If you’re shopping around for solar panels or a battery storage for your home, you’re undoubtedly come across the terms ‘kilowatt’ (abbreviated as kW) and kilowatt-hour (kWh). These terms might be a bit confusing at first, so we’ve written this article to explain these terms and make them easy to understand.
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 – although not much in Australia). 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.
Example using a ~2.5kW solar system: Instantaneous power output vs cumulative energy production over a two-day period. Peak power output is just under 2.3kW (due to standard inefficiencies), while the total amount of energy produced over the two days is just over 33kWh.
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.
Telsa’s Powerwall 2, for example, has a continuous output capacity of 5kW (higher rates possible for short periods) and a storage capacity of 13.2kWh (at the beginning of its warrantied life).
Tesla’s Powerwall is a ‘power battery’, able to instantaneously release stored energy at a relatively high rate.
Enphase’s modular AC Batteries, on the other hand, have a continuous power output rating of 0.26kW (260W) each and a storage capacity of about 1.2kWh. AC Batteries can be ‘stacked’ to increase both their storage and output capacities (basically just multiply both figures by the number of units).
Enphase’s AC Battery in action in a demo stand at the Australian Energy Storage Conference & Exhibition 2016.
Meanwhile, LG Chem’s RESU10 has a storage capacity of 8.8kWh with a continuous power output capacity of 3kW).
LG Chem’s RESU10 battery.
Other things to keep in mind when comparing battery capacity
Talking about battery storage capacity can be tricky – especially when it comes to storage capacity, which may degrade over time. Check out our article on why you should always ask for an ‘energy throughput’ figure in addition to a storage capacity (or cycle life) specification.