Depends how well insulated your tank is. If it well insulated you may as well leave it on all off-peak times, as very little energy is going to heat up your airing cupboard. If it isn’t so well insulated you probably don’t want a hot airing cupboard, in summer at least, and it’s best to heat the water as closes as you’re going to use it as possible. You need to experiment a little. I’d initially try setting it for an hour each afternoon and seeing how you go. Remember also that there’s no point heating water hotter than you need to. If you need to dilute hot water with cold you’re pouring money down the drain.
So, I’m currently with npower and my monthly direct debit has gone up by about £19 per month. I rang around some other suppliers who fully support E10 and they were coming out at least £10 more expensive per month than npower.
In that case I’m thinking that just trying to reduce my usage is the best bet to reduce costs. Heating is off for the summer so it’s just the immersion heater that will be using the most off peak. I have two meters, so a standing charge for each and two readings to send to the provider each month, if that’s helpful.
Most of my hot water usage is in the evening- I have an electric shower that I use in the morning. I currently leave the immersion on all the time, so it heats during the night then also during the boost in the afternoon. I don’t know how long it takes to heat the water or how long it retains heat for after it’s switched off. Would it be better to heat during the night only, or during the afternoon boost only or just leave it switched on all of the time?