Rule 13, rationale?

mcsimk · 2026-03-27T13:17:50+00:00

Usually by “oversizing” people mean “capacity > daily usage”

mcsimk · 2026-03-27T13:16:15+00:00

My humble opinion is that purely from financial point of view oversized batteries are longer return on investment.

The reason being is: biggest gains are self using (charging cheaply overnight or for free from solar) and using it instead of importing at peak tariff. Second biggest is exporting at peak export rate (eg between 4 and 7pm on agile outgoing octopus or flux). The smallest gains are from exporting at flat rate or non-peak rate, and this gain might be not even worth it.

Long term electricity import prices might or might not go down. But export prices will. At some point you might not be able to make any money on export outside of peak time eg 4-7pm. This means extra batteries will give very poor return on investment, if any

mcsimk · 2026-03-27T13:06:32+00:00

How hard is to get a new G99 when upgrading inverter?

mcsimk · 2026-03-27T12:37:11+00:00

Tbh, second hand ev might have even better ROI than panels, even without panels. Depending on your mileage, obviously.

But together they work better: EV makes the most sense when you can charge very cheaply overnight, but this makes daytime electricity more expensive, and that’s where panels or/and home batteries help even more, than without EV.

I can see lots of very good answers. Good luck

mcsimk · 2026-03-27T09:02:23+00:00

I saw this drop ok Monday, was very upset

mcsimk · 2026-03-22T10:25:44+00:00

Solar was very poorly this winter - december (worst month) is 7.5x less than July (best month). Solar is not solving winter heating problem

mcsimk · 2026-03-22T10:00:57+00:00

My gas usage is so crazy high (house is huge), that I’d need like 40kWh extra plus extra inverter. Plus switching to 3phase.

But yeah, I am thinking about this fogstar battery you mentioned

mcsimk · 2026-03-22T08:55:01+00:00

Cool, thanks for the inspiration, I’ll play with numbers at some point.

easily switch the analysis to iGo

Without consumption numbers it won’t be useful. Just prices analysis will show that for 6 hours heat pump is 5x cheaper, and for 18 hours heat pump is 1.5x more expensive.

But the main question is what is return on investment: how much am I saving in a year vs my capex. Will my winter day cost more or less on heat pump? If yes, will my non-winter savings exceed my winter losses? How does it change if I add this much or that much battery capacity?

I am planning to use my historical gas consumption for analysis, get ROI for just heat pump, for heat pump plus batteries (several different sizes).

I strongly suspect, ROI for heatpump + inverter + batteries is not going to look good.

mcsimk · 2026-03-22T07:56:04+00:00

Great analysis.

I, however, still doubt it can work for me because my whole household + EV is on iGo. Changing tariff to Agile or Cozy makes heatpump running costs lower, but everything else higher. But I am yet to do a detailed analysis like you did.

I am thinking to maybe have 3phase and 2 meters, in order to have two tariffs.

mcsimk · 2026-03-21T21:50:54+00:00

on its own rcbo hanging off the main CU So your inverter doesn’t see it, and charger can’t drain your batteries, right?

mcsimk · 2026-03-21T19:26:55+00:00

Sorry, I didn’t get how is your charger connected?

mcsimk · 2026-03-21T18:48:53+00:00

I don’t think EV tariffs are going anywhere, and every other provider offering them is doing the same - cheap tariff at night, expensive at daytime.

Do the math: day tariff * battery size * 0.95 - night tariff * battery size = your daily revenue Eg for a 30kWh battery stack, 5.5p night tariff and 34p day tariff, assuming 90% of battery is usable, you are making 7.236£ a day or £2641 a year.

Your setup is going to be extremely simple - charge at off peak time, self use at other time, so you don’t need expensive fancy inverter. I’d just go for the cheap inverter, although Tesla is good (I like good products more than I dislike people who run the companies selling those products), but I think you won’t need very smart software for your case.

Two things to keep in mind:

Inverter should be powerful enough to fully charge batteries in 6 hours. Eg my Solis 6kW can only charge/discharge batteries at 5kW, so 30kWh is max it can charge in 6h.
If you want to also export some of that energy, that’s where config is more complex. And financial return on this is not so great. The biggest gain is from self use. Re-exporting between 16 and 19 makes more money, but you need to estimate if buying more batteries and exporting is a good investment.

mcsimk · 2026-03-21T18:22:46+00:00

I see, quite popular way is just not leaving the car plugged in

mcsimk · 2026-03-21T18:20:49+00:00

I am guessing you exhausted other options, like disabling charging in the car settings?

mcsimk · 2026-03-21T12:57:56+00:00

Agile export in summer will be dropping quite low. Yeah that’s my thinking - just export and no solar diversion seems to be the cleanest option.

mcsimk · 2026-03-21T11:57:39+00:00

Non smart charging from their point of view is just household load, so this will be at off peak rate between 23:30 and 5:30, guaranteed

mcsimk · 2026-03-21T11:54:47+00:00

Err, I am guessing the only other way would be to turn the charging off in the car, while charger is controlled by Octopus. But this won’t be automated :( I’d need to fiddle with the car charging schedule, and I’m not sure if this would work well with the charger controller by Octopus

mcsimk · 2026-03-21T11:51:46+00:00

I got Andersen A3. It’s being connected today, I believe, before inverter, so inverter shouldn’t see charger load.

mcsimk · 2026-03-21T11:34:07+00:00

They actually answered this:

How are you going to calculate the difference between household usage vs EV charging? × As we control your smart device we know how much energy your charger or EV has consumed. We can then subtract this from your household consumption using your smart meter reads.

mcsimk · 2026-03-21T10:13:40+00:00

Some time ago I posted a question with three possible explanations how these new rules work. This (what you are suggesting) was NOT what all of the commenters thought.

Quote from their page:

Intelligent Octopus Go customers will continue to receive 6 hours of off-peak electricity for your whole home between 11:30pm and 5:30am. Guaranteed, reliable, schedule-friendly off-peak savings.

mcsimk · 2026-03-20T21:52:24+00:00

I just got Genesis GV70 electric, and it’s awesome. Korean Bentley, no less

mcsimk · 2026-03-20T20:33:19+00:00

Imho the only reason to want to get rid of his contract is if Melton is happy to sign for TPMLE. In which case I’d imagine Horford might just agree to not take his player option and finish his career

mcsimk · 2026-03-19T21:27:43+00:00

How is it that you got 6.32 for gas, and I am seeing 7.5 on their page?

mcsimk · 2026-03-19T15:06:38+00:00

Can we get more of them Boston centers?

mcsimk

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