Electricity Cost Calculator

There is one formula behind every line on an electric bill: kWh = kW × hours. Power is the rate at which a device draws electricity, measured in watts. Energy is what you are billed for, and it is power accumulated over time. So a 1,000 W appliance is a 1 kW appliance, and running it 5 hours a day for 30 days uses 1 × 5 × 30 = 150 kWh. At 17 cents per kWh that is about 25.50 dollars a month. The distinction between watts and watt-hours is the thing worth internalizing, because it explains results that otherwise look wrong. A 1,500 W space heater draws fifteen times the power of a 100 W aquarium heater, but if the space heater runs two hours a day and the aquarium heater runs continuously, the aquarium heater uses more energy over a month: 3 kWh a day against 2.4. High power for a short time can cost less than low power all the time. Anything that runs 24 hours a day is doing 720 hours of accumulating every month, and small numbers get large when multiplied by 720. That is why the honest way to explain a bill is to list devices with both numbers — what they draw and how long they run — rather than ranking them by wattage alone.

The number printed on an appliance is its rated or maximum draw, and for a large share of household devices it is not what they actually consume over an hour. Anything with a thermostat cycles. A refrigerator's compressor might be rated at a few hundred watts, but it runs only a fraction of each hour once the box is cold — and that fraction changes with the room temperature, how full it is and how often the door opens. An electric oven pulls its full rating while heating up and then cycles to hold temperature. Air conditioners and heat pumps do the same. For all of these, multiplying nameplate watts by hours plugged in dramatically overstates the cost; what you want is watts times hours actually running, which is the duty cycle. Second complication: many nameplates list amps rather than watts. Multiplying amps by voltage — 120 V for a standard US outlet, 240 V for a dryer or range circuit — gives volt-amps, which equals watts for a purely resistive load like a heater or a toaster. For motors and compressors, real power is somewhat lower than the volt-amp figure because of power factor, so the amps-times-volts route tends to overestimate. Which is why the reliable method is measurement, not arithmetic. A plug-in energy meter sits between the outlet and the device and accumulates real kWh over days, capturing every cycle and every idle period. For a 240 V appliance or the whole house, a whole-home energy monitor at the panel does the same job — installed by a licensed electrician, since it lives in the service panel. Use the nameplate for a first estimate, and measure anything whose result surprises you.

A device that is switched off can still be drawing power. Anything with a remote control, a clock, a network connection, a status light, a memory of its own settings or an external power brick keeps a circuit alive to wait for you. Individually these are trivial — a few watts each. Collectively they are not, because they run every one of the 8,760 hours in a year. A single device drawing a constant 5 W uses about 44 kWh a year, and a typical home has a great many of them: TVs and their soundbars, set-top and streaming boxes, game consoles in instant-on mode, printers, coffee makers with displays, laptop chargers left plugged in, garage door openers, smart speakers, older instant-on appliances. Standby is commonly estimated at several percent of a home's total electricity — small as a share, and real in dollars. To find yours, look for the loads that never change. If you have interval data from your utility, the flat overnight floor when the house is asleep is your baseline draw, and everything above it is activity. The fixes are unglamorous: a switched power strip for a cluster of entertainment devices, unplugging seasonal or rarely used equipment, and turning off instant-on or quick-start modes where the wake-up delay does not bother you. Note the exceptions — you would not put a refrigerator, a medical device, a security system or anything with a clock you depend on behind a switched strip. When replacing equipment, standby is part of what efficiency certification covers, so an ENERGY STAR model is generally better on idle draw as well as running consumption.

The rate to enter here is not usually the rate advertised anywhere. American electric bills separate supply (the energy itself, sometimes from a competitive supplier) from delivery or distribution (the utility moving it to you), and then add a fixed monthly customer charge, plus taxes, riders and surcharges that vary by state and utility. Quoting only the supply rate can understate the true cost per kWh substantially. The number that reflects reality is the all-in effective rate: take the bill total, divide by the kWh used that month, and use that. It automatically folds in every line item. It also changes month to month, since the fixed charges spread over more kWh in a heavy month. Then check what kind of rate you are on, because two more structures change the answer. Tiered rates charge more per kWh above a monthly threshold, which means an additional appliance is priced at your top tier, not your average. The marginal cost of running something more is higher than the average cost you calculated. Time-of-use rates price the same kWh differently by hour and season, often with a large peak-to-off-peak spread. Under TOU, when a device runs matters as much as how long. Shifting a dishwasher, laundry, pool pump or EV charging to off-peak hours reduces cost without reducing use at all — and running a 1 kW load during peak can cost several times what the same kWh costs overnight. Rates vary enormously by state and utility. EIA publishes state-level average residential prices, which is useful for sanity-checking whether your rate is typical, but for a real calculation there is no substitute for your own bill.

Add up a household's electricity and the largest shares almost always belong to things that move heat: space heating and cooling, water heating, clothes drying, refrigeration and cooking. Everything else — lighting, electronics, small appliances — is real but usually secondary, and it is where most people look first. Heating is the extreme case. A portable electric heater is capped near 1,500 W on a standard 120 V circuit, and at 1.5 kW it is one of the largest loads that will ever plug into a regular outlet. Run one 10 hours a day and it uses 15 kWh a day, which is more than many households use for everything else combined. The common belief that a space heater is a cheap way to warm a room holds only when it lets you heat one small room instead of the whole house — used as a supplement to a system already running, it simply adds. There is also a hard efficiency ceiling here. Electric resistance heat converts essentially all its electricity into heat, so no resistance heater is meaningfully more efficient than another regardless of marketing. The way to get more heat per kWh is a heat pump, which moves existing heat rather than creating it and therefore delivers more heat energy than the electrical energy it consumes. So the productive order of operations is: measure or estimate the biggest loads first, then attack them in the order of kWh rather than in the order of how wasteful they feel. A phone charger left plugged in is not the problem; the water heater, the HVAC system and the second refrigerator in the garage are. One safety note, since this page invites people to think about big loads: a 1,500 W heater on a 15 A circuit is close to that circuit's continuous limit. High-draw appliances should be plugged directly into a wall outlet rather than an extension cord or a light-duty power strip, and any warmth at the plug, discolored outlet or repeatedly tripping breaker is a reason to stop and call a licensed electrician. Circuit sizing and wiring are not estimation problems.