So you look at a light bulb and it says “100 Watts.” What does that mean?

Your power company charges you for “Kilowatt-hours” that you use.  What is a Kilowatt-hour?

Kilowatts (kW) and kilowatt-hours (kWh) – these are very clearly defined units, but also very frequently confused. kW is a unit of power, kWh is a unit of energy. Power and energy are unambiguously different from physics and engineering perspectives but are often used interchangeably in common English as well as by some reporters who should know better.


Power vs energy (vs capacity)

When it comes to solar & batteries (and electricity in general) people sometimes use the terms power and energy interchangeably, but they’re actually different.


Power (kilowatts, kW)

Power, technically speaking, refers to instantaneous output – the amount of electricity generated (or discharged, in the case of batteries) at a given moment. Basically, power is measured in watts (W), but when we talk about rooftop solar and batteries, it’s usually easier to talk in terms of kilowatts (where 1kW = 1,000W) – just as we usually talk about the weight of produce and meat in kilograms as opposed to grams.

For example, a solar power system may produce 2kW of electrical power in the morning when the sun isn’t yet fully up, but 5kW of power around midday, when the sun is shining its brightest.


Energy (kilowatt-hours, kWh)

Energy, on the other hand, is more a measure of the ‘volume’ of electricity – power over time. You’ll usually hear (and see) energy referred to in terms of kilowatt-hour (kWh) units. The place you’ll see this most frequently is on your energy bill – most retailers charge their customers every quarter based (in part) on how many kWh of electricity they’ve consumed. It also applies to solar PV systems, of course – your solar system will generate a certain number of kWh per day. Similarly, the amount of energy that a battery can store is often referred to in terms of kWh.

As a simple example, if a solar system continuously produces 1kW of power for an entire hour, it will have produced 1kWh in total by the end of that hour.


Capacity (kW for solar, kW & kWh for batteries)

Capacity is the measure of a solar system’s potential to generate power (or in the case of batteries, both generate power and store energy).


For solar PV systems

Where things can sometimes get a bit confusing is when you see a solar PV system’s size described in terms of ‘kW’ (which is why it’s also sometimes written as kilowatt-peak, kWp). Here, we’re really talking about peak output capacity. When a solar installer refers to a 5kW solar system, for example, they’re actually talking about a system that can produce at most 5kW of instantaneous power – which will happen when the system’s panels are receiving a full dose of sunlight. As mentioned above, however, the system’s actual power output will fluctuate throughout the day depending on how brightly the sun is shining.


For battery storage

Battery capacity is measured (and discussed) in both terms of kW of power and kWh of capacity – this is why you’ll hear talk about ‘power batteries’ vs ‘energy batteries’. All batteries have both power and energy capacity ratings.


Think before you flick the switch

So next time you turn on the television or leave your air conditioner running, have a think about what your energy consumption really means. It’s fascinating to consider how much energy we actually consume each day and what it equates to in other scenarios, especially since flicking a switch requires so little energy on our part.

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