Is solar battery storage worth it?

Reed_Richards

energynerd99 said:

I have looked at Air Source Heat Pumps and there are many reasons why they do not add up. Based on a high efficiency unit which can produce water tmperatures of 75degC I estimate it would cost me an extra £700 per year for a system that would be inferior in lots of ways to gas in providing a comfortable 20 degC home. More often than not I will only have 1 or 2 radiators requiring heat at any one time and some fan assisted plinth heaters which because of low water flow/ temperature wil not function correctly.  

If you aspire to water at 75degC with a heat pump you are bound to conclude that they're uneconomic.  But you don't need water to be anywhere near as hot.  A unit with a maximum water temperature of 55 to 60 C should be all you need and will give you a much higher efficiency than one providing water at 75 C.  I agree about the plinth heaters not functioning correctly but I have read that it is possible to modify them to overcome this.

Exiled_Tyke

I have more than enough hot water heating to 45 degrees.  I could get away with a lower temperature but then by the evening it's not quite hot enough for any washing up.  (water is heated on Octopus IO for 7p a unit and a COP better than two).     Octopus insisted I disconnected my plinth heaters and replaced them with rads. I'd be very interested in being able to reconnect them.   

CuparLad

Thank you for a very interesting thread.

I am in the process of purchasing a 120yo single storey house in rural Perthshire that will require quite a significant upgrade. The intention is to insulate the property to (at least) current standards, remove the existing suspended wooden floors and replace with insulated solid floors, and replace the existing oil-fired (range) heating system to radiators with underfloor heating served by a heat pump.

I’m hoping for this to be my forever home and the principal aim is for it to be warm and cost effective to heat. I work from home full time and my current electricity consumption (in a 25yo 3 bed bungalow with oil central heating) is 3,000kWh.

As part of this upgrade, I’m considering the installation of solar PV with battery storage. My current idea is to install the battery storage in the (to be constructed) detached garage. The house is orientated within 2 degrees of east-west so there is a long almost exactly south-facing roof. I don’t have an EV today and have no current plans to purchase one. The batteries are intended to provide backup in case of loss of power, store any excess solar and be topped up overnight so that I can then use the power during peak times. I’m currently with Octopus, so will undoubtedly shift to one of their suitable tariffs.

My architect, who is very supportive of the overall approach, suggested hot water storage as an alternative to batteries, but since I have one shower a day, wash my hands and intend to have a dishwasher in addition to a washing machine, it feels like that approach is just wasting the excess solar in repeatedly heating water that I won’t use.

I’m very new to the whole topic and was interested in forumites’ thoughts/opinions and any other things to consider? It seems every manufacturer has their own “ecosystem” and app, so also want to ensure I don’t end up going down a technological dead end too.

Thanks to all.

Heedtheadvice

As a starter response, very general!:

Sounds a great idea especially starting with insulation (relatively cheap! ) but don't forget about the heat loss through doors and windows etc.

Water heating energy requirement, especially with low consumption, is fairly minor compared with the space heating. If you are then all electric then most demand over a year will be  via heat pump and then your dishwasher/ washing  machines ( cooking?) plus your 'background' ( low consumption but many hours) so I would try and estimate annual consumption. Your architect might be able to advise on the space heating requirement and you could estimate the rest.

Once you have a good idea what your annual electricity consumption will be you can then consider battery size.

For a backup ( for just a day or two) supply, if you include space heating, you will need lots. In rural Perthshire you will need to factor in the frequency and durations of power loss. It could be very costly to cover periods such as after storm Arwen! I would consider another type of heating/supply for those periods. You already have oil or maybe a standby genny?

Otherwise you can possibly work on 10kWh of batts for background stuff (fridge  freezer minor electrical equipment) for a couple of days cover.

I have not as big a demand on leccy ( have has too) and further South but feel 12 kWh will meet my background needs for two days - if fully charged up.

Given heating too I would be looking at batts at least of a much higher level a bit like Solarchaser's system levels ( see other threads he has posted lots of info

To support that on one of the best tariffs maximise Solar generation.

You will probably ( almost certainly) will need G99 installation approval from the electricity  network Distributor ( not the supplier), the DNO.

Getting  a good design work done for the heat pump/property will be very worthwhile do that you are neither cold nor running inefficiently!

Keep posting on here,  many with lots of experience.

CuparLad

There is certainly still much to consider. Once I actually have my hands on the property it will be much easier to start making assessments. The windows and doors are certainly in scope for review.

I don't plan to require battery backup for the Heat Pump as there are two existing wood-burning stoves that could be pressed into action in the event of a long-term outage, so it will be more about the day to day requirements. It feels like 10-12kWh might be a suitable amount?

Heedtheadvice

Seems fair enough to me, and log burners roaring away are very nice!

Solarchaser

I'm a big fan of a log burner myself.
The wife is not, so we don't have one.

If you are ripping out stage in Cupar, id advise adding insulation into the walls as well as the floor.
Pir and studding will square up those old walls.
Underfloor heating is great, I wish I'd put it everywhere when I initially started.

Perhaps not the way you have been thinking, but I understand that heat pumps come with a buffer tank, which you then coil into and out of to remove the heat.
If this is the case, you could increase the size of this pretty cheaply, say a 300L vertical tank, and if you get one with twin coils it would do your hot water as well as your heating. A quick Google says under £1k for a 300l vertical.
This is how I run my heating and hot water, but with cheap rate immersion heating rather than heat pump.

Large stack of batteries to me makes lots of sense, if you have big solar (and I'd definitely fill that south facing roof) you will be able to get a decent return from export tarrifs on the excess solar, and then you have the batteries for backup in storms etc, but also as a source of income if set to charge on the off peak rates, and discharge on the peak rates.

Heedtheadvice

....and to follow up the Solorchaser comments batteries have rapidly dropped in price. The chap above is a fan of the Seplos DIY batts and they are relatively very cheap at the moment for Jan delivery. If I didn't already have others and I was in your position  I would be placing an order!

CuparLad

Heedtheadvice said:

....and to follow up the Solorchaser comments batteries have rapidly dropped in price.

Fingers crossed that in another 6 months, when I'll likely be purchasing, they will have dropped further!

Which leads me to another question. My idea is to have the batteries installed in a detached garage on the other side of the plot from the house as I don't really want them outside. I'm wondering if this creates issues? If the solar panels are on the roof of the house, is it reasonable to underground the DC cables across to the garage and have the inverter there, or have the inverter in the house (not ideal)? I have in my mind to have the garage as a sort of "plant room" where all this sort of stuff it kept so that it's protected from the elements.

paul991

What distance is it?