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The Difference Between kW & kWh

Updated: Nov 24, 2019

When you come across terms like, “the solar plant has 1kW capacity”, what that means is that the solar plant can generate maximum power of 1kW at its peak efficiency.

On the other hand, 1kWh, shows the energy generated or consumed in a one-hour time window. kWh is the unit by which electrical consumption is measured via meters installed at homes and offices.

But this doesn’t mean that you can only consume 1 kWh of energy in one hour. For instance, a 100W bulb will take 10 hours to consume up to 1kWh of energy, a 1000W water heater will take 1 hour to consume up to 1kWh energy, and a 15000W air-conditioning unit will take 40 minutes to consume up to 1kWh of energy.

How Do kW & kWh Affect A PV System’s Size?

When we say that a building needs 5kW of power to function in a day, it means that the building is using up to 5kWh of energy the whole day. To meet the requirements of this building through solar power, you do not need a solar power plant with 5kW capacity. You just need a solar plant with 1.5-2kW capacity instead.

How Does That Work Out?

A 1.5kW system will produce a maximum of 1.5kWh of energy in one hour when the sun shines. Depending its location, a particular building could receive peak sunshine for 3 to 5 hours. For our purposes, let’s say this building receives maximum solar irradiance for 4 hours. A 1.5kW system installed in such a building will generate approximately 6kWh of energy which is enough to meet the demand of the building.

However, there is a minor problem. The 6kWh energy was produced over a 4-hour time period but the building needs that 6kWh of energy over a 24-hour time period. The solution is battery storage. You will need to store the energy generated in a battery so that it can power the building continuously all day long.

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