Cheapest home storage battery options?

Solarchaser

And that is the other side of batteries.
Solar is free.
In winter it's not there, so you can't have free, but if you charge at the cheap rate you can have electricity at a third of the normal price

JKenH

Don’t forget round trip losses which can be in the order of 20% (see dit note below) but are usually forgotten in man-maths calculations to justify the economics of a battery. As a rough rule, to get 4kWh out your battery you need to put 5kWh in. The 5p/kWh you pay overnight on Octopus Go to top up your battery becomes 6.25p when you use it so the saving is closer to half than two thirds.

Also bear in mind salesmen usually quote the capacity of the battery gross without pointing out there is a minimum level below which the battery cannot be discharged. Effective capacity can be as low as 75% of quoted capacity for lithium or even 50% for lead acid batteries. 

Here are a couple of useful documents:

Domestic Batteries

Best Practice Guide – learnings from NEA’s Technical innovation fund field trials

https://www.nea.org.uk/wp-content/uploads/2020/11/Best-Practice-Domestic-Batteries-FINAL-27-03-2019.pdf

Batteries and Solar Power:

Guidance for domestic and small commercial consumers

https://www.bre.co.uk/filelibrary/nsc/Documents%20Library/NSC%20Publications/88031-BRE_Solar-Consumer-Guide-A4-12pp.pdf

edit: round trip efficiency typically 80% so round trip losses 20% as pointed out by @EricMears

EricMears

JKenH said:

Don’t forget round trip losses which can be in the order of 80%  

Sorry to niggle,  but it's the round trip efficiency that's of order of 80%.  Losses are only 20%

JKenH

EricMears said:

JKenH said:

Don’t forget round trip losses which can be in the order of 80%  

Sorry to niggle,  but it's the round trip efficiency that's of order of 80%.  Losses are only 20%

Well spotted Eric. I’ll amend it. Thanks. 

QrizB

Spies said:

What about overnight base loads? 2.4kw wouldn't cover mine, 4.8kw almost does, if it charged on cheap energy during the night then it definitely would. 

This is an example of why my numbers only work for me - from sunset to 10pm my load is around 400 watts, from 10pm to 6am it's only 100 watts.

Solarchaser

So in winter I charge my batteries from cheaper rate electricity. 
I pay 4.5ppkwh for 3 hours of octopus go faster.
My system is 82% round trip efficient. Or wastes 18% if you prefer.

So let's say I buy 5kwh
5x0.82 = 4.1kwh @22.5p
On normal rate its 4.1 x15p = 61.5p

61.5/22.5 =2.733 so pretty darn close to 1/3rd of the cost, certainly much closer than half.

That's real world, actual figures, not theoretical.
That's a lux ac inverter 

QrizB

Solarchaser said:

That's real world, actual figures, not theoretical.
That's a lux ac inverter 

Thanks for the evidence that temporal arbitrage works in the real world, my goal is similar but I need more battery capacity first!

Spies

Does loss only apply to charging from grid if you have a hybrid inverter? 

QrizB

Spies said:

Does loss only apply to charging from grid if you have a hybrid inverter? 

Some (not all) hybrid inverters won't charge from the grid. However there are losses in converting your string voltage to the battery voltage, and in the battery while charging, and then losses when converting it back again. These are likely to be broadly similar to the losses discussed above.

My Sofar AC-coupled inverter claims a charging efficiency of 94.1% and a discharging efficiency of 94.3%, for a round-trip total of 88.7%. I don't have a number for losses in the battery but judging from the temperature the battery gets to (about 10 deg C above ambient, when hard-pressed) I would guess low single-figure %.

JKenH

Solarchaser said:

So in winter I charge my batteries from cheaper rate electricity. 
I pay 4.5ppkwh for 3 hours of octopus go faster.
My system is 82% round trip efficient. Or wastes 18% if you prefer.

So let's say I buy 5kwh
5x0.82 = 4.1kwh @22.5p
On normal rate its 4.1 x15p = 61.5p

61.5/22.5 =2.733 so pretty darn close to 1/3rd of the cost, certainly much closer than half.

That's real world, actual figures, not theoretical.
That's a lux ac inverter 

I am on Go Faster as well but take the 5 hour option so pay 5.5p cheap rate and 13.31p normal, quite a bit cheaper than you. 

On Go, which is what I was referring to as I thought it was the more common option I would be paying 5p and 13.31p so if I bought 5kWh at 5p, that’s 25p and after 20% round trip loss I would end up with 4kWh worth 53p, so the original cost is 47% of the time shifted “value”, hence my “closer to half” comment. 

On my 5hour Go Faster tariff it would be 27.5p in and 53p out so 52%.

Just as an aside is 3 hours the best option for you with a M3LR?  You can only get around 18/20kWh net of charging losses into it in 3 hours. If I have two long trips back to back I often need to add 30kWh overnight so 5hrs is just right for me.