Kilowatts (kW) and kilowatt-hours (kWh) are two units of measure that are often confused in the world of solar and batteries. Because these to units are interrelated, it can be very confusing.
A kilowatt (kW) is a unit of power. 1 kW is equal to 1000 Watts (W). This is a measurement of how fast (or the rate at which) something uses or produces power. In the example below, each light bulb uses 100 W, or .1 kW. That means it will consume 1 kWh over a period of 10 hours because
.1 kW x 10 hours = 1.0 kWh
Solar modules (panels) or entire solar systems are often described by how much power (kW) they can generate under certain conditions.
A kilowatt-hour (kWh) is a unit of energy. Energy can be thought of as power used or produced over a period of time. This is a useful unit of measure to describe the capacity of a battery or the ability of a solar system to produce power over a certain time period. Customers are billed by electrical utility companies for how many kWh they use over a certain amount of time.
This example shows that one light bulb uses the same amount of energy in ten hours as ten light bulbs use in one hour. However, the single light bulb requires much less power (1 kW vs 0.1 kW).
So how do these two units of measure relate to batteries and solar systems?
When designing a battery backup system, we must consider a range of factors to ensure the system will perform correctly. Namely, we must ensure that:
- The battery backup can provide enough power (kW) for your appliances, lights, and other electrical devices at any given moment.
- The battery backup system can provide enough power over a period of time (kWh) when the grid might be down.
Oftentimes you will see both of these units of measurement when dealing with solar and battery systems. For example, you'll often see the size of a solar system represented in kW. This rating tells you how much power (kW) the solar system can generate (typically under certain conditions). Batteries often are described by both units of measurement. The battery's capacity is represented in kWh and it's power is represented in kW. Those two values give you an idea of how long the battery might last given a certain power demand. Read more about battery runtime and power here.