Electrical
Electricity Cost Calculator for Electricians
The running cost of a circuit or appliance, worked from whichever figures you actually have: a nameplate, a clamp meter reading, or a wattage.
Result
Power factor matters whenever you are working from a clamp meter. A meter reads current, and current times voltage gives apparent power, which is larger than the real power the utility bills for anything that is not purely resistive. Heating elements run at a power factor near one; motors, drives and switching supplies run lower, so treating a motor's measured amps as though the load were resistive overstates the cost. The three phase factor is the square root of three, about 1.732.
Get a Free Business AuditHow the calculation works
Three routes to the same answer, because the figure you have depends on how you measured it. A wattage goes straight in. Amps and volts multiply to apparent power, then the power factor converts that to the real power a meter bills, with a root three factor on three phase. Measured kilowatt hours skip the conversion entirely. From there it is kWh times your rate, with the hourly and annual figures scaled from the same draw.
What moves the number
- Power factor, which on anything inductive makes measured current overstate real power noticeably.
- Whether the supply is single or three phase, which changes the calculation by a factor of about 1.73.
- Duty cycle, since nameplate draw and average draw differ for anything that modulates or cycles.
- The rate used, because a blended figure ignores tiered and time of use pricing.
Where this one goes wrong
Treating a clamp meter reading as real power is the error that overstates running cost on every motor, drive and switching supply, because current times voltage gives apparent power and only the power factor converts it. Nameplate figures cause the opposite problem: they are maximum rated draw, so anything that cycles or modulates runs well below them most of the time, and costing a modulating load at its plate rating inflates the answer. Where the number matters, a measurement over a representative period beats both.
Before you quote
- Apply the power factor when working from a current measurement.
- Confirm single or three phase before calculating.
- Use a measured period rather than nameplate draw for cycling loads.
Where the rules come from
Questions
About this calculator
How do I work out running cost from amps?
Multiply amps by volts to get apparent power, then by the power factor to get the real watts a meter bills. For three phase, multiply by the square root of three as well. Divide by a thousand and multiply by hours for kilowatt hours, then by your rate.
Why does my calculated cost differ from the bill?
Bills include fixed charges, taxes and often tiered or time of use rates, none of which are a per kWh figure. A single blended rate will get you close on the energy component and will not reproduce the total. Demand charges on commercial accounts can be a large share and are billed on peak draw rather than consumption.
Should I use the nameplate wattage?
Nameplate figures are maximum rated draw, and many loads run well below it most of the time. For anything that cycles or modulates, a measured reading over a representative period gives a much better answer than the plate. The kilowatt hour method exists for when you have that measurement.
The formula
kWh = watts / 1000 x hours; single phase watts = amps x volts x power factor; three phase watts = amps x volts x 1.732 x power factor
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How to use this calculator
- Work it out from. Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Power draw (watts). Enter your own figure; the page starts at 1,500.
- Hours of use per day. Enter your own figure; the page starts at 6.
- Days. Enter your own figure; the page starts at 30.
- Cost per kWh ($). Enter your own figure; the page starts at $0.17.
- Number of these. Enter your own figure; the page starts at 1.
The result updates as you type, so you can try a figure, see what it does to cost over the period, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Work it out from | A wattage |
| Power draw (watts) | 1,500 |
| Hours of use per day | 6 |
| Days | 30 |
| Cost per kWh | $0.17 |
| Number of these | 1 |
What comes out
| Result | Value |
|---|---|
| Cost over the period main result | $45.90 |
| Kilowatt hours used | 270.0 |
| Power draw | 1,500 |
| Cost per hour of running | $0.26 |
| Cost per day | $1.53 |
| Cost if run this way all year | $558.45 |
Running this draws 1,500 watts and uses 270.0 kWh over the period, which costs $45.90. That is $0.26 an hour, or $558.45 a year at the same duty.
How sensitive the answer is
Holding everything else steady and changing only cost per kwh, this is what happens to cost over the period. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per kWh ($) | Cost over the period |
|---|---|
| $0.09 | $24.30 |
| $0.13 | $35.10 |
| $0.17 example above | $45.90 |
| $0.26 | $70.20 |
| $0.34 | $91.80 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Work it out from
- Choose the unknown quantity. Only the inputs used by that calculation are shown.
- Power draw (watts)
- A number, between 0 and 500000, starting at 1,500.
- Hours of use per day
- A number, between 0 and 24, starting at 6.
- Days
- A number, between 0 and 3650, starting at 30.
- Cost per kWh ($)
- A money amount, between 0 and 5, starting at $0.17.
- Number of these
- A number, between 1 and 10000, starting at 1.
What the results tell you
| Result | What it is |
|---|---|
| Cost over the period | The headline result. A money figure. |
| Kilowatt hours used | A decimal figure. |
| Power draw | A number. |
| Cost per hour of running | A money figure. |
| Cost per day | A money figure. |
| Cost if run this way all year | A money figure. |
Before you rely on this number
Conductor sizing, voltage drop and load figures are governed by code, and the code takes precedence over any calculator. Ambient temperature, conduit fill, continuous load factors and the specific installation all change the answer. Treat this as a first pass, not a design.
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