Amps to Watts Converter: Watts at Any US Voltage with kWh Cost

The formula is Watts = Amps x Volts x Power Factor. For resistive loads with PF 1.0: Watts = Amps x Volts. Examples: 10A at 120V (resistive) = 10 x 120 x 1.0 = 1,200W. 15A at 240V = 15 x 240 = 3,600W. 20A at 120V with PF 0.90 = 20 x 120 x 0.90 = 2,160W. The reverse formula (watts to amps) is: Amps = Watts / (Volts x PF). Both conversions require knowing the voltage; without voltage, you cannot convert between amps and watts. The energy conversion then follows: kWh = Watts x Hours / 1,000.

15 amps at 120V equals 1,800 watts for a purely resistive load (PF 1.0): 15A x 120V x 1.0 = 1,800W. For an inductive load with PF 0.85: 15A x 120V x 0.85 = 1,530W. At 240V, 15 amps equals 3,600W (resistive). Common 15A, 120V loads: space heater (1,500W pulls 12.5A), hair dryer (1,875W pulls 15.6A at full heat – exceeds safe load for a 15A circuit), microwave (1,000-1,500W pulls 8.3-12.5A). A single 15A circuit should not simultaneously carry loads that total more than 1,440W (80% of 1,800W) on a continuous basis per NEC guidelines.

30 amps at 240V equals 7,200 watts (7.2 kW) for a purely resistive load: 30A x 240V x 1.0 = 7,200W. A 30A/240V circuit is typically used for: electric clothes dryers (approximately 5,000 to 5,600W actual draw, less than the 7,200W circuit capacity, providing headroom); portable air conditioners and window ACs (up to 7,000 BTU at 240V); electric tankless water heaters in smaller applications (larger models use 40A to 150A at 240V); and Level 2 EV chargers in the 6-7 kW range (though most residential EVSE installations now use 40A, 50A, or 60A circuits). The circuit requires a 30A double-pole breaker and AWG 10 copper conductors (three conductors: two hot, one ground, with or without neutral depending on the load).

To calculate kWh from amps: (1) Convert amps to watts: W = A x V x PF. (2) Multiply watts by hours of operation: Wh = W x hours. (3) Divide by 1,000 to get kWh: kWh = Wh / 1,000. Example: 8 amps at 120V (PF 1.0) running 5 hours per day: Step 1: 8 x 120 x 1.0 = 960W. Step 2: 960 x 5 = 4,800 Wh. Step 3: 4,800 / 1,000 = 4.8 kWh per day. Annual: 4.8 x 365 = 1,752 kWh per year. At $0.14/kWh: $245.28 per year in electricity cost for this one load.

A clamp meter (clamp ammeter or clip-on ammeter) measures AC current by sensing the magnetic field generated by current flowing through a conductor, without requiring the circuit to be broken. The hinged jaw of the clamp meter opens and closes around a single conductor (not a cable with multiple conductors – clamping around a cable with both hot and neutral cancels out the magnetic fields and gives a false zero reading). The magnetic field intensity is proportional to the current flowing, allowing the meter to display amps directly. Common clamp meters for residential electrical work: Fluke 323 (approximately $100, 400A AC), Klein Tools CL110 (approximately $35, 400A AC), and Milwaukee 2235-20 (approximately $50, 600A AC). Clamp meters can measure amps safely on live circuits without interrupting power, making them invaluable for diagnosing unknown loads and verifying circuit loading. After measuring amps, use this calculator to convert to watts by entering the measured amps and the known circuit voltage. Most modern clamp meters also directly display watts and power factor if you probe both voltage and current simultaneously.

For a 240V appliance: Watts = Amps x 240V x PF. For resistive loads (electric range heating elements, baseboard heaters, electric water heaters): use PF 1.0 exactly. For motor-driven 240V appliances (central AC, heat pump, pool pump, submersible well pump): use PF 0.85 to 0.92. Example: 240V well pump rated at 12A with PF 0.85: 12 x 240 x 0.85 = 2,448 watts (2.45 kW). Running 6 hours per day: 2,448W x 6h / 1,000 = 14.7 kWh/day. At $0.14/kWh: 14.7 x 365 x $0.14 = $751 per year in pump electricity. This is why whole-home energy audits frequently flag well pumps and older single-phase motors as high-value efficiency improvement targets.

A modern full-size refrigerator typically draws 1.0 to 2.5 amps at 120V on a continuous basis when the compressor is running, with compressor run cycles averaging 30 to 50 percent of the time. The nameplate may show a higher “rated” amperage (typically 3 to 8A) which represents the peak draw during compressor startup. Average energy use for a modern full-size (18-22 cubic foot) refrigerator is 400 to 600 kWh per year, corresponding to an average watts of 46 to 68 watts (600,000 Wh / 8,760 hours = 68.5W average for a 600 kWh/year unit). This is why a 600 kWh/year refrigerator drawing an average 68W does not need its own dedicated circuit by code (though the NEC requires the outlet be on the small appliance circuit). Older refrigerators from the 1980s and early 1990s typically consume 1,200 to 2,000 kWh/year due to poor insulation and inefficient compressors. Replacing a 1990 refrigerator with a current ENERGY STAR model can save 1,000 to 1,500 kWh per year and $140 to $210 annually at $0.14/kWh.

The Kill-A-Watt (P3 P4400 or P4460) is a popular US plug-in energy monitoring device that measures amps, volts, watts, VA, power factor, frequency, kWh, and running cost for 120V appliances. To use it: plug the Kill-A-Watt into any 120V outlet, then plug the appliance into the Kill-A-Watt’s outlet. The device continuously measures and displays the electrical parameters. Press the KWH button to see accumulated energy consumption; set the cost per kWh with the up/down arrows and it will show accumulated cost and project annual cost. The Kill-A-Watt accurately measures real power (watts), making it ideal for finding the actual consumption of devices whose nameplate wattage does not reflect typical operation (like computers, TVs, gaming consoles, and refrigerators that cycle). It is less useful for 240V appliances (refrigerators are 120V but ranges and dryers are 240V). For 240V appliance monitoring, a clamp meter measuring each hot leg (and summing the readings) or a whole-home energy monitor is required. The Kill-A-Watt P4460 version adds a data logging timer feature useful for seeing how consumption varies over a day or week.

A solar charge controller regulates the charging of batteries from solar panels. The amp reading on a charge controller display typically shows the charging current flowing from the panels into the battery bank. To convert this to watts: Charging watts = Amps (on display) x Battery bank voltage (12V, 24V, or 48V). A 30A reading on a 24V system: 30 x 24 = 720 watts of charging power. A 40A reading on a 48V system: 40 x 48 = 1,920 watts. MPPT (Maximum Power Point Tracking) charge controllers also show the PV input current and voltage separately from the battery output current, because the MPPT algorithm converts higher-voltage, lower-current PV power to lower-voltage, higher-current charging power. A 60V PV input at 15A (900W) into a 24V battery results in approximately 34-37A charging current (900W / 24V, minus conversion losses of approximately 5-8%). The 12V and 24V voltage options in this calculator are specifically designed to help solar and RV users convert DC amp readings from their charge controllers and shunts into watts for system design and monitoring.

Both AC (alternating current) and DC (direct current) measure current in amps, but the nature of the current flow differs. DC amps flow in one direction continuously; AC amps oscillate back and forth at 60 cycles per second (60 Hz) in the US. The watts formula (W = A x V x PF) applies to both, with PF = 1.0 for DC. For AC circuits with reactive loads, PF falls below 1.0 due to the phase shift between voltage and current waveforms. AC amps are measured as RMS (root mean square) values, which represent the equivalent DC value for heating and power purposes; a 10 ARMS AC current delivers the same heating effect as 10A DC. Clamp meters measure AC RMS current by sensing the magnetic field from the alternating current; they cannot typically measure DC current unless they have a Hall-effect sensor (labeled as “DC clamp” or “True DC” capability). If you need to measure DC amps in a solar or battery system without breaking the circuit, use a Hall-effect clamp meter or install an inline shunt (a precision low-resistance resistor) with a compatible battery monitor (Victron SmartShunt, Renogy BT-1 compatible shunt) to measure current as a voltage drop across the shunt.

Cost per hour = Watts / 1,000 x $/kWh. Since Watts = Amps x Volts x PF, the formula is: Cost per hour = (Amps x Volts x PF) / 1,000 x Rate. Examples at $0.14/kWh: 10A at 120V (PF 1.0): (10 x 120 x 1.0)/1,000 x $0.14 = 1.2 kWh x $0.14 = $0.168 per hour. 15A at 240V (PF 0.90): (15 x 240 x 0.90)/1,000 x $0.14 = 3.24 kWh x $0.14 = $0.454 per hour. 50A at 240V (PF 1.0, electric range at full load): (50 x 240)/1,000 x $0.14 = 12 kWh x $0.14 = $1.68 per hour at maximum draw. Use the optional hours input in this calculator to project the daily, monthly, and annual cost more accurately than estimating by hourly rate, since most appliances do not run at constant load for full hours.

A subpanel (also called a sub-panel, load center, or distribution panel) is a secondary electrical panel fed from the main panel, used to distribute power to a detached garage, workshop, ADU (accessory dwelling unit), basement finish, or other area remote from the main panel. The subpanel’s breaker size (in the main panel) and the feeder wire gauge determine the maximum power available in the subpanel. A 60A/240V subpanel breaker with AWG 6 copper feeder has a total capacity of 60A x 240V = 14,400W (14.4 kW). A 100A/240V subpanel with AWG 4 copper feeder has 100A x 240V = 24,000W (24 kW). The NEC requires subpanel feeders to be sized using load calculations (Article 220 demand factors), not just adding up all individual breaker ratings (since not all loads run simultaneously). Planning a subpanel for a workshop: add up the wattages of the maximum expected coincident loads (the largest tools and machines that might all run at once), convert to amps using this calculator, apply the NEC 125 percent factor for continuous loads, and size the feeder breaker and wire for that ampacity.

Doubling the voltage at the same wattage halves the current, which means smaller (less expensive) wire can carry the same power. This is why high-voltage transmission lines carry enormous power with relatively thin conductors: 345,000 volts at 1,000 amps carries 345 MW while a 120V circuit at 1,000 amps (which would require enormous conductors) carries only 120 kW. In residential and commercial applications: a 2,400W load at 120V draws 20A, requiring AWG 12 wire. The same 2,400W at 240V draws 10A, requiring only AWG 14 wire. This provides real cost savings on long runs (workshops in detached garages, pools, HVAC equipment at far corners of a home). For DC systems at 12V vs 24V vs 48V, the current ratio is even more dramatic: the same 1,000W load draws 83A at 12V (requiring large AWG 4/0 cable) versus 42A at 24V (AWG 2/0) versus 21A at 48V (AWG 4). Off-grid solar system designers universally prefer 48V systems over 12V for any system above approximately 2 kW precisely because of this wire sizing and efficiency advantage.

Inrush current (also called starting current or locked-rotor current, LRA) is the large surge of current drawn by a motor or transformer at the moment it starts up, before the rotating magnetic field is established. Inrush currents for single-phase motors are typically 5 to 8 times the full-load running amperage (FLA), lasting from a few milliseconds to several seconds depending on motor type and load. A refrigerator with 2A running current may draw 12 to 16A at startup. A central AC unit with 10A running current may draw 50 to 80A for a fraction of a second. Why inrush matters: it determines UPS, generator, and inverter sizing (which must supply the full inrush current peak), affects nuisance tripping if breakers are sensitive, and drives the motor starting circuit design for large commercial motors. For home circuit sizing, standard thermal-magnetic breakers tolerate inrush currents because of their time-delay characteristics; you do not size the circuit for inrush but for running amperage. For generator and inverter sizing, you must account for inrush: the generator or inverter’s kVA peak rating must exceed the inrush VA demand of the largest motor that will start on it. The Generator Sizing Calculator and Inverter Size Calculator in this hub specifically account for motor starting surge requirements.

Power factor correction (PFC) reduces the reactive component of a load’s current draw, bringing the power factor closer to 1.0. This means the same real power is delivered with less total current, reducing I-squared-R losses in conductors and transformers. Utility companies apply power factor surcharges to commercial and industrial customers with low power factor (typically penalizing customers with PF below 0.90 or 0.95) because low PF loads increase current in the distribution system without increasing billable kWh. Residential customers in the US are generally not charged for power factor directly; utilities absorb the reactive power cost for residential accounts because individual residential reactive demand is small and unpredictable. Active PFC circuits in modern power supplies (computers, LED drivers, variable-speed drives) automatically correct power factor to 0.95 to 0.99. Passive PFC uses capacitor banks to cancel inductive reactive power. Power factor correction does not reduce your residential electricity bill (since you are billed for real power, kWh, not apparent power, kVAh), but it does reduce conductor losses and can reduce wire sizing requirements in industrial settings. The main practical benefit for residential customers is in generator and UPS sizing: equipment with PFC draws less kVA for the same kW, requiring a smaller generator or UPS.

The most accurate method for measuring total home electrical load is a whole-home energy monitor that installs current transformers (CTs) on the main service conductors at your electrical panel. Products like the Sense Home Energy Monitor, Emporia Vue 2, and Eyedro Home Energy Monitor clamp around the two main hot conductors entering your panel and continuously calculate total watts (and individual circuit watts for the Vue 2) by measuring both the current and voltage waveforms. These devices report total watts and kWh in real time via smartphone app and cloud logging, making it easy to see minimum loads (2 to 5 AM standby draw), peak loads (cooking + HVAC + EV charging simultaneously), and daily/monthly kWh totals that match your utility bill exactly. Installation requires briefly working near the main panel with the service conductors energized (since CTs clamp around the live wires; the panel itself can be de-energized after the CTs are installed if desired). Many utilities also provide real-time or interval (15-minute or 60-minute) kWh data through online portals, which can serve as a whole-home monitor without any additional hardware. Converting the kWh reading from any of these sources to average watts is simple: total kWh for the day divided by 24 hours gives average watts for the day.