Note: this post was written in 2019 and a lot has changed since then in terms of energy prices!

One way to minimise the cost of running an EV is to use an off-peak electricity tariff. Mind you, there’s not much to save. Even at the average domestic price of 15p per unit (kWh); 10,000 miles would cost you approximately £300. Not per month, that’s per year. There is a gadget that can cut even that low cost down hugely, and ensure that your EV is powered in the greenest, most grid-friendly way possible. It’s the “Ohme” smart cable.

How? Well, the cost of wholesale electricity can vary hugely depending on the time of day and customer demand. Most tariffs hide this from the customer – but if you use most of your electricity off-peak then you can take advantage of this volatility with the right tariff.

Octopus has an “Agile” tariff, which varies by the half-hour. It is capped at 35p, and peaks between 4 and 7pm. Generally it’s been around 10p for the last couple of months, July and August. The average price paid in the UK is around 15p. The UK has a lot of wind turbines, 31GW installed, which is similar to the demand- so very windy periods, especially with little demand, such as overnight, result in low prices. Typically, it seems that an Atlantic storm mid-week produces enough wind energy to bring prices to zero. Add in solar in the summer, and afternoon prices can be low too, 5p is not unusual. The price isn’t limited to zero- it can go negative! In December 2018 prices dropped to zero on one occasion, and -2.4p on another. December 2019 has seen two consecutive nights of negative prices as a storm crossed the country; -1p for a few hours on a Sunday morning then as low as -5.09p, on Monday morning. Of course periods of low prices also correspond to low carbon – a nice bonus. Historical Octopus prices can be seen on the website

So, if prices vary so much, how can the EV driver make sure that they use the cheapest periods? Most cars have a charge timer, and setting this to charge between, say 1 to 5am will generally work well. A better way is an intelligent charge cable, and Octopus have teamed up with Ohme to offer a half-price Ohme cable.

The Ohme cable has a mobile connection to pull the tariff from Octopus, and also get the car’s state of charge from the API – if that information is available for your car (Tesla, Nissan, BMW are supported). Hyundai don’t yet offer this. A phone app interface to the Ohme enables the user to set an amount of charge to add, as well as other options such as prioritising electricity cost or carbon content. This happens to enable another missing Hyundai Ioniq feature; the ability to stop charging before it reaches 100%, to be kinder to the battery. It just takes a bit of thought to work out how many battery percent you need to reach, say, 90% full. If your car has an API you can simply ask it to charge to a certain percentage or range.

The result of this is on your electricity consumption can be seen below (using an Open Energy Monitor). I asked for 25% by 7am. The Ohme is clever enough to use only the cheapest periods, and better than that, modulates the charge rate too. Overnight, prices varied from 2.6p to 8.8p, so this can make a big difference over time. Note that smallest variations in the graph aren’t the car, they’re fridges turning on and off, as this shows the whole house consumption. Now, the cheapest period was 3:30-4:00 and I noticed that Ohme chose the earlier period, 2:30-3:00 to charge at the maximum charge rate, 6.6kW. Ohme say this is a bug due to a rounding error that will be fixed.

The results of smart charging on Agile, compared to using our previous Octopus off-peak tariff are easy to calculate for the off-peak EV charging, however the overall impact on the total annual electricity bill won’t become clear until we have been on Agile for 12 months.

For EV charging in the above example, 8.25kWh gets us 38 miles and cost just over 25p, this would have cost 74p on our old off-peak tariff. The downside would be charging at peak rates, at peak time in winter – for example if the battery was flat at 5pm and the car was needed. This could cost almost £3- however in practise if I needed to, I’d use a rapid charger just a mile away instead at £1!

The financial benefit of the Ohme will probably pay back the purchase price (£199) in maybe 1 year, again this depends on the actual electricity prices. The convenience aspects and the ability to avoid a 100% charge, extending battery life, are more of an incentive to me.

A log of “plunge pricing” events (zero-cost or below) that Agile customers could take advantage of:

8 December 2018: -2.65p

25 December 2018: 0p

8 December 2019 -1.9p

9 December 2019: -5.57p

14 January 2020: 0.05p (as close to zero as we have seen on a weekday night!)

31 January 2020: -0.07p

2 February 2020: 0.0p

10 February 2020: 0.16p

11 February 2020: 0.86p (and under 3p for 3 hours on a weekday)

…so you can see that, as more wind turbines are installed, these plunge-pricing events are becoming more frequent.

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