IHD’s are notoriously poor at displaying correct on peak/off peak rates.
( mine has gone in the recycling)

 

More likely to be an issue with the IHD's clock - Octopus bills are detailed in UK local time, so for now they are BST.

 

Surely in 2025 it is within the wit of an IHD to keep its clock right and aligned with GMT/BST!

 

have you ever seen how complicated the timezone files are? Keeping millions of such embedded system updated with that is very difficult.
The IHD specification for tariffs isn't sufficient for the half-hourly time-of-use tariffs like Agile, so Octopus don't bother trying to keep the tarifff data sent to the smart meter IHD accurate, presumably because they'd have to send millions of tariff messages to the smart meters every half an hour.

 

My IHD is a guide only for me to glance at in the morning. If it's above £1.00 first thing I know the car has charged. Even then sometimes Octopus has shifted the charging before midnight and the IHD seems too low so I check my cars SOC just in case.

The IHD has no idea about Octopus intellignet charging at odd time slots when there's surplus electricity so some days it's pounds over the actual charge to my account.

 

I had thought on how to check GMT/DST - I looked at the half-hourly charges for March and this is what the bill shows :
00:00- 00:30 6.67 0.10 0.660
00:30 - 02:00 6.67 0.10 0.647
02:00 - 02:30 6.67 0.12 0.820
02:30 - 03:00 6.67 0.10 0.667
03:00 - 03:30 6.67 0.79 5.287
03:30 - 04:00 6.67 0.40 2.680
04:00 - 04:30 6.67 0.18 1.187

There is a mising hour at 00:30 to 2:00
So it looks like the clock changes at Octopus so the cheap period stays the same time of day for humans

 

That must make it irritating for those using simple time switches to control things, as they rarely seem to have automatic BST switching in my experience. Our washing machine built-in timeswitch does have the option for automatically adjusting for BST, but we leave it off as our off peak times are locked to GMT. What I do find useful are the Mainslink switches I started using a year or two ago. They work just like timeswitches but are operated by radiolink from a master transmitter that is hard wired to the meter 5th wire. This saves faffing with time switches, as whenever the meter is in off-peak mode the Mainslink switches all turn on, so no need to adjust times etc.

A few months ago I fitted this same Mainslink system for a chap with a Zappi. Very conveniently the Zappi has an input that accepts a mains timeswitch signal to turn it on or off. This works brilliantly, the chap uses the Zappi in off-peak mode all the time and doesn't have to worry about times etc.

 

You might want to adjust to original post to be clearer that you're specifically talking about intelligent octopus Go (presuming you are based on signature). There is no general answer for all octopus tariffs.

Interestingly the FAQ for regular Go explicitly answers this question (off peak is always in your local time, so always 00:30-05:30) but the IOG FAQ does not. In my experience it operates the same way.

The Go FAQ also explains the IHD issue

 

Yes I'm on IOG but I'm talking about the guranteed cheap period not just for car charging. The car charging takes care of itself. Our old system was locked to GMT. Then we had timeswitches to turn things on but that doesn't work with modern appliances. Yes I can set a timer on the machines themselves but the old way was simpler (Luddite warning!) turn on the power and the machie starts.

 

The 23:30 - 05:30 cheap period (which is only those times on IOG. It's at different times on other tariffs) is not locked to GMT.

 

My IHD reports 7p all day for IOG so I ignore all costs from it.

That said, I like my IHD (originally supplied by EdF when they did the smart meter fit in 2023) because I can leave it on a screen that shows live electricity consumption, or going negative for export, and last half hour for gas, all in kW.

I can usually see at a glance if the car is charging.