Power

 If it was necessary to calculate how much energy (total) was consumed in a day in electrical systems, where maybe exist also accumulators such as batteries, for example, in order to identify the correct size of the photovoltaic panels, I have to calculate the energy used instantly and then multiply it by the time.

Power

We start from the Electrical Power (electrical workflow per unit of time), which is the product of Voltage and Current, and the formula is the following:

 P = V * I

Where: P = Power and the unit of measurement is the Watt; V = is the Voltage and is measured in Volts; I = current and measured in amperes.

So if the Current remains unchanged and the Voltage varies, for example, 15 Amps and 20 Volts, they produce 300 Watts of Power (P = 15 * 20 = 300 W), while if the current remains the same, but the Voltage varies, for example, 48 Volts, the Power will become 720 Watts (P = 15 * 48 = 720 W), and the difference is very big.

Let's say that the current (A) is used to calculate the cable size, the regulators, and also the batteries, while the Power (W) is used to understand how much "work" the Current produces based on the Voltage you choose.

All this reasoning is fine when it happens instantaneously, but it is clear that a 50 Watt light bulb kept on for one hour consumes less than the same light bulb kept on for 2 hours, even if the power is always 50 Watts.

Energy

For this reason, I have to find a method which I can calculate, in this case, comparing the consumption or production as had happened in a single hour. This ratio is the kilowatt-hour (kWh) or, even if less used, the Watt-hour (Wh), which are the units of measurement of Energy.

Energy (amount of energy available thanks to the flow of electrical charges in a conductor) is calculated using this formula:

E = P * t

Where: E = Energy (kWh or Wh); P = Power (Watt); t = is time and is measured in hours.

A clarification: Energy is expressed in joules (J). Although energy is almost always expressed in multiples of the joule; and the most used is the kWh (kilowatt-hour) which represents the power of 1 kW delivered for one hour, or also the Wh which is the total energy provided by a power of one watt (W) for an hour (h).

In summary, a 50 Watt light bulb kept on for one hour will consume 50 Wh of energy, and that kept on for 2 hours will consume 100 Wh.

Summary

 Electric Power (electrical workflow per unit of time) is the product of Voltage and Current, and the formula is the following:

P = V * I

The unit of measurement is Watts.

Energy (amount of energy available thanks to the flow of electrical charges in a conductor) is the product of power over time:

E = P * t

The unit of measurement is kilowatt-hours (kWh) or watt-hours (Wh).

Dictionary

A = current flowing at this moment is like the flow of water in a pipe that supplies the vessel.

Wh or kWh = total energy used, produced or accumulated; it is like the bank account.

W = instantaneous power produced or consumed; it is how much work we can do simultaneously.

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