day night meter

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 I recently looked at moving from a basic electricity meter to a day/night meter, and got a little caught up in the maths, so thought I write it here.

Where I live, your electricity is made up of a standing monthly charge, and a usage charge.  you by default get a meter which is an all day meter and not aware of day/night.  though smart meters are coming  (smart tarriffs some other time in the future).

For free you can change your meter to be a day night meter, where the day tariff is higher than the all day tariff, but the night tariff is a lot cheaper.  The standing rate is different, but lets ignore that or now.

There is a magic point where your % of electrcity used during the day vs night, will be equal to the cost of the all day tariff, and expressing this lets you figure out if you will save money moving from all day to day/night.  The maths looks like this

(night rate - all day rate) / (night rate - day rate) = %day  (the percent of you 24hr usage that must be in 'day' time

In my case, standard all day rate is 0.1426 c per kwh , day rate = 0.154 and night rate = 0.0839

using that math above, 84% is the magic number.  if I used < 84% of my daily electricity during the day, I will save money.  My current usage is 70/30 so savings will be achieved :)


Now, about standing rates ... for this example, the all day standing rate is 0.52c a day, and day/night is .70c a day, this can be expressed that to save this 18c difference a day, moving 3kwh from day to night would give 0.18/(0.1426-0.0839) = 3.06 or 3kwh.


The final answer is that I must be 84% or less usage is day, and an additional 3kwh must move to night to save money

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