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# PV System Size Advice

• 04-11-2020 4:04pm
Registered Users Posts: 194 ✭✭

I was hoping someone could help me calculate the PV system size I should go for.

My house uses a A2W HP for heating and hot water. We recently changed electricity provider so I lost all my historic data. All they can tell me now is over a 14 month period I averaged 580 KwH each month.

Obviously I used more over winter than summer but I'm not sure how much more would be likely.

At a guess, I'm going to go with 70% summer usage so 70% of 580 is 406.
Then 30 days in a month so 406/30 = 13.5 KwH.
Then 70% of 13.5 Kwh for day use = 9.45 KwH.

So if I got a 9 Kw Solar PV system, chances are I would use all the electricity generated each day for the year.

Is that logic sound (I'm very new to this)?
Also, is 70% for summer usage of the average reasonable enough?

Thanks for the help

• Registered Users Posts: 574 ✭✭✭

Your math is not quite as reliable or as predictable as that.

Here's another way of looking at it - How big is your roof? What orientation is the roof?

As a reference, my 16-panel 5.2kwp solar array has so far generated 17kwh today. Which is a pretty good showing for November, production for me would peak during summer at 30-35kwh per day - often more than I can use, but I've got the EV outside that it works out well.

What make/model of heat pump? For me, my heat pump may has very little to do with my solar, due to the fact that I run it mostly at night rate, and it doesn't haven't any EDDI-type excess solar integration for heating water. I try to time my mid-afternoon water temp top-up at a time when there is usually either battery buffer built up, or decent afternoon solar, but that's as close as that integration gets (This is with a Daikin, other makes may have better integration, but the usual immersion type diverters aren't a reliable bet here)

Again, the heat pump can be a red herring, as the solar production and heating demands are often at odds - loads of solar in summer, very little need for heating. Lots of need for heating at night in winter, with no solar available.

I would try to separate out your heat pump usage, and find your other usage, and figure out what power you would be able to make use of on production. Can your washer/dryer during the day on sunny days? Electric cooker or gas?

• Registered Users Posts: 194 ✭✭

Thanks for the reply.
Here's another way of looking at it - How big is your roof? What orientation is the roof?

House is SE facing semi-d, 200 square meters with standard pitched roof.
What make/model of heat pump?

9KW Samsung Series Gen 5. I don't have any smart setup with this at all.
Can your washer/dryer during the day on sunny days? Electric cooker or gas?

I work at home all day and yes the washer/dryer is on during the day. There is no gas/oil in the house, everything runs of electricity.

Based of your setup then, even a 5kwp setup would be more than I can use most days. Is your house SE facing as well by any chance?

• Registered Users Posts: 574 ✭✭✭

Not too different to me, I'm 150degrees to the South, so just about SSE facing.
11kw Daikin, 2600sq/ft, A2 rated, and ditto work from home.

My daily usage is rarely below 30kwh (excluding EV charging), which is high (too many tvs, computers, and gadgets means high 'idle' usage) so I'm rarely short of production. I'd put more solar on if I could.

I do have an EV also, so on those big solar days, I've usually got somewhere to 'put' the power.
Also, with 2 small kids, there's always washing/drying to be done.

• Registered Users Posts: 194 ✭✭

That's great, thanks for the info. Much appreciated.

I'll start thinking/researching around a 3-4 kwp system and go from there

• Moderators, Home & Garden Moderators Posts: 4,495 Mod ✭✭✭✭

That's great, thanks for the info. Much appreciated.

I'll start thinking/researching around a 3-4 kwp system and go from there

As you are an all electric house, If you have the space, go as big as you can, At least 4kwp but more like 5 or 6kwp.

• Registered Users Posts: 194 ✭✭

graememk wrote: »
As you are an all electric house, If you have the space, go as big as you can, At least 4kwp but more like 5 or 6kwp.

Yeah I have the roof space for it, just a question of cost really.

• Moderators, Society & Culture Moderators Posts: 35,548 Mod ✭✭✭✭

Yeah I have the roof space for it, just a question of cost really.

Spend extra on additional panels now, dont mind the battery for now.
You use the power generated in real time with A2W HP, so you need the generation rather than the storage for now.

• Registered Users Posts: 194 ✭✭

Yeah, I think it will just be panels and inverter for stage 1. I'll overspec the inverter for future proofing.

Maybe add a hot water diverter down the line if it makes sense.

Batteries will be the last thing and I'm not sure they will really work out for my use case anyway.

• Moderators, Sports Moderators Posts: 17,897 Mod ✭✭✭✭

Go for as many panels as you can afford, totally gloomy overcast today and I'm still covering the house load no problem

• Registered Users Posts: 194 ✭✭

I may as well use this thread rather than start another.

During my research I came across the below image located at:
https://www.pluginsolar.co.uk/?product=plug-in-solar-3-5kw-diy-solar-power-kit-with-roof-mount-for-tile-or-slate-roofs

The quote that caught my eye was With standard Solar PV kits, electricity generation is dependent on the panel that is performing the worst

Does that mean if I had 20 panels on the roof and 1 became completely 100% shaded/covered, that no energy would be produced from the entire system?

• Closed Accounts Posts: 1,997 ✭✭✭

I may as well use this thread rather than start another.

During my research I came across the below image located at:
https://www.pluginsolar.co.uk/?product=plug-in-solar-3-5kw-diy-solar-power-kit-with-roof-mount-for-tile-or-slate-roofs

The quote that caught my eye was With standard Solar PV kits, electricity generation is dependent on the panel that is performing the worst

Does that mean if I had 20 panels on the roof and 1 became completely 100% shaded/covered, that no energy would be produced from the entire system?

Yes you need micro inverters like solar edge or emphases to over come this. Shading of a tree or chimney can do the same.

• Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 62,736 CMod ✭✭✭✭

Yep. It's quite shocking (or not ). I put my hand on one panel (covering maybe 3-4% of the panel) and production dropped by 50%. This drop will occur in the whole string, even if you have 100 panels in that string

You either need to have an optimizer on any panel that gets shading, or use individual micro inverters per panel, rather than a string inverter

• Registered Users Posts: 194 ✭✭

My concern is more around moss and leaves on the roof as we have a lot of tall trees around us.

Optimisers seems to be half the price of mini inverters but I'd prob need to put one on each panel. Need to get the biggest panels I can to buy less optimisers I guess :rolleyes:

• Moderators, Sports Moderators Posts: 17,897 Mod ✭✭✭✭

I looked at optimizers for my 10 panel string with heavy shading, that's a lot of optimizers and they have a limited life span so I decided not to go for it, my system is too big for mini inverters.
Guess there's an element of you just have to accept elements like tree shading, I like looking out at the trees outside my house so just accept the negative impact they have on the applicable string...

• Registered Users Posts: 194 ✭✭

Yeah I accept that I may be better of without them but where did you see they have a limited lifespan? The info I found suggests the warrenty is longer than some panels:

SolarEdge Warranty Period: 25 years for the optimizers and 12 years for the inverter respectively

https://www.solaris-shop.com/blog/power-optimizers-everything-you-need-to-know/

• Moderators, Sports Moderators Posts: 17,897 Mod ✭✭✭✭