What day/night ratio makes Economy 7 worthwhile?

Macc

Helping my mum go through her bills at the moment. She has an Economy 7 meter. She doesn't do anything to take advantage of the night rates and her elec use is roughly 85/15 day/night use in an average month. No electric car or anything like that. 

If she made an effort to use things at night (dishwasher, washing machine, chargers etc) how much can she realistically get that ratio down to? It's a house of 3 adults. 

Seems to me it'd be unlikely to be worth it and she'd be better off going on a flat rate tariff? She's with OVO, currently her day rate is 29.03, night rate is 19.08. I can't see what their Oct 1st day/night rates are, not sure if they have announced it yet. 

QrizB

As with all these things, it depends. How are you with maths?

Last autumn I posted a formula for working out how much energy you needed to use on the each rate for it to work out cheaper (see here).

If my formula doesn't make sense, here it is in someone else's post from 2007. You might find that they've presented it better than I did. Here is their working:

This formula is the one to use to calculate what percentage of your electricity consumption needs to be during the night hours in order for an Economy 7 tariff to be worthwhile:

100 x (D - S) / (D - N)

Where D is the day rate on Economy 7,
N is the night rate on Economy 7
S is the standard rate for users not on Economy 7.

If your night-time electricity usage is greater than this then it is worth using Economy 7
If your night-time electricity is less than this then it is not worth using Economy 7.

This formula excludes any differences in standing charges - so it is not 100% accurate.

If you can give details of your mum's tariff, I can work it out for you.

Keep in mind that if she's on a variable tariff it will change in a little over a fortnight and the answer could well be different when it does.

[Deleted User]

So some examples - using the newly published EdF rates with QrizB's equation:

East Midlands (47.31p day, 6.90p night, 33.02p standard) - you need to use more than about 35% at night.
South Scotland (41.09p day, 16.95p night, 33.76p standard) - you need to use more than about 30% at night

Macc

Thanks guys, this forum is such a great help.

So I've worked it out that she actually is better off right now on E7 surprisingly (or I might have got the maths wrong!)

Day rate is 29.03, night rate is 19.08, flat rate would be 28.03 (28.03 is the price cap price atm right?)

So on that basis I make it she only needs to use over 10.05% at night for her to be better off? I guess because of the small difference between the day rate and the standard rate? Have I got that right?

She is on the variable rate so be interesting to see how things change on Oct 1st. Have OVO announced their E7 prices yet?

QrizB

Macc said:

Day rate is 29.03, night rate is 19.08, flat rate would be 28.03 (28.03 is the price cap price atm right?)
So on that basis I make it she only needs to use over 10.05% at night for her to be better off? I guess because of the small difference between the day rate and the standard rate? Have I got that right?

28.02p/kWh sounds about right.

And yes, using your numbers I think she needs to use at least 10.05% at night rate too.

macman

Basics: how is the property heated and hot watered? The things that make E7 feasible are NSH's and an immersion heater, both running on cheap rate E7.
Obviously, it's the winter usage on heating that is the biggest chunk of annual usage, so an 'average month' has to be taken from a 12 month period to be representative.
It depends on tariff, but using just 15% on cheap rate is unlikely to pay.

pensionpawn

Agree with the above however if you have solar panels it's almost a no brainer to go for E7. Import during the day is minimised / eliminated with the advantage of the lowest rate available overnight to run under floor heating just before waking, the dishwasher, the washing machine.......