Your bill totals every appliance together, so the number that shocks you each
month points at nothing. You need one device broken out, one month at a time.
That decomposition is a four-field calculation, and our electricity cost
calculator runs exactly the version described below.
The formula our calculator runs
The tool takes four inputs. Wattage in watts, hours of use per day, days of
use per month, and your price per kilowatt-hour. It then computes three
energy figures and three cost figures.
daily kWh = (watts x hours) / 1000
monthly kWh = daily kWh x days
yearly kWh = monthly kWh x 12
each cost = the matching kWh figure x price per kWh
The division by 1000 converts watt-hours to kilowatt-hours. A 100-watt device
running for 8 hours consumes 800 watt-hours, which is 0.8 kWh. Your utility
bills kilowatt-hours, so that is the unit the tool outputs.
The two default values in the tool are 30 days per month and a price of 0.15
per kWh. The price is a placeholder, and the next section shows why replacing
it matters more than any other step.
A worked example you can check by hand
Enter a 100-watt bulb, 8 hours per day, 30 days, and 0.15 per kWh. The tool
returns 0.8 kWh per day and 24 kWh per month. The daily cost is 0.12 and the
monthly cost is 3.60.
The yearly line shows 288 kWh and 43.20. Every one of those numbers follows
from the formula above, so you can verify them with a phone calculator in
under a minute. If a different calculator gives you materially different
numbers for the same inputs, one of the two tools is not using this formula.
Rounding is fixed in the code. Daily kWh is rounded to three decimals.
Monthly kWh, yearly kWh, and every cost figure are rounded to two decimals.
Where the yearly number runs short
The yearly figure multiplies your monthly figure by 12. If you leave days at
the default 30, the tool models a 360-day year. A device that genuinely runs
every single day gets understated by five days of consumption per year.
Apply this rule for always-on devices. Divide 365 by 12 and enter 30.42 in
the days field. Your twelve modeled months then cover 365.04 days, which
removes the gap. For devices you use a fixed number of days, such as an
office printer running 22 working days, enter the true count and accept that
the yearly line extrapolates those same months twelve times.
This is the honest limit of the design. The tool models one average month
and scales it. It does not know that heating runs in January and sits idle in
July.
What the calculator cannot see
Three blind spots cause the result to drift from your real bill.
First, nameplate wattage is not always real draw. A refrigerator label states
the compressor's peak power, but the compressor cycles on and off. An air
conditioner, a dryer, and anything with a thermostat behaves the same way.
If you own a plug-in energy meter, use this workaround. Measure the device's
kilowatt-hours over one full day. Multiply that number by 1000 and enter it
as the wattage, then set hours per day to 1. The formula then reproduces your
measured daily kWh exactly, and the monthly line multiplies it out correctly.
Second, the tool takes one wattage and one runtime. It cannot split a device
into active and standby consumption. If standby draw matters to you, measure
it separately and run the calculation twice.
Third, the price field must be your all-in rate. The tool applies a single
rate to every kilowatt-hour. It does not model tiered pricing, time-of-use
windows, delivery fees, or taxes. Copy the effective rate from a recent bill
by dividing the total charge by the total kWh on that bill. If you enter a
bare energy-only rate, your estimate will land below what you actually pay.
One interface detail is deliberate. If you click Calculate and nothing
appears, one of the four fields is empty or zero. The code rejects those
inputs silently instead of showing an error message, so check the fields
before you assume the tool failed.
Steps to run an appliance audit
1. Open your latest bill and divide the total charge by the total kWh. Enter
that rate in the price field.
2. Pick the five devices you suspect cost the most.
3. Find each wattage on the label, the manual, or the power brick.
4. Estimate honest hours per day for each device, not best-case hours.
5. Run each device through the calculator and write down the monthly cost.
6. Sort the list and act on the top three entries first.
Checklist before you trust the number
- Price per kWh is the all-in rate from a real bill.
- Wattage matches the nameplate or a meter reading.
- Hours per day reflect actual use, including idle hours.
- Days per month is 30 for typical use and 30.42 for always-on devices.
- Cycling devices were measured with a plug-in meter.
- The sum of your line items is compared against the bill total once.
If your line items sum far below the bill, the difference is fixed fees,
tiered rates, or devices you forgot to enter. That residue is itself a
finding, and it tells you where the next measurement belongs.
Run your own numbers through our electricity cost calculator at
/electricity-cost-calculator. If your measured appliance costs disagree with
the formula's output for the same inputs, send us the four inputs and the
meter reading, because that pair exposes either a tool bug or an interesting
device behavior worth documenting.
