Formula
Energy is watts × hours per day × days, divided by 1,000. Cost is that energy times the rate you enter. A kilowatt-hour you type in replaces the product, so a number copied off a meter is not run through the watt fields.
kWh = watts × hours × days ÷ 1,000
cost = kWh × rate
There is no live utility rate, no tier, and no tax. If your bill splits delivery and supply, add those into the single rate before you type it, or the cost will be short. Watts from a voltage and a current, before the hours, are the Ohm’s law calculator.
Worked examples
600 W, 5 hours a day, 30 days, $0.15/kWh. 90 kWh and $13.50.
A meter that already says 450 kWh, at $0.16. The cost is $72.00. The watt fields are ignored once kWh is above zero.
A 10 W load left on all month. 10 × 24 × 30 / 1,000 = 7.2 kWh, $1.08 at $0.15.
Reference
1,000 watt-hours = 1 kWh. A day-long 1,000 W load is 24 kWh.
Questions
What does a 600 watt load cost at 5 hours a day?
Over 30 days that is 90 kWh. At $0.15 per kWh the energy charge is $13.50. The rate is whatever you type. This page does not look up a utility tariff.
Can I start from a kWh figure I already have?
Yes. Enter the kilowatt-hours and leave watts at 0, or enter a kWh above zero and it wins over the watt calculation. The cost is still kWh times your rate.
Does this include demand charges or taxes?
No. It is energy times a single rate. Demand, tiers, taxes, and fees are not modeled.
Sources
- The kilowatt-hour definition: watts × hours ÷ 1,000.
- The price is an input. No tariff is fetched.