How to use the energy tariff comparison calculator
- Enter your average monthly electricity use in kilowatt hours, taken from a few recent bills.
- For each tariff enter the fixed monthly charge and the price per kilowatt hour.
- If a tariff has a cheaper or dearer first block, enter the size of that block in kilowatt hours and the price after it. Use 0 for a single price.
- Read the cheapest tariff at your use and the second table, which shows the ranges of use where each tariff wins.
Formula
monthly cost = fixed charge + the first block of kWh x price + kWh above the block x price after the block
yearly cost = monthly cost x 12
two tariffs cost the same where their monthly cost lines cross
Each tariff is a line that bends once, at the end of its first block. Between the bends and crossings the cheapest tariff stays the same, so the tool lists each range with its winner.
Three tariffs at 650 kWh a month
A flat rate costs $12 a month plus $0.15 a kWh, so $109.50 for 650 kWh. A tariff with a $5 charge pays $0.13 for the first 500 kWh and $0.19 after, which gives $5 + $65 + $28.50 = $98.50. A tariff with a $30 charge and $0.11 a kWh costs $101.50.
The second tariff is cheapest at $1,182 a year, which is $132 less than the first and $36 less than the third. At 250 kWh the second one still wins, at $37.50 a month, while at 1,400 kWh the third wins at $184.00 a month because its low price per kWh outweighs its higher fixed charge.
Fixed charge against price per kilowatt hour
A tariff with a high fixed charge and a low price suits heavy users, and one with a low charge and a higher price suits light users. The crossing table makes this plain, since it shows the use at which the best tariff changes. If your use is close to a crossing, your choice matters little, and other things such as contract length and exit fees may decide.
To find how much your appliances use, try the Electricity Cost Calculator, and break a bill into its parts with the Electric Bill Breakdown Calculator. If you work from home, the Home Office Percentage Calculator helps with splitting the cost.
Averages hide seasons
The yearly cost here is twelve times the monthly cost at your average use. If your use is much higher in winter or summer, the real yearly cost on a tiered tariff is different, because months above a block cost more. Run the tool at your low use and at your high use to see the range.
Assumptions and limits
- Use is the same every month of the year.
- Each tariff has a fixed charge, a first block and a price after the block, and nothing else.
- Taxes, credits, time-of-use pricing, demand charges and exit fees are not modeled.
- The prices on this page are examples. Enter the rates from the tariff you are looking at.
Frequently asked questions
What do I enter when a tariff has just one price?
Enter the price per kilowatt hour and set the first block to 0. The price after the block is then ignored.
Why does the second table have several rows?
It lists each stretch of monthly use in which one tariff stays cheapest. A new row starts where the cheapest tariff changes, and the last row runs on to higher use than you are likely to need.
Where do I find my monthly use?
Your bill lists kilowatt hours for the month. Average several months, or use the highest and lowest to see how far the answer moves.
Does it include the cost of a smart meter or a switching fee?
No. Add one-off charges to your own comparison by dividing them by the months you plan to stay and adding the result to the fixed charge.