Hi all,

I'm looking to probe the assembled brain trust for advice on the best tariff options for my imminent energy systems upgrade - installers on-site this month. A couple of caveats up front - to keep this on topic 😉 

• I know tariff design is changing - there's a lot more flexible tariffs these days, and there's national discussions re: zonal and half-hourly pricing - let's go with what's available or is expected soon.
• My set-up and final components may not be to your liking. However, after nearly three years of effort, I've finally managed to get this project off the starting blocks. Could I have chosen differently? Absolutely. However, one aspect people on the internet don't discuss is that if you tell the installer what to install, you become much more responsible for the system working, i.e. "but sir, you told us your heat loss calculations were better/more accurate than ours. We sized the heat pump to your specs, it's not our fault if it's undersized". I have to hold my installers responsible, not offer them a get-out clause if there is work to be done to optimise the system. I realise this is a fine balance with cowboy or inexperienced installers who don't know or care about what they install.

We have a 4-bedroom bungalow in the north-east of Scotland that has been insulated within an inch of its life. We currently have an oil boiler and a boiler stove connected to a 500-litre thermal store that provides heating and mains pressure DHW. There's 6 kW PV on the roof with a 3.6 kW Solis inverter. I've put off exporting until this current project is finished - then I may consider it. I'm still working on the "every electron generated is a prisoner" mentality, but am open to the option that it might be more cost-effective to export PV during the day and import at a cheap rate overnight.

So what will I have by the end of the month?

• 9 kW Kensa ground-source heat pump, with a 200m borehole, 300 litre hot water tank, and a 90 litre buffer (that's going to trigger people, but it is specified by Kensa, so again it's safer to go with the recommendation)
• 7 kW Zappi EV charger - though the EV won't be along until later in the year. Likely to be a Mini, or something of a similar size, for mostly local trips. Have a petrol car for long trips.
• 8 kW PV array (another 2 kW to the current system)
• Tesla Powerwall3 plus an expansion pack, giving 17 kWh of storage.

We are currently with OVO on a standard tariff and have no smart meter. Before the heat pump, EV, and potential export, we have gone from 6,200 kWh a year to 4,800 kWh with the current PV. That means that we have used about half of the electricity generated. I'm guessing our future demand will be in the region of 8 to 10,000 kWh with the heat pump and EV, but I will have to wait a year to get good data.

My current thinking is to be in a position all year round to never need to use standard or peak rate electricity.

In the summer, we use about 10 kWh daily, and the current PV array generates between 10-15 kWh per day from Apr to Sep. The upgraded PV will likely provide for all our needs (including the EV), so an overnight tariff to top up the battery in readiness for cloudy days is what is needed.

In the winter, our current demand goes to 16-18 kWh per day, and PV averages about 1.5 kWh (6 on the rare good day). Future winter demand will jump hugely, I just don't know by how much yet. The working assumption here is that we charge the battery to full overnight, topping up with whatever PV we manage to get. I need to check how quickly the battery can be charged, but I'm guessing this will be the limiting factor. So, again, an overnight tariff that has a long enough duration to fill the batteries.

After writing it all down, it looks like I need a tariff that allows as much overnight charging as possible, but that doesn't tie me into specific equipment, like an EV charger or heat pump manufacturer. Octopus used to be the go-to supplier for innovative tariffs, but it looks like others are catching up quickly. What are people's thoughts?

One final thought from me, I might end up pulling more current than my cut-out fuse can handle with overnight demand from an EV charging, a battery charging, and the heat pump ticking over. Is this really a risk? I have just been upgraded from a 60 A to 80 A fuse. The 80 A fuse can go over 100 A for a while, but every winter night for 6 hours? Not sure.

Thanks in advance for your input