Watt Converter

Watt Converter changes watts into kW, horsepower, BTU/h and 14 other power units. It also finds watts from volts and amps or from VA, units of electricity, and microwave cook time.

What the Watt Converter Works Out

The Watt Converter takes one power value and shows it in 18 units at once. Type 1,500 W and it reads 1.5 kW, 2.012 mechanical horsepower and 5,118 BTU per hour. A fourth card gives the current, which is 12.5 A on a 120 V circuit.

Five more conversions sit in the same dropdown. Volts and Amps to Watts covers DC, ohms, single-phase and three-phase supplies, and Watts to Amps runs the other way. VA and Watts handles a UPS or generator rating. Watts and Units (kWh) gives electricity use and cost, and Microwave Time rescales a cooking time to your oven.

Each result shows its working under Calculation Steps, so the factor and the division are on screen. The factors come from NIST Special Publication 811, Appendix B.8, the US reference table for unit conversion.

Watt Conversion Factors

A watt is one joule of energy each second. It is the SI unit of power, and power is the rate at which energy is used or work is done. A watt conversion is a multiplication or division by a fixed factor, because every other power unit is a set number of watts.

To change watts into another unit, divide by the watts in one of that unit. To go back to watts, multiply instead. For 1,500 W in mechanical horsepower the working is 1,500 ÷ 745.6999, which gives 2.012 hp.

So 1 W is 0.001 kW, and 1,000 W is exactly 1 kW. To convert watts to kW, divide by 1,000. To calculate 1 kW from smaller loads, add their watts until the total reaches 1,000. A 200 W load equals 0.2 kW, 0.2682 hp and 682.4 BTU per hour.

Horsepower needs care because four versions are in use. Mechanical horsepower is 745.7 W and metric horsepower is 735.5 W, so a 100 PS engine is 98.63 hp. Engine ratings feed into the Power-to-Weight Ratio Calculator, where the wrong horsepower shifts the ratio by 1.4%.

The BTU in the table is the International Table BTU, and the calorie is the thermochemical calorie of 4.184 J. Radio work uses dBm, a log scale where 0 dBm is 1 mW and 30 dBm is 1 W. The same scale runs through the EIRP Calculator and the Signal-to-Noise Ratio Calculator.

In mechanical terms, 1 W lifts 1 kg by 10.2 cm every second against standard gravity. The volts-and-amps and VA modes print this lift figure beside the joules delivered each minute. The same link between weight, height and energy runs through the Potential Energy Calculator.

Watt Formula for Volts and Amps

People who look for a watt converter with volts usually hold two of three values. Volts measure electrical pressure and amps measure current. Watts are the product of the two, and that product is the basic watt formula.

$$P = V \times I$$

Here P is power in watts, V is voltage in volts and I is current in amps. A 12 V battery supplying 20 A delivers 240 W, and that is the example loaded in the Volts and Amps to Watts mode.

Ohm’s law has three forms, which are V = I × R, I = V ÷ R and R = V ÷ I. Putting them into the watt formula gives the three formulas of power. They are P = V × I, P = I² × R and P = V² ÷ R, where R is the resistance in ohms.

The Supply and Known Values list has an option for each of the three. With 20 A through 0.6 Ω the result is 240 W, and the load card adds the 12 V across it. With 12 V across 0.6 Ω it is the same 240 W, and the card shows the 20 A instead.

On AC the product of volts and amps is apparent power, and only part of it does work. The real power formula adds the power factor, which is the cosine of the phase angle in OpenStax University Physics, section 15.4.

$$P = V \times I \times \text{PF}$$

Three-phase power uses the line-to-line voltage and the square root of 3. A balanced 400 V supply at 20 A and a power factor of 0.85 carries 11.78 kW. When the voltage is measured from one line to neutral, the factor becomes 3.

$$P = \sqrt{3} \times V_{LL} \times I \times \text{PF}$$

$$P = 3 \times V_{LN} \times I \times \text{PF}$$

The same load entered as 230 V line to neutral reads 11.73 kW. The two answers differ a little because 400 ÷ √3 is 230.9 V, not 230 V. The voltage label in the tool follows whichever three-phase option is picked.

Is 1 watt equal to 1 volt or 1 amp?

No. Watts, volts and amps are three different quantities. One watt is 1 V driving 1 A, so a watt figure needs both. A 10 W load on a 1 V supply would draw 10 A, and 1,000 W is 1 amp only on a 1,000 V supply.

A voltage alone has no wattage. 12 volts equals watts only once the current is known, and 1,000 volts is simply 1 kV until amps flow. In the same way, 20 W is 5 V only when 4 A flows, and at 9 V the same 20 W needs 2.222 A.

1 Amp in Watts at 120 V and 220 V

One amp is as many watts as the circuit has volts. So 1 amp is 120 watts at 120 V and 220 watts at 220 V. The figures below are for DC or a power factor of 1, such as a heater or a kettle.

The table covers most searches for a 1 amp watts calculator. 20 amps is 2,400 W at 120 V and 4,600 W at 230 V, and 1 kW draws 8.333 A at 120 V. Every row assumes a power factor of 1, so a motor or a fluorescent fitting will draw more amps than the table shows.

Watts to Amps and Breaker Size

The Watts to Amps mode divides the power by the voltage, and on AC by the power factor as well. It takes DC, single-phase and both three-phase voltage options, with power in watts, kilowatts or horsepower.

$$I = \frac{P}{V \times \text{PF}}$$

The loaded example is 1,800 W on a 120 V single-phase supply at a power factor of 1, which draws 15 A. One kilowatt at 120 V draws 8.333 A at a power factor of 1 and 10.42 A at 0.8. A second card repeats the answer at 240, 208 and 230 V.

The first card multiplies the current by 125 percent. NEC 210.20(A) sizes a breaker at 125 percent of a continuous load, which is one expected to run three hours or more, as EC&M explains in its guide to branch-circuit overcurrent protection.

For the 15 A example the breaker figure is 18.75 A, so a 15 A breaker is too small if the load runs that long. The same card shows the load as 100% of a 15 A circuit and 75% of a 20 A circuit.

VA to Watts on a UPS Label

A UPS or generator is sold by its VA rating. One volt-ampere is 1 W only at a power factor of 1, so the watts it can supply are the VA times the power factor. That also means 1 kVA is 1,000 VA, never 1 watt, and it supplies up to 1,000 W.

$$P = S \times \text{PF}$$

S is the apparent power in VA. At a power factor of 0.8, 600 VA is 480 W and 1000 VA is 800 W. The rest is reactive power, which for the 600 VA case is 360 VAR at a phase angle of 36.87 degrees.

The power factor is printed on the UPS or generator plate, so read it there before picking a column. The Watt Converter builds the same table for any rating in VA, kVA or MVA and marks the row for the power factor you enter.

To size a UPS from the load, pick W or kW as the unit and the result flips to VA. A 480 W load at a power factor of 0.8 needs 600 VA. The Circuit Voltage field sets the amps row, so 600 VA reads 5 A at 120 V.

Watts to Units of Electricity

One unit of electricity is 1 kilowatt-hour. A watt is a rate and a unit is an amount, so no fixed number of watts makes 1 unit. It takes 1,000 W running for one hour. That is why 1 kW equals 1 unit only after a full hour of use.

$$\text{kWh} = \frac{W \times h}{1000}$$

The US Energy Information Administration gives the example of a 40 W bulb used for five hours, which comes to 0.2 kWh. The same 40 W bulb needs 25 hours to use one unit. This sum is what a 1 unit to watt converter has to do.

The Watts and Units mode loads a 750 W appliance used 6 hours a day. That is 4.5 units a day and 135 units in 30 days. At a sample price of $0.16 per unit the 30-day cost is $21.60, and the appliance uses one unit every 1 h 20 min of running.

A bill of 200 units of electricity means 200 kWh, which is 720 MJ of energy. Spread evenly over 30 days it equals a steady load of 278 W. Enter the rate from your own bill in dollars, rupees, pounds or euros to turn units into cost.

The same mode works backward from a meter reading. Pick units per 30 days, enter 135 units used over 6 hours a day, and it returns 750 W while running. Units per day is the other option for a single day’s reading.

Watts and watt-hours are not the same thing. A watt-hour is 3,600 J of energy, while a watt is 1 J every second. Rated watts are also the most an appliance draws, so a fridge or an iron that cycles on and off uses fewer units than the rating suggests.

Tons, BTU and AC Size in Watts

How many kW is a 1.5 ton AC?

A ton of refrigeration is 12,000 BTU per hour of heat removed, which is 3.517 kW. A 1.5 ton AC therefore has 5.275 kW of cooling capacity. This is heat moved out of the room, and the electricity drawn at the plug is a smaller figure set by the unit’s efficiency.

What size AC suits a 12×12 room?

The ENERGY STAR room air conditioner chart lists 5,000 BTU per hour for 100 up to 150 square feet with an 8-foot ceiling. A 12×12 room is 144 square feet, so it sits in that row. In other units that is 1,465 W of cooling, or 0.4167 tons.

ENERGY STAR adds 10 percent for a very sunny room and 4,000 BTU per hour for a kitchen. It also adds 600 BTU per hour for each person beyond two. A sunny 12×12 room therefore needs 5,500 BTU per hour, which the converter shows as 1,612 W.

How much is the AC bill for 1 hour?

The bill comes from the input watts on the nameplate, not from the tonnage. An air conditioner drawing 1.5 kW uses 1.5 units in an hour of steady running. At $0.16 per unit that is $0.24, and the compressor cycling off will bring the real figure lower.

Watt Converter for Microwave Times

Pack instructions are written for one oven wattage, and a lower wattage oven needs longer. The Watt Converter microwave mode scales the time so that watts times seconds stay the same as the recipe.

$$t_2 = t_1 \times \frac{W_1}{W_2}$$

In this formula t1 and W1 are the recipe time and wattage, and W2 is the wattage of your oven. A recipe of 2 min 30 s at 1,100 W becomes 3 min 56 s at 700 W, which is 1 min 26 s longer.

The USDA Food Safety and Inspection Service notes that a higher wattage oven cooks faster and that ovens vary in power and efficiency. If the wattage is not on the door or the back plate, its time-to-boil test gives an estimate.

In that test, one cup of ice-cold water that boils in under 2 minutes means 1,000 W or more. Boiling at about 3 minutes means 650 to 700 W. Treat the converted time as a starting point, allow standing time, and check that reheated food reaches 165 °F.

Watts on a Plug or Adapter

A search for a watt converter plug usually means a travel voltage converter or a power adapter. Both carry a rating in watts or VA, and the appliance must draw no more than that rating. The appliance label gives volts and amps, and their product is the watts.

A label reading 120 V and 12.5 A is a 1,500 W appliance. A charger marked 5 V and 4 A is 20 W. A converter rated in VA should be checked in the VA and Watts mode, since its watts are lower whenever the power factor is below 1.

The Watt Among SI Units

What are the 7 base SI units?

NIST lists the seven SI base units as the second, meter, kilogram, ampere, kelvin, mole and candela. The watt is not one of them. It is a derived unit, equal to one joule per second, or one kilogram meter squared per second cubed.

What is the SI unit of AC?

Alternating current is measured in amperes, like any other current, and its frequency is measured in hertz. AC power is still measured in watts, with volt-amperes for apparent power. The joules behind each watt are the same ones worked out in the Kinetic Energy Calculator and the other Energy, Work and Power Calculators.