Home batteries *Can* save you money

Solarchaser

I was going to add to this thread, but its been closed for some reason.
https://forums.moneysavingexpert.com/discussion/6111409/a-year-with-home-batteries-and-more-solar-and-an-ev#latest

I've edited this thread title to something more accurate 

Having started writing this, I've realised it's going to be a very long post, so I'll break it into a few posts.

Anyway, at the end of October last year I had my smart meter replaced, handily this coincided with the replacement of my lux inverters within a day of each other.
This gave me the opportunity to run for a year with comparable data rather than relying on just the CT clamp reporting from the lux.

I have import data from the last 7 years as I started recording it when I got 4kw fit system of solar panels and that shows the following from start of November to start of November, so 2015 is actually nov2014 to Nov 2015 etc etc

2015 - 4929kwh
2016 - 5003kwh
2017 - 4743kwh
2018 - 5239kwh

2019 - 4013kwh
First year of batteries, sofar me3000sp and 3x pylontech for a total of 6kwh useable

2020 - 3316kwh
Second year of batteries, changed from sofar to lux, and from 6kwh useable to 17kwh useable approximately half way through the year

2021 - 4239kwh
Third year of batteries, now have working parallel lux and 20kwh useable batteries.

As you can see, having more batteries has increased my consumption.
OK that's true, but like any daily express article, you need to look past the headline, the reality here is I've been buying cheap rate electric from octopus go and filling my batteries with off peak leccy.
So have I bought more? Yes
Have I paid more? No, I've paid less.

Flicking back to 2019, a couple of more things changed, at the start of November I also got my first electric car, a little Nissan leaf, its done about 15k miles in the 3 years, and a little of that has been public charging, so I think if we say 4m/kwh so 4000 miles =1000kwh then say 12k of that 15kmiles has been home charging, id think that would fair, so essentially from 2018 on, I'd add around 1000kwh to my yearly consumption.
About a 1/3 of the year through 2019 I added a second solar array which produces around 4000kwh on top of the fit array producing around 3300kwh.
So from 2019 I'm producing around 7300kwh of solar.

It would have been great from a statistical viewpoint if I had added these things a year at a time, as it would be clearer which made the difference,  but I was trying to keep my cost down rather than going for statistics

So anyway back to the lux vs smart meter.
Smart meter import says 4239
Lux import says 4050, accuracy of 92%
Smart meter says export of 2083
Lux says export of 1902, accuracy of 92%

Smart meter records 2152kwh on rate 1(cheaper) and 2087kwh rate 2(normal rate)
My cheaper rate is go faster 20:30 - 23:30 @4.5p/kwh
My normal rate is 14.2p/kwh (apologies to those I told it was 15p, I thought it was until I checked my bill)

Solar fit meter says solar production of 3407 (bright middle part of the year)
Lux says solar on fit of 3347. Accuracy of 98%

Continued on next post

pete-20-11

You also use more electric converting from AC to DC and back to AC again - vs using it live?

Something like 90% efficiency on a round trip? Some lost to heat? 

But yes, the key is buying it cheap and/or free from solar. 😀

Solarchaser

The problem I've had until this last year, is that I wasn't sure if it was the extra solar installed in 2019, or the batteries which made the significant reduction in my bill.
Between 2015 and 2018 my average was 4979, so adding on the car at 1000kwh my yearly consumption should have been 5979, let's just say 6Mwh for ease.
In 2019 it dropped to 4013, and there was no way of being sure whether it was the batteries or the solar which made the best reduction. 
My sofar didn't give details of battery usage, so I decided to go for lux inverters instead around a third of the way into 2020.
Come the end of 2020 and I'd increased my battery capacity from 6kwh to 17kwh and nothing else changed, so it was definitely responsible for the drop from 4013kwh in 2019 to 3316kwh in 2020.
However due to being supplied the wrong inverters (twice) the data I had wasn't consistent enough to draw any conclusions. 

Now in 2021 I've had a full year of running 20kwh of batteries and on one set of inverters and it tells me that the batteries have charged a total of 5258kwh, and discharged a total of 4323kwh at an efficiency of 82%

So this year they have supplied 4323kwh to run the house.

Continued on next post

Solarchaser

So how much of the battery charging has been on cheap rate and how much for free (solar) because that's where the devil is in the detail.
If its 4323 of solar then its all gravy and I've saved £613 (4323 x 0.142).
But if its all on bought cheap rate then I've bought 5258 x 0.045 =£236 and only saved the difference of the cheap rate to the normal. So 14.2 -4.5 = 9.7p  saving between cheap rate and normal 9.7 x 4323 = £419  so 419 - 236 = £183 which is quite a bit less.

The truth is I can't say 100% where its all gone.
I can say I've bought 2152kwh on the cheaper rate, but that doesn't mean it's all went in the batteries, some will be running the house, showers etc, and some will be charging the car.
The cheap rate is all year as well, so while from Feb to October I could count on one hand how many times I charged the batteries from mains rather than solar,  there would still be house usage at night.

From looking at the data of solar and export and import I'd conservatively guess at 75% of the bought electric is for charging the batteries.

So 75% of 2252 = 1614 of charging at cheap rate for a house supply of 1323 from discharge.
1614 x 4.5p = £73 spent for a saving of 1323x 14.2p = £188 a difference of £115
This leaves 2709 of solar or free energy which returns 2709 x 14.2p = £385
Combined saving for the year of £500

Now I have to admit I've done this a little off the hoof and there is a good chance I've confused myself here, so feel free to point out of if done something daft.

Caveats... I decide each day based on weather and state of charge (when I remember) whether to charge the batteries during cheap rate, or let solar do it, and this has an unfortunately large amount of human error.

The last two days the weather has supposed to be dull and raining so I've charged the batteries fully, and have exported more than 5kwh each day because it was brighter than expected.

So some sort of weather monitoring that a. Works and b. Doesn't rely on me remembering would definitely up my savings

Edit.  To sort messed up figures

Petriix

Surely it makes sense to only ever charge the batteries to 75% full from the grid to allow space for surprise sunshine, and just use a bit less if it's not sunny? 

Solarchaser

Petriix said:

Surely it makes sense to only ever charge the batteries to 75% full from the grid to allow space for surprise sunshine, and just use a bit less if it's not sunny? 

Not if you have 20kwh of batteries and the house uses an average of 25kwh a day in the darkest months.

I do like the idea of "just use a bit less", clearly you have never met my wife 😉

Petriix

Solarchaser said:

Petriix said:

Surely it makes sense to only ever charge the batteries to 75% full from the grid to allow space for surprise sunshine, and just use a bit less if it's not sunny? 

Not if you have 20kwh of batteries and the house uses an average of 25kwh a day in the darkest months.

I do like the idea of "just use a bit less", clearly you have never met my wife 😉

Well, I'd be shifting my demand into the off-peak period as much as possible. There are certain appliances that don't have to run every day etc. But I take your point. 

arty688

Solarchaser said:

Petriix said:

Surely it makes sense to only ever charge the batteries to 75% full from the grid to allow space for surprise sunshine, and just use a bit less if it's not sunny? 

Not if you have 20kwh of batteries and the house uses an average of 25kwh a day in the darkest months.

I do like the idea of "just use a bit less", clearly you have never met my wife 😉

How much of that 25Kwh is off peak though anyway ? In the winter I would say run your house like you don't have a battery use as much off peak as possible.

Solarchaser

Yeah its a good theory.
It's like me saying to the family to have showers at the off peak time too....

The reason I got batteries was to save money sure, but also because I knew I couldn't control the house use.
If I could control house use then there would be little use for batteries as we would be optimised for solar and off peak.

So the house runs as if we had no solar, and no off peak, as that is what gives a far more harmonious married life.

arty688

Charging the car putting the washing machine and dishwasher off peak is no big inconvenience?

Solarchaser

Ahh you must live in a different family arrangement than me.
I have 3 young kids and an OCD wife.
My washing machine and tumble dryer are on 5 times a day each on average.

We do (well I do) charge the car off peak and when remembered the dishwasher goes on off peak, or next day mid day if I forget.

If I was retired, had no kids or lived alone, the things you list would be no issue, but then I wouldn't need batteries if that was the case.
It's horses for courses