Is it still worth exporting your home battery?

[Rolfe]

I'm talking about Octopus reducing its export tariff to 12p (from 15p). This reduces the differential between the import price (7p off-peak) and the export price to 5p. It was 8p, of course.

It was my assumption that at the 8p differential it was worthwhile exporting the remains of my home battery last thing in the evening, and recharging at the off-peak rate. Most people seem to have been doing this, so I guess that was a reasonable conclusion. Now, though, I'm hearing people question whether it's actually worth it, taking into account transmission losses - which of course are always borne by the consumer, both ways. I don't really know how to estimate this though. Do our resident experts have any opinion about this?

 

[Everest]

I've measured the round-trip efficiency of my inverters to be about 85% (i.e. 230V AC -> 48V DC -> 230V AC).

So, yes still worth it

 

[Rolfe]

Thanks for that. Could you perhaps show more of your working, and maybe quantify what the actual profit per kwh of "arbitrage" is likely to be?

 

[SteveMSJ]

I've measured the round-trip efficiency of my inverters to be about 85% (i.e. 230V AC -> 48V DC -> 230V AC).

So, yes still worth it

However, if you include a cost per kWh for the degradation of your batteries as say:
BatteryCost/(capacity x cycle life)
I think you’ll find it’s no longer worth it at 5p difference between import and export.

 

[Everest]

Could you perhaps show more of your working

You "buy" 1kWh for 7p, but that gives you 0.85kWh to export, which at 12p/kWh pays you back 10.2p. So, assuming your efficiency is similar, a gain of 3.2p per kWh.

However, if you include a cost per kWh for the degradation of your batteries as say:
BatteryCost/(capacity x cycle life)
I think you’ll find it’s no longer worth it at 5p difference between import and export.

IMHO, cost per kWh is minimal. Early studies of LiFePO4 chemistry indicates that calendar ageing will likely degrade your batteries before cycle count will, as you are unlikely to cycle the batteries fast enough for usage to be the biggest factor. Time, of course, will tell. We can revisit this in 2036

 

[SteveMSJ]

IMHO, cost per kWh is minimal. Early studies of LiFePO4 chemistry indicates that calendar ageing will likely degrade your batteries before cycle count will, as you are unlikely to cycle the batteries fast enough for usage to be the biggest factor. Time, of course, will tell. We can revisit this in 2036

I’ve put a note in my diary

 

[Rolfe]

3.2p. It's fairly marginal, but at least if I do it I know I'm not actually losing! In the summer it comes to about 25p per day, assuming I'm exporting about 8 kwh.

The comment about battery degradation reminded me of a spat I had on another forum with an American guy who of course knew everything, because he had all these qualifications and could build a battery from toothpaste and old beer cans, you know the sort. I was showing the setup I have which allows me to harvest the solar generation that would otherwise be clipped on very sunny days. He threw an absolute fit about my battery exporting - which could be seen on the graph, even though that wasn't what my post was about - and banged on and on about the years I was stripping from my battery's life, until I blocked him.

I pointed out that if I didn't do that, my battery would spend pretty much all its life above 80% charge, and all of its life there at midsummer. How was that better than cycling it? Even though it's an LFP. More ranting about the damage I was doing. I got a bit testy and questioned whether he actually knew what I was talking about, so he blocked me. Such is life.