Solar PV vs solar hot water: high electricity use household

minnow · 29-03-2020 11:20am #1

Hi all,

I would like to install solar panels to reduce my electricity costs but am not sure whether to install solar hot water tubes or solar PV. I'm hoping to draw on your experience to advise me in my circumstances.

We have a modern well-insulated house with air-to-water heating system, underfloor heating and mechanical heat recovery ventilation but have high electricity consumption, approx 9700 kWh per year over past 2 years (350kWh, >1000kWh per month summer,winter). I assume that our high consumption comes from a variety of reasons:
- all heating/cooking is electrical
- house occupied all day
- heavy use of oven
- use of dryer
- largish house

I would like advice on whether it's better in my case to install solar PV to reduce import needs or to install hot water tubes to reduce the use of the heating/hot water system by feeding hot water into tank.

Thanks for any advice.

KCross · 29-03-2020 12:14pm

Your hot water costs should not be high since you have an efficient heat pump. Therefore, Solar tubes would never pay for itself.

Solar PV all the way. Get it as cheap as possible with no batteries (since house is occupied all day) and no hot water diverter. You will need to tell installers those requirements as they will try to sell them to you.

unkel · 29-03-2020 10:30pm

minnow wrote: »

I assume that our high consumption comes from a variety of reasons:

No, there is just one reason. You are heating your house with electricity.

The good news is that you are saving yourself EUR1000-2000 per year by not heating your house with oil / gas

I presume you have a night rate meter already? If not, give yourself an enormous kick up your own arse and arrange to get it first thing in the morning (it is free)

ted1 · 29-03-2020 10:38pm

unkel wrote: »

No, there is just one reason. You are heating your house with electricity.

The good news is that you are saving yourself EUR1000-2000 per year by not heating your house with oil / gas

I presume you have a night rate meter already? If not, give yourself an enormous kick up your own arse and arrange to get it first thing in the morning (it is free)

My gas bill is 600 a year. That’s a pretty wild statement to make a out saving 1000-2000 a year.

OP your heat pump is the big use. It’s COP decreases when you need it most. i.e when it’s colder you get less output.

Solar PV will also do the same. It’ll power your HP less in the winter when your heat pump is less efficient and needed more.

installing Solar to reduce your electricity bill is a bit dubious.

The pay back period is 10+ years. So you’ll have a big out lay ( probably paying it back monthly ) and you don’t see any saving ms till 2030.

If it’s well insulated, why do you need to heat it ? There’s something not right.

rolion · 30-03-2020 1:37am

@minnow

Im not an expert just a DIYer.

Can i ask how old is your house,to see if you qualify for the PV grant.
is it suitable for "some" type of solar systems,in relation to lcoation, position, shadows,obstructions ?
Are you good at DIY ?
How big is the existing hot water / heating cylinder ?
What type of HP you have,where is the internal element connected ?
Not lastly,what type of budget are we taking about ?

You could look at a mixture of:

-new 500l cylinder to act as a buffer for the water,from HP and from Solar Tubes
-new grant very cost effective subsided 40 / 60 Joule solar tubes with a 300l cylinder that will act as the main for all dhw and ch/ufh
-probable DIY route for a minimum 5kw array installed power to keep you happy_ish winter and happy with a assumed FIT in the good harvesting days to cover most of the house from 9am until 6pm
-lots of controllers and automation,zoning and sensors with timers heat controls and solar PV diverter integrated smart and intelligent with the HP
-use the heat recovery with an intake connected to a "heat source" buffer and HP ... and warm the house with a nice 20ish 24/7.

But,if house well insulated,can't really see the very high consumption.
Keep in your mind that the PV panels works in opposition to your demand: less generation when you need more in winter VS more generation when you dont need that much in the summer (without now FIT).
The solar tubes can act as the booster for lower temperatures in the main cylinder / buffer during winter times and can do most of the hot 80 degrees water from march until well in to november,helping or eliminating the HP usage.

Take care.

PS

i have all of your elements done DIY, except the modern house
These days,the solar PVs covers most of the day time consumption
From next week,i hope to be able to power off the gas boiler and rely mostly on the solar tubes
Heat recovery does the job keeping warm and/or cold the interior of the house.

unkel · 30-03-2020 12:56pm

ted1 wrote: »

My gas bill is 600 a year. That’s a pretty wild statement to make a out saving 1000-2000 a year.

It's obviously a rough value :rolleyes:

I use the average Irish household quantity of m3 of gas per year and my bill is about a grand. If your house is big, you use more. If your house is well insulated, you use less. If you like it to be 23C, you use more, if you use lots of hot water, you use more, etc.

garo · 31-03-2020 3:03pm

I don’t see the usage as massive. I don’t use electricity for heating or cooking, have LED lighting, house is not large and yet my annual consumption is close to 5000kWh. Another 4700 to cook and heat the house via a A2W pump is not exceptionally large. OP since the house is occupied all day if you think you can stagger equipment use during the day PV is a reasonable option. As rolion says it’s all about your budget and DIY capabilities. And if you qualify for the solar PV grant. If you can get 5kW of solar and a non-hybrid inverter so no battery or diverter installed for 5k you should get a reasonable payback. Mind you it would still be a 10+ year payback but you should be able to cut your bills in half or thereabouts.
The solar tubes option that rolion presents is doable but requires getting several things right. If you have the patience and skills then go for it.

ted1 · 31-03-2020 11:40pm

garo wrote: »

The solar tubes option that rolion presents is doable but requires getting several things right. If you have the patience and skills then go for it.

Don’t bother with tubes. The payback calculations don’t factor in yearly services, or the fact that you end up dumping hot water when you don’t need it.

In fairness by nephews love the hot water going into the paddling pool in the summer, a complete waste

Calahonda52 · 01-04-2020 9:20pm

winter - summer = 650kWh/month= heating= 21kWh/day
Am guessing your heat pump is say 27kW with a winter COP of 2.5 so input power of 10.8 for for 2 hrs = 21 kwh so i would surmise that all is well
what is the annual energy bill/sqm working out at?

Calahonda52 · 03-04-2020 9:14pm

Kathyel wrote: »

I have had one of these for two years now. It lowered my electric bill almost 40 percent.

One of what?

fenris · 01-06-2020 12:37pm

Solar tubes, sound great but are an absolute pain to get anyone to service or fix problems, installers are tripping over themselves to do an initial install and help with grant applications but they nowhere to be see if you have a problem afterwards that is any more complex than changing the glycol. I have a Kingspan system (tubes and controller) that worked well for about a year and has given me nothing but trouble since then, including leaking with Glycol dripping back along the pipes to drip into my hotpress!
So to me a fault can happen with any system but the fact that there is no proper after sales service to support the solution is a real deal breaker, I am in Greystones so well within the catchment area of most of the big players, I have spent over 2K on unsuccessful maintenance of the system including a replacement pump, not to mention the days lost to waiting at home for various geniuses to turn up, tell me that they need to go onto the roof but aren't allowed by their company but will try just bleeding the system instead. I spoke to a Kingspan rep who told me that he would send someone to take a look for €450 plus labour and parts, in other words piss off and leave me alone - WTF!!!!
So the nett result of trying to do the right thing is a big waste of cash, lost days of work, an hotpress full of destroyed clothes and bedding with a bloody Dalek in it taking up most of the useful space.
So my advice is run a mile from solar water unless you are actually working as an installer, in which case you would know not to bother installing one anyway.

unkel · 01-06-2020 9:49pm

Some true points there, Fenris, but you have obviously dealt with a shower of incompetent service companies. I too had a malfunctioning system. It was caused by a leak in the attic (probably caused by ourselves moving stuff around in there) and it was fixed including a complete replacement of the glycol and some anti-leak fluid for EUR250 incl VAT

And if you want to do it on the cheap, any plumber should be able to fix a solar thermal system really. It is a very basic system

I had my system installed in 2016 when there were no subsidies on PV and PV panels and other parts were still extremely expensive. With the huge subsidies on solar PV nowadays and super cheap panels and other parts, I would recommend a (very large) PV system over a solar thermal system myself

spose · 02-06-2020 12:29am

I have both. Tubes for 12 years and still going strong. Panels for 6 months. If I was starting from scratch now I’d probably just go with the PV with the larger cylinder, preferably with a top and bottom immersion for the iboost

roy rodgers · 04-06-2020 7:48pm

I work on both thermal and p.v and I have to say the solar thermal has turned into an nuisance for us with leaks coming up after 3 years or so. The main reason for this to happen is the extreme heat that is being produced at the manifolds on days like we had over the past few weeks. The way seai calculated the size of the system had to correspond to the size of the foot print of the house and the direction of the roof. Which was a flawed calculation because it wasnt address the household needs so most of the installs are actually oversized for the households needs I'm finding now.
I dont recommend solar thermal unless it did make sense for someone who wants to heat a swimming pool or the likes!! But solar pv is the better way to go. I'd be a big fan of the bigger systems to get your return back quicker..

unkel · 05-06-2020 12:35pm

roy rodgers wrote: »

I work on both thermal and p.v and I have to say the solar thermal has turned into an nuisance for us with leaks coming up after 3 years or so. The main reason for this to happen is the extreme heat that is being produced at the manifolds on days like we had over the past few weeks.

Hold on a second, that only happens on cheaper - often Chinese - systems. A high end Kingspan system doesn't let the temps go over 95C or the tubes will vent themselves. So there is no pressure at all on the system. In cheap systems the temps can go as high as 200C, which causes extreme pressure within the system, leading to the problems you describe.

dathi · 07-06-2020 11:30am

unkel wrote: »

Hold on a second,. In cheap systems the temps can go as high as 200C, which causes extreme pressure within the system, leading to the problems you describe.

only if the installer didn't bother their arse installing a heat dump. 12 year old Chinese system (with heat dump) still going strong collector never above 95

unkel · 07-06-2020 1:32pm

Surely all non self-venting systems must have a heat dump? Otherwise they'd just explode during a nice few days in summer when you're away for your holidays (not using any hot water)

Solar PV vs solar hot water: high electricity use household

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