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.

 

[GaryMG4]

Funny enough I was wondering how the round trip efficiency would effect arbitrage with the reduced rate and how low Octopus would have to go before you got below the break even point. I guess you could look back over the figures for past year and substitute 12p for 15p to see how much you might 'lose'.

 

[Rolfe]

Way too complicated.

 

[GaryMG4]

Way too complicated.

I guess there's no way to separate battery export from solar. I did a similar calculation of deemed export (FiT) vs going to SEG, which I believe some companies demand if you add a battery to a FiT solar. I could be around £300 a year better off (less now at 12p), but SEG prices appear to be on a downward trajectory so that could vanish quite quickly. My goal this year is to increase my self consumption of solar as that is likely to be the most beneficial.

 

[Rolfe]

Of course it's different if you have a feed-in tariff.

 

[Rolfe]

Well, I think this just answers the question. The differential between import and export is back up to where it was - actually a nominal 8.5p now, as opposed to 8p before the reduction in the export tariff.

 

[Rolfe]

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.

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

Would you mind redoing the sums for the new price? Is it simply 3.5p more?

 

[Everest]

Is it simply 3.5p more?

yes

 

[Rolfe]

Thought so. So, actual gain of 6.7p for every unit arbitraged. I'll take that.

 

[Ferretusmaximus]

Depends on your view and your hardware configuration. I don't have gas so I've tried to optimise my configuration for dependency on electric. I've invested in enough battery capacity to cope with the coldest February day (nice not to be dependent on the grid) so charge the batteries every night at 8p kWh. If I was using the grid I would be paying 25p kWh so for me I'm saving 17p per kWh. At 30 kWh+ each February day that's £5+ per day at peak.I then regard the solar production and feed-in tariff as "profit". The night rate also charges the car very cheaply so win, win, win.

 

[Rolfe]

It's really all about the difference between import and export rates, and transmission losses. When I started the thread last week my difference had just been cut to only 5p (from 8p), and I wondered if that would all be swallowed up by transmission losses. Everest calculated that I'd still be 3.2p ahead on the deal.

Today, though, I got the news that the difference is about to go back up to 8.5p, which leaves me 6.7p/unit ahead for my battery export. Most days I have 50-80% left in my battery in the evening, depending on usage.

Isn't this fun!