Worth its own thread, because the calculator is only as good as what goes into it.
The usual suspects, in the order they catch people out:
- The rate. The calculator defaults to the price cap, and if you are on a fixed tariff or a time of use tariff, your real unit rate is not that.
- The standing charge. It is on the bill whatever you run, so a bill that went up by more than the calculator said may not have gone up because of the unit at all.
- The hours. Almost everybody underestimates these. A thermostat cycling through the night is not the same as the hours you think you ran it.
- The unit's rated power against what it actually draws. A machine working harder in a hot room draws more than its label suggests.
If you post your unit, your rate, and roughly how long you ran it, somebody can sanity check the arithmetic with you. If the gap is still there after that, it is worth knowing about.