New build eco-house solar pv and battery sizing

[dan_b]

Hi chaps
I have a friend who is in the process of designing a new build which he wants to be as eco as possible - they're going for passiv-haus levels of insulation and heating demand, but the bit they're now trying to work out is the size of the solar and battery they should look to install.

Roof size wise with modern 400W panels he could probably get 16kW of panels on a south-facing roof.  But there must come a point where there's really no value in putting extra panels on and spending more money on storage or other self-consumption devices, and of course there's no FiT payment anymore?

I have said to him to consider perhaps a smaller south-facing array but then to add some west-facing panels to extend the evening generation profile, but basically - for those of you here who are effectively zero-import on electricity, or actually fully off-grid,  what sort of sized PV and battery array are you looking at as a rough rule of thumb?

They'll get a decent sized battery system like a Tesla PowerWall, and they're looking at the SunAmp heat batteries for hot water storage plus occasional heating requirements.   But they're intending not to have a gas connection at all, so cooking would be electric.  They're looking at a  heat pump for occasional heating requirements in the winter.

Is it really as simple as "fit as much solar as you can", or is there a point where there's really no point adding any more, and indeed you'd be better off spending on extra batteries?

Any thoughts/ suggestions?

[Fintray]

Dan

I've 14kWp of PV (3.87 which gets FIT's) and a Powerwall 2 battery and in the last 3 months have imported about 6kWh from the grid, A PV diverter has supplied all my hot water and extra power has been used to supply heating when required. There is also the option of going for the Outgoing Octopus tariff from Octopus Energy which gives a flat rate 5.5p/kWh or a variable rate if you go Agile.


So if your friends have passiv-haus levels of insulation their heating requirements should be pretty minimal and if they go for A++ rated appliances their electricity requirements could be very reasonable meaning more to export if they can get approval for that level of export, failing that they could install SolarEdge inverters and limit their export.

[A.L.]

hello,

- I would limit it to an annual output 2x their current/likely annual appliance use. The summer excess can heat the hot water initially with an EV in short order to soak up the rest, maintain their eco-credentials. After all it will always be possible to add more later (although that will be subject to VAT). As for batteries, 4 usable days storage should be the max.

[brackwell]

I think a lot of this depends on where you live,no.of people, no/mileage of EVs. and other large personal loads

I live on the south coast and if i was building paasivhaus standard i would not have a heating system relying on passiv solar, and a pellet stove and/or leccy storage heaters (on E7) for just in case.

Ken

[nowty]

Solar PV is so cheap now that I would "fit as much solar as you can" as you will never have enough in winter, you will wish you had more in Spring / Autumn and you can always sell excess back to the grid in summer. This is especially so if your friend is going for a Powerwall as this will help soak up the excess generation.

[dan_b]

Thanks that’s basically what I thought - I have suggested he might want to try and pop some West facing panels in to stretch the generation profile out a bit.
With panels now routinely in the 300+ watts rating does anyone care about panel efficiency anymore or is it really just down to get as much kWp as you can for the lowest acquisition cost?

[Iain]

Hi

I have suggested he might want to try and pop some West facing panels in to stretch the generation profile out a bit.

Possibly less important with a battery as the battery will "Stretch the profile"

Iain

[billi]

Exactly as Iain said , if the  array is on the south facing roof then  at that size it will still deliver   some   at  the morning and evening  ( when  the kettle is on  for coffee in morning or the pizza oven  in the evening )

Maybe me sentimental , it hurt me to   have the  el. oven on in the evening  and too much power came from the battery  .... but  i have  only 4 kW PV

the tesla  power wall  is far too small and far too expensive  , a 100 kWh   industrial fork lift battery similar price ,  will suit a 10 -15 kw PV much better  , for  beside electricity , for heating , hot water and electric driving

a 15 kW PV is doing about 20 kWh on average a day in November  in South UK  , so ....



Billi

[Scruff]

What are the benefits of a battery for a grid connected installation?

[billi]

well what are the benefits of  a big PV then in times of no FiT ?

[nowty]

Hi

I have suggested he might want to try and pop some West facing panels in to stretch the generation profile out a bit.

Possibly less important with a battery as the battery will "Stretch the profile"

Iain

I agree that if you have a small PV array, then a battery will stretch the profile, thus better to have it all facing south, but I find that if you have a large array it helps reduce the peak power generation problem if you spread it around and its more efficient to use the power as its generated rarther than store it in batteries.

My last lot of PV has gone on my NE facing roof. Partly cos there is no more room elsewhere for it !, but also because it helps me run my heatpump from very early in the morning (Mar to Sep) without solely relying on the batteries.

[Scruff]

well what are the benefits of  a big PV then in times of no FiT ?

Competitive cost-effective power.

A battery is an efficiency reduction device of arguable financial merit. Data to prove otherwise from existing installations is not something I've witnessed. I'd love to see some. 
Can it compete with cables and direct drive? Why does it compete with cables and direct drive?
Looks like an expensive endeavor in proprietary electron farming.

Genset offsetting is an entirely different fish of kettles.

[billi]

Scruff , there is a other point too in my opinion

While there was a war in Irak   and the the german government decided at the time to send  soldiers there , there was a movement of  people  called "no blood for oil"   many business people of that movement refused  to pay their tax  at those years

If anyone   refuse to   connect to the grid  and tries not  to joining criminal politics , like it is happening in the the UK and Germany right now , with  minor or close to nil  incentives  towards a faster  conversion towards a fossil-fuel  free politics  , then   i am still convinced to  advice people to cut the line and do not  wait .

That was my decision  15 years back  to not connect to the Irish Grid

So being off grid is more viable and cost efficient nowadays  then ever ,  sure  it needs  a situation that suits   and some investment upfront ( that does not mean its more expensive then  a grid  connected idea )  , but with those factors of electricity ,  heat  and hot water ,  electric charging  one can  utilize  own sun power pretty   good  and  with a sunny smile

Billi

[dan_b]

Right so to summarise the advice here is now
Big arrays are pointless
Storage is pointless
Going off grid is the only solution
But only if you have a big array and a big battery?

[knighty]

I'd go for as much PV as possible... but as you said split some onto the other side of the roof... maybe keep most of it south facing?

do you know what his export limit is? - it could be an option to miss out the battery for now and cover both roovs in PV, probably more cost effective (with 5p/kwh for export)

then... if he wants batteries add them later when they're cheaper?  (prices are falling pretty quick?)