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Hello.

Sis has just looked at a house with a possible view to purchase. It has just had 30 PV panels fitted along with one battery, and this is used to run the fully electric CH 'wet' system. (This has only been installed for 1 month, so owners have little idea of how effective it is going to be.)

The owner admitted that they were recommended to have a second battery fitted, but turned it down as it would have cost a further £6k.

I'm afraid I have no details of the system other than what I've outlined above, but is it possible to estimate how 'effective' such a system would be in annual use? Ie - what are the chances of it covering, say, half the total energy use? More? Less?! And how much difference would a second battery make? (How much should an additional battery cost?)

It's a detached 3-bed modernised schoolhouse in north Devon.

Many thanks.
 
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  • Thread Starter Thread Starter
  • #2
No thoughts on how effective/useful a 30-panel system with one battery should/could be? :)
 
It is impossible to reply to you as you haven't provided enough information.
30 panels is a huge amount for a standard UK home, most aren't big enough to fit that amount on, therefore people need panel details, what output wattage are they. It can range from 100W to 400W.
Then what size is the battery, it could be 1 x 2kWh battery or 1 x Tesla powerwall @ 13.5kWh.
The variables are just too large to give you any advice without more details.
 
Simply,

1- Look at your bill and sum up the KWhs in a year.
2- Search for "Solar radiation" (per m^2)in your location hourly, daily, monthly, yearly; (whichever available; more precise data you will get if find more precise data).Unit: (KWh/square meter)
3- measure the area your panels fit.

4- multiply 2 and 3.
5- multiply 4 by panels' efficiency (8-17%)

6- If 5 is not possible, measure 1 panel and find it's efficiency based on its wattage!


7- Now, you have calculated overall power production.


Batteries!

1- Find how many days continuously you do not have a sunny day.
2- Calculate your load (house) in each hour after sunset.
3- Now, you know how much your backup battery should be.

Hope you succeed!
 
  • Thread Starter Thread Starter
  • #6
Gulp - thanks, Mr Eng :)

I now have the info on the system. It consists of:

30 x Zanussi 330W panels (Estimated annual output 8,774 kW/h )
1 x Growatt Twin Tracker inverter (when I looked around the house, there were actually 3 of these in a row...)
1 x Triple Power 4.5kW Battery with SE - which I understand is 'Social Energy'
1 x 'Social Energy' controller
1 x Voltage Optimiser (gives the home a controlled 232 - 239V supply)


The house also has a hot water storage cylinder coupled to this, so I presume excess PV will be used to heat this up. An electric boiler provides the CH.

Based on current energy prices, they've been told the annual savings from the PV & battery equates to £990. The Social Energy contract should see them gain a further £516pa, but this obviously depends on the home users' actual power demand. The voltage optimiser is claimed to provide a further £158 of annual savings.

If these estimates are anywhere near accurate, then it looks as tho' this house would cost an additional ~£975 of electricity pa to run (estimated leccy bill before PV was £220 pm = £2640pa), which would make it a fairly cheap detached rural house to heat and use that doesn't have access to natural gas.

Does that all make sense? And would an extra battery or two also be worthwhile?

Thanks for any thoughts on this.
 
30*330 = 9900 Watts is the system's size. While, PV's efficiency varies between 8-30. Also, the location's solar radiation is important in the calculations.

Therefore, if they say 8774 kwh, you have no choice but accept it because you do not have efficiency and solar radiation info and panels' area.

1 year equals to 8765 hours. If you want to do some calculation:
8765 h * 30* 330 Watts = 86,773.5 Kwh! in a year.
Where you know that it is not correct because Sunlight is not available 24/7.

Locations radiation is available online you can access but that gives you energy received in a unit area. You can use it to calculate PV output if you have the efficiency and area values.
For Example:
This is data from Ontario, London station using Google Maps (click on a place to get the location) then CLIMWAT(Select stations in a country and extract data) and then CropWat (open extracted data to see graphs, etc.) software.
You can also use HOMER Pro 1 month free trial to do some calculations.

Mj convert to KW using online tools.

MonthSunRadiation
hoursMJ/m² (per day )
(can be converted to KWh
using online tools)
January
2.3​
5​
February
3.4​
7.7​
March
3.9​
10.9​
April
5.7​
15.8​
May
7.3​
19.8​
June
8.3​
21.9​
July
8.7​
22​
August
7.5​
18.7​
September
5.4​
13.4​
October
4.2​
9.2​
November
2.4​
5.4​
December
1.7​
4.1​
Average
5.1​
12.8​
 
  • Thread Starter Thread Starter
  • #8
30*330 = 9900 Watts is the system's size. While, PV's efficiency varies between 8-30. Also, the location's solar radiation is important in the calculations.

Therefore, if they say 8774 kwh, you have no choice but accept it because you do not have efficiency and solar radiation info and panels' area.

1 year equals to 8765 hours. If you want to do some calculation:
8765 h * 30* 330 Watts = 86,773.5 Kwh! in a year.
Where you know that it is not correct because Sunlight is not available 24/7.

Locations radiation is available online you can access but that gives you energy received in a unit area. You can use it to calculate PV output if you have the efficiency and area values.
For Example:
This is data from Ontario, London station using Google Maps (click on a place to get the location) then CLIMWAT(Select stations in a country and extract data) and then CropWat (open extracted data to see graphs, etc.) software.
You can also use HOMER Pro 1 month free trial to do some calculations.

Mj convert to KW using online tools.

MonthSunRadiation
hoursMJ/m² (per day )
(can be converted to KWh
using online tools)
January
2.3​
5​
February
3.4​
7.7​
March
3.9​
10.9​
April
5.7​
15.8​
May
7.3​
19.8​
June
8.3​
21.9​
July
8.7​
22​
August
7.5​
18.7​
September
5.4​
13.4​
October
4.2​
9.2​
November
2.4​
5.4​
December
1.7​
4.1​
Average
5.1​
12.8​
Thanks again, Mr Eng.

I have been given a copy of the documents provided to the vendor when they proposed the system. It is pretty comprehensive with charts showing the effects of partial shading throughout the day from the neighbouring building, their own house's 'ridge' etc etc, and it was on that basis that they detailed the anticipated generation. Eg there are three panel zones, with one being of 6.6kWp size, a KK (postcode, roof orientation and pitch) of 959 and an SF (shading factor) of 2. This is repeated for the other two zones. From this is they anticipate a total annual Kwh of 8,773.

In addition, the system is signed up for 'Social Energy' and I understand this essentially means that when the home's PV and stored energy capability exceeds the domestic requirement, the energy provider can take that excess energy for the grid. Eg, in this current period of high sun and low use, the battery is almost always fully charged, so they'd anticipate the energy company would pinch some of that excess power during, say, the early morning and tea-time grid demands. In return, when little energy is being produced by the PV system and the domestic energy demand is high - as it would be on cloudy days in cold weather - the energy company will sell it's power at times of low demand at a much cheaper rate. Eg it would charge up the battery overnight at a low cost.

They have estimated that this 'SE' benefit is not far off the main benefit from the actual PV generation; £990 pa for solar-generated savings, and £516pa for the SE-generated savings.

Yes, we need to take these calculated figures on trust to some degree and I know that various elements can affect its performance such as each year's climate and the living conditions of the occupiers. What I am trying to figure out, tho' is if it's possible to anticipate what additional benefit can be gained by adding a further, say, two batteries, given that the single one fitted is only 4.5kWh, and has been spending these last few weeks in a state of near permanent full charge.

I did phone up the installers - Project Better Solar - to ask them if they could re-run their initial calcs with 2 additional batteries fitted, but they - pitifully - cited 'data protection' as their reason for 'no'. They are happy to do this after we buy the hoosie...

The vendor did admit that they'd been advised at the time to install more batteries, but their renovation costs had already reached a point where they decided to say 'enough'.

My sis would like to have an idea - before considering making an offer - if an additional outlay of around £6k (for two batteries) should be on the cards.

Basically, assuming the figures given are accurate enough - annual generation of 8,773kWh, PV savings of £990pa, SE savings of £516pa on a single battery - is it possible to estimate the additional benefits of a second or third battery?

Many thanks.
 
is it possible to estimate the additional benefits of a second or third battery?
Batteries are for backup purposes such as stabilizing the power quality, supporting the load during nights and rainy days where there is not enough sunlight.

So, any extra battery would not increase your power production but increase supporting your load.

For instance, if you see you are not consuming all the power produced by the PV during a day (which is normal: power production can be higher than the load), on the other hand, the energy stored in the batteries is not able to cover all the power consumption during night, so you are using electricity from a grid, you need to think about increasing your BSS (Battery Storage System).

Could you provide me with supporting information of the PV system?
 
  • Thread Starter Thread Starter
  • #10
Batteries are for backup purposes such as stabilizing the power quality, supporting the load during nights and rainy days where there is not enough sunlight.

So, any extra battery would not increase your power production but increase supporting your load.

For instance, if you see you are not consuming all the power produced by the PV during a day (which is normal: power production can be higher than the load), on the other hand, the energy stored in the batteries is not able to cover all the power consumption during night, so you are using electricity from a grid, you need to think about increasing your BSS (Battery Storage System).

Could you provide me with supporting information of the PV system?
Thanks for sticking with this, Mr Eng :)

I've attached the most relevant pages as requested.

I think I understand the gist of the system - the PV panels provide leccy when it can, and this is used to run whatever in the house, with the excess being used to charge up the batteries and then provide any remaining to the grid (although I also understand that SE can mean the grid can take some over the battery at times).

Since no PV system will provide a constant output - unless the sun is always fully shinning... - then the battery will act as a buffer; it'll charge when there's surplus generation and release it on demand when there isn't.

All I'm really trying to work out is, if we accept that the installed system is as described and with a generation as outlined in these docs, is it possible to estimate the additional benefit of a second 4.5kWh battery? And a third?

It's pretty obvious to me that it will be of benefit, since a single battery will (presumably) only be capable of running a 2kW appliance (tumble drier, W/M or oven, for example) for just over two hours (assuming for argument's sake the appliance is drawing a full load during that time - which most usually won't). In our current sunny spell, the battery seems to be fully charged by mid-morning (probably wasn't 'flat' before in any case), so the PV system is providing no further benefit to the house - unless you were to time washings and stuff for during the day. But when it comes to doing your cooking when you come home, with ovens and hobs drawing multiple kW and with the PV output dramatically falling due to the sun's angle, the single battery will likely be fully depleted pdq.

I understand one of the benefits of SE is that the energy supplier will provide the home owner with cut-price leccy during periods of low demand on the grid. Eg if your batteries are 'flat' they will charge them up overnight for around half the 'normal' leccy cost. This would obviously be of greatest benefit in winter time when a fully charged battery in the morn would be great, but 2 or 3 batteries would be better still.

On bright sunny winter days, the batteries would also have a decent chance of being charged up during the day, so could well provide the bulk of heating when the occupants come home in the eve - but a single battery would soon be depleted.

Thoughts? And are the figures provided below realistic?

Many thanks.
 

Attachments

Last edited:

Gary K

-
DIY
If the battery is fully charged quite early in the day, then I would program the washing machine/dishwasher etc to come on during the day so that you make use of the available power
 
  • Thread Starter Thread Starter
  • #12
If the battery is fully charged quite early in the day, then I would program the washing machine/dishwasher etc to come on during the day so that you make use of the available power
Thanks Gary.

Yes, a lot of how effective these systems can be comes down to how well it's used, and any way to take advantage of surplus PV energy is a bonus.

We've just found out that the hot tank - a new system installed at the same time - does not get heated up automatically when there's surplus PV energy available. I presume it must get heated overnight at the lower rate instead, but it still seems crazy to have 'omitted' the PV source, unless there is a reason?

I wonder if it's part of the Social Energy deal? The energy supplier obviously wants access to the excess power when it wants it, and then - as I understand it - allows the house to buy it back at a lower tariff, but I can't get my head around this; why can't anybody fit batteries and charge them up overnight at the much lower rate in any case, 'economy 7' style?!

Man, this is confusing.
 

Paignton pete

-
Arms
Esteemed
You could save a lot of money by buying a solar iboost.

Approx £400 fitted.

You basically fit it to the hot water cylinders immersion and it measures when there is excess electricity flowing backwards through meter. ( wasted excess energy the pv produce you are not using). It then allows the amount wasted amps to be redirected into the immersion heater.

e.g.
If the pv is producing 500w the house is using 347W then the solar iboost will use the excess 153W to heat up the water in the cylinder.
As the input and use change during the day the solar iboost would alter the input to compensate.

Basically you are storing the otherwise wasted electric as hot water.
 
  • Thread Starter Thread Starter
  • #14
You could save a lot of money by buying a solar iboost.

Approx £400 fitted.

You basically fit it to the hot water cylinders immersion and it measures when there is excess electricity flowing backwards through meter. ( wasted excess energy the pv produce you are not using). It then allows the amount wasted amps to be redirected into the immersion heater.

e.g.
If the pv is producing 500w the house is using 347W then the solar iboost will use the excess 153W to heat up the water in the cylinder.
As the input and use change during the day the solar iboost would alter the input to compensate.

Basically you are storing the otherwise wasted electric as hot water.
Thanks PPete.

I was ill-informed - the EPC report suggested improvements, one of which was 'solar water heating' which sis took to mean the PV system not currently being used to heat the water. When I read the EPC, I realised it was referring to the addition of the other type of solar panel...

So the existing system will almost certainly already have provision to heat the tank with surplus leccy.

Can anyone make an informed guesstimate as to the possible benefit of an additional battery(ies)?

Cheers.
 

Gary K

-
DIY
Thanks Gary.

Yes, a lot of how effective these systems can be comes down to how well it's used, and any way to take advantage of surplus PV energy is a bonus.

We've just found out that the hot tank - a new system installed at the same time - does not get heated up automatically when there's surplus PV energy available. I presume it must get heated overnight at the lower rate instead, but it still seems crazy to have 'omitted' the PV source, unless there is a reason?

I wonder if it's part of the Social Energy deal? The energy supplier obviously wants access to the excess power when it wants it, and then - as I understand it - allows the house to buy it back at a lower tariff, but I can't get my head around this; why can't anybody fit batteries and charge them up overnight at the much lower rate in any case, 'economy 7' style?!

Man, this is confusing.
The cost of the batteries is prohibitive. Basically you would save very little when you factor in the cost of replacements every so many cycles x the cost of the energy saved
 
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