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I am wondering why is it called Harvesting energy and why not harvesting power? Energy is measured in Joules/s and power is Watt but the result is watt. For example if we harvest energy from sun or from wind or from heat or from vibration all of these energy sources gives a output in the form of Volt and Current. So basically what we get is power.

So why don't we call harvesting power from sun or from other sources of energy?

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    \$\begingroup\$ Energy is not measured in joules per second - it is measured in joules. Watts are joules per second. \$\endgroup\$ – Andy aka May 26 '14 at 9:35
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To answer the question, it's called energy harvesting because in most systems the rate of collection (input power) is very different from the rate of use (output power).

Most such systems run intermittently, so a key parameter is how much energy the the application consumes each time it runs (output power times run time). This then determines how often the application can run, because it takes a certain amount of time to collect at least that much energy (input power times collection time) in the energy storage device (capacitor or battery).

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Power is the rate at which energy is being consumed. An energy harvester stores energy rather than consuming power.

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Power is the amount of energy in a specified time, usually in the form Joules (energy) per second (time).

Let's say you have a 60 W light bulb. That is its power. Let's say you turn it on for a day (24 hours). It needed 60 x 24 = 1440 Wh = 1.44 kWh energy.

Energy is the capacity to do work while power is the rate at which work is done (diffen.com).

Let's say you can lift a 10 kg weight easily but you don't do that. That is your power. Let's say you lifted a 10 kg weight 30 times. In this situation you spent energy.

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