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Is a home battery a good investment with rented roof solar panels

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(@bontwoody)
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Im trying to decide whether to recommend getting a home battery to my brother in law who has just had an 8kW heat pump installed. My gut feeling is no and my preliminary calcs support this but i just want to get a few opinions. He has 4kWish of solar panels but under a deal where he just gets the free energy and no payments.

Im starting from the idea that he will save by charging them cheap rate. Assuming he was on Octopus Cozy, that could be two full charges at about 12p saving per unit. (£1.20/day on a 5kWh battery).

There is also a saving through the battery taking up the slack when the usage exceeds production. This is hard to quantify, Ive guessed 50p/day.

Assuming an installation cost of £4500, this gives a payback of 7.25 years.

Any thoughts? Thanks in advance

image

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Transparent
(@transparent)
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Any plans like this will need to take more account of what's going to happen to energy strategy in the future.
Almost everything I'm currently working on is related to devices and regulations that do not yet exist.
That facilitates some very useful discussions with senior planners at companies like National Grid and Wales & West Utilities (a licensed Gas Transporter).  🙂 

Despite the announcements by DESNZ on 15th March, it is clear that the energy planners are still working towards Nodal Pricing.
That's because energy networks have to be built on the science, rather than political policy.

So the location of your brother-in-law's house matters.

 

This post was modified 2 months ago by Transparent

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(@bontwoody)
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@transparent 

Thanks for the reply. It seems he has managed to get onto the new OVO tariff at 15p kWh for all heat pump usage. That knocks the benefits back again. I think maybe its a decision for a later date.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@misterb)
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@bontwoody 

if the solar on his roof is 'available' for home use, with the excess being fed back to the grid for the benefit of whoever 'owns' the PV system, then this surely would include charging his battery, same as it is for heating his immersion heater, heating his home etc. 

whether its a viable option for his heat pump though is a different matter. we have an air to air heat pump (mini split type), which allows it to run in reverse and cool the room it is fitted in. This is particularly useful when the sun is shining as we get free air con, however when the sun isnt shining and we need heat, then obv running it isnt free. 

Thats where the storage battery comes in to play, by charging up overnight at 7.5p (we bought a used PHEV mitsubishi outlander, as we needed a new car) we can run our mini split at that rate throughout the winter

However, its all related to how much energy he will need to run his heat pump AND how much he pays for it and thats the tricky one as prices can go up or down. it may be that to be able to run his heat pump off a battery in winter every day, might require a massive amount of battery storage and he might be better off spending the money on insulating his home to a very high standard, it may also prove more beneficial in the long term by adding value to his property?

 

 


   
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(@bontwoody)
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@misterb 

The deal is that the solar panel providers get the FIT payments so he can do what he likes with the produced electricity (use it or lose it). The 15p per kWh OVO tariff applies at any time to the use of the heat pump (I guess they meter it seperately?) so this takes away the benefit of filling the batteries at cheap rate. (He is unlikely to get an EV anytime soon so cant get a 7.5p rate overnight)

What is considered a reasonable payback time for batteries? I was thinking 5 years but maybe that is a little too short?

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@misterb)
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Joined: 11 months ago
Posts: 139
 

with a larger battery than your suggested 5kw at a lower price that you allowed for, the payback time will obviously reduce!!!! though its not just a battery, its an inverter and associated cabling etc. and installation

BUT if he is paying 15p per Kw at anytime the heat pump is in use (and the general thinking of heat pumps is to leave them on 24/7) then Im not so sure that the savings would be that sufficient to warrant a home battery system - unless of course they use a lot of electricity outside of the time they have their heat pump on. 

 


   
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(@bontwoody)
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@misterb 

thanks. I think that mirrors my thoughts. 

What you say about bigger batteries is interesting. I have a 5kWh battery, it’s a modular system so I could add another 5kWh easily but I can’t make it add up financially. I only really start using the grid at night in winter. 

The best argument I can make is that currently I go to 95% DOD and perhaps it might be better to lower that? 

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@misterb)
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Joined: 11 months ago
Posts: 139
 

Check out Fogstar UK prices. Their own 'designed' battery is a really good option and works with most inverter protocols 

https://www.fogstar.co.uk/collections/solar-battery-storage/products/fogstar-energy-15-5kwh-48v-battery

 

We are all electric and our circumstances made it viable to buy a PHEV and go onto the intelligent octopus go rate. As we are also on the FiT scheme it's now becoming viable to sell all our PV production at 15p per KW and buying it all in at 7.5p per KW overnight and storing it for use.

This post was modified 2 months ago 2 times by MisterB

   
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(@bontwoody)
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2920 kWhs
Joined: 2 years ago
Posts: 416
Topic starter  

@misterb lol. That’s exactly my situation too. My battery is huawei, I’ll have a look at the fog star thanks.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Transparent
(@transparent)
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Posted by: @misterb

Their own 'designed' battery is a really good option and works with most inverter protocols

Can you elaborate on that comment please?

Does that Fogstar battery require communication with an inverter in order to 'work'?
And, if so, then to what end?

Or is it that the BMS within Fogstar's battery has the capability to report cell voltages and SoC (State of Charge) to an App which is provided with an inverter?
Ie, it's nice to have, but not essential?

What could the Fogstar battery do if the chosen inverter exhibited a fault which created a message available via Modbus or CANbus?
Imagine it's something like "mains voltage outside specification"?

What about the other way around?
Suppose the Fogstar battery had an alert message like "cell-7 has reached the threshold of 46.5v during discharge".
What, if anything, might an inverter be expected to do?

 

Are we possibly in a situation whereby manufacturers of inverters and batteries are asserting that their devices are in some way 'compatible', without either

  • stating what benefit is offered by that feature?
  • providing any warning or safety assistance in a form that a customer would expect to respond to?

Save energy... recycle electrons!


   
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(@misterb)
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Joined: 11 months ago
Posts: 139
 

@transparent 

I am not selling the battery its an option  for them to consider. I'm sure they are adult enough to make their own judgement and enquiries as to it's suitability...


   
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Transparent
(@transparent)
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Yes, I can see that.
... and I'm not objecting to your post or having a go at you.

My intention is to allow this forum to consider whether the 'compatibility' claims being made by manufacturers of batteries, BMSs and inverters are of any real value.

Sunspec offers a set of standards which are voluntary and allow manufacturers to design, test and supply equipment within a common framework.

But are these standards giving end-users a level of functionality which is beneficial to us.
Or are they more akin to self-congratulatory accolades that seem attractive to potential customers, whilst actually delivering negligible benefit?

Save energy... recycle electrons!


   
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