Energy cost comparison

Nobody replaces a working appliance for the fun of it, and nobody switches supplier for the paperwork. The question in both cases is the same one: what does a year of this look like on the bill, and is the difference big enough to bother with. This page takes two sides — a wattage and a price each — and reports the yearly gap between them. Change only the wattage and you are comparing two appliances. Change only the price and you are comparing two tariffs on the appliance you already own. Change both and you are doing what most people actually do, which is switch the thing and the supplier in the same month and then wonder which change did the work.

How it is calculated

saved = (W₁ × h × d × p₁ ÷ 1000) − (W₂ × h × d × p₂ ÷ 1000)

Each side is watts turned into kilowatts, held for the hours a day and the days a year you set, then priced at its own rate per kilowatt hour. The saving is the first side minus the second, so a positive number means the option you are weighing up is cheaper and a negative one means it is not. The share is that saving as a percentage of what the first side costs, which is the figure worth quoting because a saving of fifty a year means something quite different on a bill of eighty than on a bill of two thousand.

Source: NIST SP 811 (2008), Table 2, SI coherent derived units — watt (W), the SI unit of power: 1 W = 1 J/s

Questions people ask

How do I compare two tariffs rather than two appliances?

Put the same wattage on both sides and change only the prices. The energy saved then comes out as zero, which is correct — a cheaper tariff moves the bill and not the meter — and the money saved is the whole answer. It also works the other way: the same price on both sides with different wattages compares two machines on one tariff.

Why is my answer negative?

Because the option you are weighing up costs more than the one you have. The sign is kept rather than hidden: a negative saving is a real result and often the useful one, since it tells you the upgrade you were about to buy is a downgrade on running cost. The line underneath names which side is cheaper so there is no reading the minus sign the wrong way round.

Does this tell me whether a new appliance pays for itself?

Not on its own, but it gives you the number you need. Take the yearly saving here and divide the price of the new appliance by it, and that is roughly how many years the swap takes to break even — ignoring installation, disposal and the fact that the old machine still works. If the answer runs past the expected life of the new one, the case for buying it is not a financial case.

Should both sides really run the same hours?

For most comparisons yes, because the point is to hold use constant and vary the machine or the price. It is not always true — an efficient heater may need to run longer, a faster kettle shorter — and where that matters this page will understate the difference. In that case work each side out separately on the running cost page and compare the two yearly figures by hand.

What about the standing charge when switching supplier?

This compares the per-unit part only, and on a tariff switch the standing charge is often where the difference hides: a lower unit rate with a higher daily fee can be worse for a light user and better for a heavy one. Work out the yearly gap in standing charge separately — it is a daily figure times 365 — and set it against the saving here before deciding.

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