Watts to kWh Calculator

To convert watts to kilowatt-hours, multiply the wattage by the number of hours it runs, then divide by 1,000: kWh = W × h ÷ 1000. A 1,000 W appliance running for 5 hours uses 1,000 × 5 ÷ 1,000 = 5 kWh. At an electricity rate of $0.17 per kWh, that costs about $0.85. The calculator above computes kilowatt-hours, watt-hours, and cost in one step.

The key idea is that watts measure power (the rate of energy use) while kilowatt-hours measure energy (power accumulated over time). To turn one into the other you must include time. A 100 W bulb and a 1,000 W heater can consume the same energy if the bulb runs ten times longer — kWh captures exactly that trade-off, which is why utilities bill in kilowatt-hours rather than watts.

Energy is power multiplied by time. The conversion to kilowatt-hours is:

Breaking it down:

• Watt-hours (Wh) = watts × hours. A 60 W bulb for 10 hours is 600 Wh.
• Kilowatt-hours (kWh) = watt-hours ÷ 1,000. That same 600 Wh is 0.6 kWh.
• The ÷ 1,000 converts watt-hours to kilowatt-hours because 1 kW = 1,000 W and thus 1 kWh = 1,000 Wh.

If your device is already rated in kilowatts, skip the division: kWh = kW × hours directly. For instance, a 1.5 kW heater for 4 hours uses 1.5 × 4 = 6 kWh. This is the single most useful shortcut on your electricity bill, because the meter records exactly this quantity — the running total of power drawn multiplied by the time it was drawn, integrated continuously and rounded to whole kilowatt-hours for billing. To turn amps into the kilowatts you need here, use the amps to kW calculator; to find a device's wattage from its voltage and current, use the volts to watts calculator.

These examples show energy and cost for typical devices, using a $0.17/kWh rate.

Example 1 — Space heater: A 1,000 W heater runs 5 hours. Energy = 1,000 × 5 ÷ 1,000 = 5 kWh. Cost = 5 × $0.17 = $0.85 per day. Over a 30-day month that is 150 kWh and $25.50.

Example 2 — Always-on router: A 12 W router runs 24 hours a day. Energy = 12 × 24 ÷ 1,000 = 0.288 kWh daily. Over a month: 0.288 × 30 = 8.64 kWh, about $1.47. Small loads add up when they never turn off.

Example 3 — Refrigerator: A fridge averages 150 W but cycles on and off, running roughly 8 effective hours per day. Energy = 150 × 8 ÷ 1,000 = 1.2 kWh daily, or about 36 kWh and $6.12 monthly.

Example 4 — 60 W bulb left on all day: 60 × 24 ÷ 1,000 = 1.44 kWh, costing $0.24 per day. Replacing it with a 9 W LED cuts that to 0.216 kWh and about $0.04 — an 85% reduction.

Energy is quoted in several related units, and mixing them causes big errors:

• Watt-hour (Wh) — one watt sustained for one hour. Used for small devices and battery capacity.
• Kilowatt-hour (kWh) — 1,000 Wh. The standard utility billing unit.
• Megawatt-hour (MWh) — 1,000 kWh. Used for large facilities and generation.

Battery capacity is often given in amp-hours (Ah), which you convert to watt-hours by multiplying by voltage (Wh = Ah × V). A 100 Ah, 12 V battery stores 1,200 Wh = 1.2 kWh. This links directly to how long a battery can power a load: a 1.2 kWh battery running a 100 W device lasts about 12 hours (before efficiency losses).

Understanding the distinction between power (watts) and energy (kWh) is the foundation of every electricity bill and every off-grid design. For the underlying relationships between voltage, current, resistance, and power that generate these watts, see Ohm's law explained.

Once you have kilowatt-hours, cost is a single multiplication:

Electricity rates vary widely — from around $0.10/kWh in low-cost regions to $0.30–0.45/kWh in high-cost areas and during peak time-of-use periods. The U.S. residential average sits near $0.17/kWh, the calculator's default. Check a recent utility bill for your exact rate, and note whether you pay tiered or time-of-use pricing, which changes the effective cost per kWh depending on total usage or time of day.

To estimate a device's yearly cost: multiply daily kWh by 365, then by your rate. A 100 W device running 24/7 uses 2.4 kWh/day × 365 = 876 kWh/year, costing about $149 at $0.17/kWh. This is how phantom loads and inefficient appliances quietly inflate a bill — and why the kWh figure, not the wattage alone, tells the real story. For whole-home load planning, convert appliance amperage with the amps to kW calculator first.

1. Enter the power in watts. Use the device's nameplate rating or a plug-in energy meter reading. If rated in kilowatts, multiply by 1,000 first.
2. Enter the run time in hours. Use actual operating hours, not clock hours. A fridge "runs" only when its compressor cycles on — estimate effective hours accordingly.
3. Enter your electricity rate. Find $/kWh on your utility bill. The default is the U.S. average of $0.17/kWh.
4. Read energy and cost. The primary result is kWh; secondary outputs show watt-hours, estimated cost, and projected monthly energy.

The calculator updates live. Adjust run time to compare scenarios, or change the rate to model peak versus off-peak pricing.

Converting watts to kilowatt-hours is the core of energy budgeting:

Home energy audits. Adding up each appliance's kWh reveals where your bill comes from and which upgrades (LED lighting, a heat-pump, efficient appliances) pay back fastest.

Solar system sizing. Off-grid and grid-tied solar designs start from daily kWh consumption. Summing every load's watts × hours gives the daily energy the panels and batteries must supply.

EV charging costs. An electric vehicle drawing 7,200 W for 8 hours consumes 57.6 kWh — at $0.17/kWh, about $9.79 for a near-full charge. This tool converts the charger's wattage and time directly into cost.

Data centers and business operations. Server and equipment power multiplied by continuous run time drives operating expense and cooling requirements, where even a few watts per unit scale into large annual kWh totals.

Battery and backup sizing. Because battery capacity is measured in watt-hours (and kilowatt-hours), the watts-to-kWh relationship tells you how long a battery can carry a load and how large a bank you need for a target runtime. A 5 kWh home battery running a 500 W refrigeration and lighting load lasts roughly ten hours before efficiency losses; sizing for a full day of essentials means multiplying each load's watts by its hours, summing the kWh, and adding a margin for inverter and depth-of-discharge losses. This is the same energy accounting utilities use, applied to stored rather than purchased power.

The value of the watts-to-kWh conversion becomes obvious when you tabulate what everyday appliances actually cost to run. The table below uses realistic wattages and typical daily run times, priced at the U.S. average of $0.17/kWh. Your own numbers will scale directly with your local rate — halve them at $0.085/kWh, double them at $0.34/kWh.

The pattern that emerges is instructive: high-wattage devices that run briefly (a microwave, a toaster) often cost less per day than modest-wattage devices that run continuously (a refrigerator, an aquarium pump, a set-top box). This is the core lesson of energy versus power — a 1,500 W space heater running four hours (6 kWh, about $1.02/day) dwarfs a 5 W phone charger left plugged in all month. When hunting for savings, look first at the product of wattage and hours, not wattage alone.

Once you can convert watts to kilowatt-hours, you have the tool to find and eliminate waste systematically. The approach is straightforward: inventory each significant load, compute its monthly kWh (watts × hours × 30 ÷ 1,000), rank them, and attack the biggest consumers first.

Target the heavy hitters. Heating, cooling, water heating, and refrigeration dominate most bills because they combine high wattage with long run times. Upgrading an old refrigerator, adding a heat-pump, or improving insulation moves far more kWh than swapping a few bulbs — though LED lighting still pays back quickly.

Hunt phantom loads. Devices in standby — chargers, game consoles, cable boxes, always-on speakers — can total 50–100 W continuously, which is 36–72 kWh per month, or roughly $6–12. A smart power strip that cuts standby power converts directly into kWh savings you can compute with this calculator.

Shift to off-peak time. If you are on time-of-use pricing, running dishwashers, laundry, and EV charging during cheaper off-peak hours lowers the effective rate applied to those kWh, even though the energy consumed is unchanged. Model the difference by entering your peak and off-peak rates separately.

To build a complete picture, first convert each appliance's amperage into watts or kilowatts with the amps to kW calculator or the volts to watts calculator, then bring the wattage here to translate it into monthly energy and cost. This two-step workflow turns nameplate numbers into a dollar figure you can act on.

• Forgetting to divide by 1,000. Watts × hours gives watt-hours; you must divide by 1,000 for kilowatt-hours.
• Using clock hours instead of run hours. Cycling loads like fridges and AC units only draw power part of the time; use effective operating hours.
• Ignoring standby/phantom draw. Many devices consume watts even when "off"; include standby hours for accuracy.
• Confusing kW with kWh. kW is power (instantaneous); kWh is energy (over time). A 5 kW load is not 5 kWh unless it runs exactly one hour.
• Using an outdated rate. Time-of-use and tiered pricing can make your marginal rate far higher than the average.

For related power and energy conversions, use the volts to watts calculator, the watts to amps calculator, and the amps to kW calculator.