Power is an electrical quantity that is measured in watts and is the rate at which a circuit absorbs or produces energy. We know that light bulbs and heaters absorb energy and that the higher their wattage value, the more energy they will consume. Likewise, batteries and generators produce power and the higher their electrical rating, the more power they can deliver to the load. The unit of electrical power is the watt, and its symbol is a large letter “P” indicating constant DC power or a lowercase letter “p” indicating time-varying AC power.

Electrical power is related to energy, which is the ability to do work. It can also be defined as the rate of energy transfer. If one joule of work is absorbed or delivered at a constant rate of one second, then the corresponding power will be equivalent to one watt, so the power P can be defined as 1 joule/sec = 1 watt. So we can say that one watt is equal to one joule per second and electrical power can be defined as the rate of work or the transfer of energy.

Likewise, we can define energy as watts per second or joules. So if power is measured in kilowatts (thousands of watts) and time is measured in hours, then the unit of electrical energy is the kilowatt-hour (kWh) and 1 kWh is the amount of electricity used by a device rated at 1000 watts in an hour.

Kilowatt-hours are the standard units of energy used by the electricity meter in our homes to calculate the amount of electrical energy we use and therefore how much we pay. So if you start an electric fire with a 1000 watt element and leave it on for 1 hour, you will have used 1 kWh of electricity. If you were to light two electric fires each with 1000 watt elements for half an hour, the total consumption would be exactly the same amount of electricity: 1kWh. So consuming 1,000 watts for one hour uses the same amount of energy as 2,000 watts (twice as much) for half an hour (half the time). So, for a 100-watt bulb to use 1 kWh or unit of electrical energy, it would need to be on for a total of 10 hours (10 x 100 = 1000 = 1kWh).

So now we know that the unit of power is the watt with the power absorbed by an electrical circuit given as the product of voltage, V, and current, which gives:

P (watts) = V (volts) x I (amps)

Furthermore, by substituting Ohm’s Law into the above equation, we can also define a constant DC power as:

P (watts) = I^2 (amps squared) x R (resistance)

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P (watts) = V^2 (voltage squared) / R (resistance)

So there are three possible formulas for calculating electrical power in a circuit. If the calculated power is positive (+P), then the circuit or component absorbs the power. But if the calculated power is negative, (-P) the circuit or component delivers power, that is, it is a source of energy.

rated power

Electrical components are given a “power rating” in watts that indicates the maximum rate at which the component converts electrical energy into another form of energy, such as heat, light, or motion. For example, a 1/4 W resistor, a 100 W bulb, etc. Therefore, electrical devices use energy to convert one form of energy into another, for example, an electric motor will convert electrical energy into a mechanical force.

Electric motors and other electrical systems have an efficiency rating defined as the ratio of the power converted to work to the total power consumed by the device. Efficiency is expressed as a decimal fraction, but is usually defined as a percentage value such as 85% efficiency. So we can define efficiency as equal to power output divided by power input x 100%.

The efficiency of an electrical device or motor will always be less than one (100%) due to electrical and mechanical losses. If an electrical device has an 85% efficiency rating, only 85% of the input power is transformed into mechanical work, the other 15% is lost as heat or other losses.

Appliances such as washers, dryers, refrigerators and freezers also have energy efficiency ratings that indicate their use and energy cost. These ratings are given as “A” for efficient and “G” for less efficient.

So remember, the more efficient the device, the less energy it will consume and the more money you will save, as well as being good for the environment.