying panels needlessly as you were in effect giving it all away in May-Sept for nothing.

With FIT inplace (even if they drop it to say €0.10/kwh) you'll make back your money for sure on every panel you buy, so it makes sense to "go big".

Batteries are a different ball game. I can see both sides to this. I'm of the opinion "get one"....but that's sort of paying the hit up front and then for years your going to have smaller bills as you'll use your night rate during the day time. How much it will it save is very dependent on the size of the battery, your usage profile, the rates you are on with your supplier......but for me with Energia my 8.5Kwh battery saved me €29 in November just gone over if I didn't have one.

paulbok · 17-12-2023 9:05am

I'd also echo 'filling the roof' with panels to start with and not worry about a sweet spot, particularly if you plan to add EV and HP in the future. Most of the installation cost is due to the actual call out and roof access, so that will cost much the same for 10 panels as for fifteen (assuming a regular enough roof). Getting an installer back to add 5 more down the line will cost way more per panel.

Over sized arrays will produce far more than you can use or export in the good days in the summer, but will maximise self use in spring and autumn, and 'help' in winter.

Planning restrictions on roof installs are gone unless you have a listed building.

The only thing insurance related to worry about, is to add the full cost of the installation/equipment to your rebuild costs when calculating that.

To maximise ROI at the moment, exporting as much as possible is the way to go, but as others have said, the rates currently on offer won't last forever so make sure you get a hybrid inverter so you can add a battery down the line if you don't get one initially, and have them do the wiring for one as well. Then if/when you do decide to get one, it's just a 'plug and play' install.

An Eddie will also cut your oil usage esp for hot water from March to Sept. Though at the moment an exported kW is worth more than it's equivalent in oil. Again, when the export rates get cut, that will reverse

A battery now will lengthen your ROI, but I'd be in the camp for having one. There is probably a sweet spot for a battery, depending on your usage and PV production.

I've a 6.84kWh array with a 6kW hybrid inverter & a 4.8kW battery, with higher than average usage. The sweet spot for me would have been around a 7kW battery, but with the cost of batteries 2 X 4.8 kW would make more sense. I do plan to increase battery storage at some stage to make better use of night time charging, once I move to a smart meter. I may go down the diy route but more than likely will get an off the shelf one.

Solar PV microgeneration sweet spot

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