An EV tariff offering very cheap overnight electricity can look like an obvious choice if you charge an electric car at home.

You might see an off-peak rate that's dramatically lower than a normal electricity unit rate and conclude:

EV tariff = cheaper electricity bill.

But that isn't necessarily true.

The important question isn't simply:

How cheap is the EV charging rate?

It's:

What will all of your household's electricity cost on the tariff?

Because while an EV tariff may offer very cheap electricity during certain hours, the rate you pay at other times can also matter.

For some households, shifting a large amount of electricity into the cheap period could produce substantial savings.

For others, the saving on the car may be partly—or potentially completely—offset by what happens to the rest of their electricity consumption.

How does an EV energy tariff work?

There isn't one universal EV-tariff design.

One common structure gives you:

a cheaper off-peak electricity rate

and:

a higher normal or peak electricity rate.

The cheaper period is usually intended to encourage EV charging when electricity demand is lower.

Some tariffs also allow other household electricity used during those hours to receive the cheaper rate.

Others work differently and may apply a special rate or credit specifically to eligible smart EV charging.

That's why comparing EV tariffs purely by their advertised EV charging rate can be misleading.

You need to understand what electricity qualifies for that rate.

The cheap EV rate is only half the calculation

Imagine an EV tariff offers:

Off-peak rate: 8p/kWh

while another tariff charges:

25p/kWh all day.

The 8p rate immediately looks much cheaper.

And for electricity that genuinely receives that rate, it is.

But suppose the EV tariff charges:

29p/kWh during peak hours.

Now there are two effects:

Saving: electricity moved to 8p/kWh becomes much cheaper.

Cost: electricity remaining at 29p/kWh becomes more expensive than the 25p single-rate alternative.

The question becomes whether the first effect is greater than the second.

A worked household example

Let's use a hypothetical household with:

Non-EV electricity consumption: 2,500 kWh/year

EV charging consumption: 2,000 kWh/year

Total electricity consumption:

4,500 kWh/year

We'll compare two simplified tariffs.

Standard tariff

Unit rate:

25p/kWh

Ignoring standing charges initially:

4,500 × £0.25 = £1,125

Estimated annual electricity cost:

£1,125

EV tariff

Peak rate:

29p/kWh

Off-peak rate:

8p/kWh

Suppose all 2,000 kWh of EV charging takes place at the off-peak rate, while the household's other 2,500 kWh remains at the peak rate.

EV charging:

2,000 × £0.08 = £160

Other household electricity:

2,500 × £0.29 = £725

Total:

£160 + £725 = £885

Compared with the standard tariff:

£1,125 − £885 = £240

In this example, the EV tariff is approximately:

£240 a year cheaper

before considering standing charges and any other tariff differences.

That's a meaningful saving.

But notice what produced it.

It wasn't simply the existence of an 8p EV rate.

It was the combination of the household's electricity consumption and how much of that consumption qualified for the cheaper rate.

What if you don't charge very much?

Now imagine the same household only uses:

500 kWh/year charging the EV at home.

Its other household electricity remains:

2,500 kWh/year

Total consumption:

3,000 kWh/year

On the standard 25p tariff:

3,000 × £0.25 = £750

On our hypothetical EV tariff:

EV charging:

500 × £0.08 = £40

Other electricity:

2,500 × £0.29 = £725

Total:

£765

The EV tariff is now:

£15 more expensive

despite offering electricity for the car at just 8p/kWh.

That's the important point.

A spectacularly cheap EV rate doesn't automatically produce a cheaper household tariff.

There is a break-even point

This gives us another useful Likewise Compare break-even calculation.

Our two tariffs are:

Standard tariff:

25p/kWh

EV tariff:

29p peak / 8p off-peak

Every kWh moved from the standard tariff to the EV off-peak rate saves:

25p − 8p = 17p

But every kWh of ordinary household electricity remaining at the EV tariff's peak rate costs an additional:

29p − 25p = 4p

With 2,500 kWh of ordinary household electricity, that extra peak-rate cost is:

2,500 × £0.04 = £100

How much off-peak EV charging is needed to recover that £100?

£100 ÷ £0.17 ≈ 588 kWh

So in this simplified example, the household needs roughly:

588 kWh of off-peak EV charging per year

before the EV tariff begins to beat the standard tariff.

Above that point, the EV tariff increasingly wins.

Below it, the standard tariff wins.

That's much more informative than simply saying:

“8p EV charging.”

Your mileage matters

The amount of electricity your EV needs depends partly on how far you drive and how efficient the vehicle is.

For illustration, suppose an EV averages:

3.5 miles per kWh

and you drive:

7,000 miles a year.

Approximate electricity required would be:

7,000 ÷ 3.5 = 2,000 kWh

If most of that charging happens at home and qualifies for the off-peak rate, a cheap EV tariff could have a significant effect.

But someone driving only 2,000 miles a year—or regularly charging at work or public chargers—might put much less electricity through their home EV tariff.

So two households with the same car could reach completely different conclusions.

Where you charge matters too

Annual mileage isn't enough on its own.

Suppose you drive 8,000 miles per year but:

• charge extensively at work

• use public rapid chargers regularly

• receive workplace charging

• rarely charge overnight at home.

Your home EV electricity consumption might be considerably lower than another driver travelling the same mileage.

For tariff comparison purposes, what matters is not merely:

How far do you drive?

It's:

How many kWh will actually be charged at home at the discounted rate?

Can the rest of your house use the cheap rate?

This can materially change the calculation.

Some EV tariffs provide an off-peak window during which other household consumption can also benefit from the cheaper rate.

For example, current Octopus Go terms provide a five-hour overnight rate, while Intelligent Octopus Go provides a six-hour off-peak period and applies that night rate to other household electricity used during those hours.

That potentially creates additional savings if you can schedule things such as:

• washing machines

• dishwashers

• tumble dryers

• battery charging

• water heating

during the cheap period.

So our earlier calculation could actually underestimate the benefit for a household capable of shifting significant consumption overnight.

But again, that depends on behaviour.

Not every EV tariff works the same way

This is especially important when comparing suppliers.

An EV tariff might provide:

a whole-house off-peak window

while another might provide:

a special price or credit for qualifying smart EV charging.

For example, OVO's current Charge Anytime terms describe an effective kWh rate for qualifying smart charging, implemented through a credit mechanism, rather than simply treating all household electricity during particular hours identically.

Those aren't directly comparable simply because both are described as EV propositions.

You need to understand the charging rules behind the headline rate.

Check whether your car and charger are compatible

Some smart EV tariffs also have eligibility requirements.

These can include:

a compatible smart meter

half-hourly meter readings

a compatible EV or charger

use of the supplier's app

allowing the supplier to control or schedule charging.

For example, Intelligent Octopus Go currently requires compatible technology and smart charging integration; its terms also require customers to provide charging preferences through the app.

So the cheapest-looking tariff isn't useful if your charging setup doesn't qualify for it.

What happens if you need to charge during peak hours?

This is another practical consideration.

Imagine you normally charge overnight but arrive home with a nearly empty battery and need the car again in two hours.

If you manually charge immediately, that electricity may be billed differently from scheduled off-peak charging.

Current Intelligent Octopus Go guidance, for example, says an urgent boost outside the cheap period is charged at the applicable peak rate.

If that happens occasionally, it may make little difference.

If it happens frequently, the real economics of the tariff could look different from the idealised calculation.

Standing charges still matter

As we explained in Energy unit rates vs standing charges: what actually matters for your bill?, unit rates aren't the entire tariff.

You should also compare:

standing charge × 365 days

for each tariff.

Suppose our EV tariff saved £100 through its unit rates but had a standing charge £50 a year higher than the alternative.

The actual estimated advantage becomes:

£100 − £50 = £50

not £100.

The tariff should therefore be compared as a complete package.

Solar panels and home batteries can change the calculation again

A household with solar panels or a home battery may have a very different electricity profile from one without them.

A battery could potentially charge during cheap periods and supply some household consumption later.

Solar generation may reduce the amount of grid electricity required during daytime periods.

Export tariff compatibility can also matter: some EV tariffs have restrictions or specific compatible export products. Octopus, for example, sets out compatibility rules between its EV and export tariffs.

At that point, simply comparing two import unit rates becomes increasingly inadequate.

You need to model the household's actual energy flows.

What should you compare before choosing an EV tariff?

Rather than asking only:

“What's the cheapest EV charging rate?”

look at:

Your household

• annual non-EV electricity consumption

• estimated annual home EV charging

• when you normally use electricity

• how much consumption you can realistically shift

The tariff

• peak unit rate

• off-peak unit rate

• off-peak hours

• standing charge

• fixed or variable

• exit fee

The charging rules

• whether the cheap rate applies only to the EV or the whole home

• vehicle compatibility

• charger compatibility

• smart-meter requirements

• what happens when you charge outside scheduled periods.

Then calculate the estimated annual cost on the same basis.

The cheapest EV rate isn't necessarily the cheapest tariff

EV tariffs demonstrate particularly well why headline rates can be misleading.

8p/kWh sounds dramatically better than 25p/kWh.

And for electricity genuinely charged at 8p, it is.

But your household doesn't necessarily consume all of its electricity at 8p.

The meaningful comparison is:

What does the EV cost to charge?

plus:

What does the rest of the household's electricity cost?

plus:

What are the standing charges and other relevant tariff costs?

That produces an estimated household cost that can actually be compared.

Eventually, the comparison should do this calculation for you

This is precisely the kind of decision Likewise Compare is being built to make clearer.

An EV household shouldn't have to choose a tariff because one supplier advertises an eye-catching overnight rate.

The better comparison would understand how much electricity the household uses, how much the EV is likely to consume, when that electricity can be charged and what happens to the rest of the household's energy costs.

Then it can answer the question that actually matters:

Is this EV tariff cheaper for this household?

Not:

Does this tariff have the cheapest-looking EV rate?

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