Energy Consumption Calculator estimates monthly kWh, daily and annual electricity costs, duty cycle, active yearly hours, and savings from shorter runtime for household appliances.
How Wattage, Hours, and Days Turn Into a Monthly kWh Number
The energy consumption calculator runs on one relationship: energy equals power multiplied by time. Everything else on the page is that same idea applied at different time scales, plus a cost multiplier at the end. There’s no code table or compliance threshold behind it — it’s the same math your utility meter is doing every second.
Four inputs drive the result. Power (P) is the device’s draw, entered in watts or kilowatts. Hours per day (H) and days per week (D) describe the schedule, and each has a hard ceiling — hours can’t exceed 24, days can’t exceed 7, since neither unit can hold more time than that.
Rate (R) is your price per kWh in dollars. Power first gets converted to kilowatts, then the calculation runs forward through daily, weekly, annual, and monthly totals:
$$P_{kW} = \frac{P_{watts}}{1000}$$
$$E_{daily} = P_{kW} \times H \qquad E_{weekly} = E_{daily} \times D \qquad E_{annual} = E_{weekly} \times 52$$
$$E_{monthly} = \frac{E_{annual}}{12} \qquad \text{Cost} = E \times R$$
Fifty-two weeks stands in for a year instead of twelve separate months, then the annual figure gets divided by twelve at the end. That ordering matters less for the math than it does for consistency — it keeps every downstream number, daily through annual, built off the same weekly base.
Running a 1,500-Watt Space Heater Four Days a Week: A Worked Example
Say you’ve got a 1,500W space heater. Cold mornings only — you run it 5 hours a day, 4 days a week, and your rate is $0.17 per kWh. Here’s how those four numbers move through the calculation.
| Step | Calculation | Result |
|---|---|---|
| Convert to kilowatts | 1,500 W ÷ 1,000 | 1.5 kW |
| Daily energy | 1.5 kW × 5 h | 7.5 kWh |
| Weekly energy | 7.5 kWh × 4 days | 30 kWh |
| Annual energy | 30 kWh × 52 weeks | 1,560 kWh |
| Monthly energy | 1,560 kWh ÷ 12 | 130 kWh |
| Monthly cost | 130 kWh × $0.17 | $22.10 |
| Annual cost | 1,560 kWh × $0.17 | $265.20 |
Notice that daily and monthly cost aren’t independent calculations — monthly cost is just the monthly kWh figure carried through the same multiplication by rate. Every dollar figure on the page traces back to that one 7.5 kWh daily number.
What Your Duty Cycle and Continuous-Equivalent Load Actually Tell You
A monthly kWh total on its own is hard to judge without something to measure it against. U.S. households currently average somewhere around 860 to 900 kWh a month, so the 130 kWh heater from the example above would account for roughly one-seventh of a typical whole-house bill — not the dominant expense, but not trivial for a single appliance run part-time.
Duty cycle answers a different question: not how much energy, but how often the device is actually on. It’s active hours divided by 168 (24 hours × 7 days), expressed as a percentage. The heater’s 20 active hours a week gives a duty cycle just under 12% — it’s idle nearly seven hours for every one it runs.
Continuous-equivalent load takes the opposite approach. It spreads the annual kWh total evenly across all 8,760 hours in a year and asks: what constant wattage would produce that same total?
For the heater, that works out to about 178 W — a small fraction of its 1,500 W nameplate rating, because it’s only running part of the time. This number is what lets you compare an appliance used a few hours a week against something that draws less power but never turns off, on equal footing.
The cost-per-active-hour figure, $0.26 in the example, strips out schedule entirely. It’s just kilowatts times rate — useful when you’re comparing two appliances and want to know which one is more expensive to run, independent of how many hours you happen to use either one.
Why Wattage Swings Your Bill Far More Than Extra Minutes of Use
Hours and days are capped. Wattage isn’t. That asymmetry is the biggest thing to understand about what actually moves your result. Together, hours (max 24) and days (max 7) can multiply your energy use by at most 168 — that’s a device running every hour of every day, the calculator’s absolute ceiling for schedule. Wattage has no such limit built in. Swapping a 60W lamp for a 1,500W heater is a 25x jump with the schedule held constant, dwarfing anything an extra hour or two of runtime could do.
Practically, that means trimming wattage — a more efficient appliance, a lower-power setting — usually saves more than trimming an hour of use here or there, unless that hour represents a large share of the device’s total runtime. Switching the power unit toggle between watts and kilowatts doesn’t change this dynamic at all; it only changes how you enter the same underlying number, converting one direction or the other so the stored value stays consistent.
If you’re trying to size a circuit or a breaker rather than estimate a bill, {{ADD_INTERNAL_LINK}} covers the ampacity and wire-gauge side of that question — this tool only handles consumption and cost.
Common Questions About Estimating Energy Use and Cost
How do I calculate my electricity cost from watts?
Convert watts to kilowatts by dividing by 1,000, multiply by the hours the device runs per day, then multiply that daily kWh figure by your rate per kWh. A 1,500W heater running 5 hours a day works out to 1.5 kW times 5 hours times your rate, with no extra formula required once you have those three numbers.
What’s the difference between kW and kWh?
kW measures how much power a device draws at any given instant, while kWh measures how much energy it consumes over a stretch of time. A 1,500W heater always pulls 1.5 kW while it’s switched on, but it only uses 1.5 kWh once it’s been running for a full hour.
Why doesn’t my calculated cost match my utility bill?
This calculator assumes one flat rate per kWh, but many utilities charge tiered rates that climb once you pass a usage threshold, on top of fixed delivery charges and taxes that have nothing to do with consumption. Treat the output as a per-device estimate, not a reconciliation of your full bill.
How much does it cost to run a 1,500-watt space heater?
At 5 hours a day, 4 days a week, and $0.17 per kWh, a 1,500W heater runs about $1.28 a day, $22.10 a month, and $265.20 a year. Change the wattage, the hours, or the rate, and the cost scales directly with whichever number moved.
What is duty cycle in an energy calculator?
Duty cycle is the share of all possible hours in a week that a device is actually switched on, worked out as active hours divided by 168, the total hours in seven days. In the heater example above, 20 active hours out of 168 gives a duty cycle just under 12%.
What does continuous equivalent load mean?
It’s the constant wattage a device would need to draw nonstop, all day every day for a year, to use the same total energy it actually uses on its real, intermittent schedule. It’s a way to compare an appliance that runs occasionally against one that genuinely runs around the clock, like a refrigerator compressor.
Can I use this calculator for appliances with variable power draw, like a refrigerator?
Yes, but plug in an average wattage rather than the nameplate rating, since appliances like refrigerators cycle their compressor on and off instead of drawing constant power. Using the running wattage as if it were constant will overstate the actual consumption by a wide margin.
What electricity rate should I use if I don’t know mine?
Pull the per-kWh rate off a recent utility bill, since it’s normally broken out separately from delivery charges and taxes. National averages sit somewhere around 17 cents per kWh as of early 2026, but your actual rate can differ by several cents depending on state and season, so use your own bill whenever you have it.
What This Calculator Won’t Tell You
These figures are for planning and comparison, not a substitute for your actual bill or for electrical code work. The calculator assumes a flat rate and steady wattage, so it won’t capture tiered pricing, delivery fees, or appliances with fluctuating draw; for anything involving circuit sizing, wiring, or installation, follow your local electrical code and have the work done or inspected by a licensed electrician.