The home storage revolution is here, and there are plenty of options when it comes to home batteries that you can install. In this article, we'll talk about battery capacity - what it is, why it matters (or doesn't), and how battery models stack up against one another.
When manufacturers or installers talk about battery capacity (or energy capacity), they usually talk about one of two metrics a battery is rated on: total capacity and usable capacity. We'll get into why those are different further down. For the time being, it's all just "capacity."
At its core, battery capacity means the amount of energy stored in a home battery, measured in kilowatt-hours (kWh). Here's a complete definition of energy capacity from our glossary of key energy storage terms to know:
The energy capacity of a storage system is rated in kilowatt-hours (kWh) and represents the amount of time you can power your appliances. Energy is power consumption multiplied by time: kilowatts multiplied by hours to give you kilowatt-hours. To understand the energy sizing of batteries, you need to know how long you want to run your appliances with your battery. Running many appliances for a long period of time would require a larger battery from both a power and energy (capacity) perspective while running fewer appliances for a shorter period would require a smaller battery from a power and energy perspective.
To give a sense of the energy usage of different appliances, keeping ten CFL light bulbs on for six hours uses nearly 1 kilowatt-hour of electricity (10 CFLs * 15 Watts per bulb * six hours). A television or refrigerator may use 1 kilowatt-hour of electricity over 24 hours, depending on how often the TV is turned off and on and to what temperature the refrigerator is set. On the other hand, running a central air conditioner could use 10 kilowatt-hours per day.
Batteries are rated for two different capacity metrics: total and usable. Because usable capacity is most relevant to the amount of energy you'll get from a battery, we like to use usable capacity as the main "capacity" metric to compare storage products.
Also, from our energy storage glossary, see how the two terms differ below:
Total capacity (kWh)
How much electricity is stored in the battery in total when fully charged. Expressed in kilowatt-hours, this is an energy metric that demonstrates the amount of electricity that would be available if you could fully discharge your battery all the way to zero.
Usable capacity (kWh)
The amount of electricity stored in the battery that is available for consumption. Batteries have their own native load requirements. In other words, batteries require a certain amount of electricity to continue running. As a result, not all the electricity stored in a battery is available for you to consume. If you try to use beyond the usable energy of a battery, it may have detrimental impacts on the health and longevity of your battery.
Like any other battery, the more energy it can store, the more stuff you can usually power with. Of course, there are other factors (like power rating and chemistry), but as a general rule of thumb, more capacity means more devices powered for longer periods.
For home batteries, this means that if the grid goes down, you'll have power for longer with a bigger battery. It also means that if you're using your home battery to avoid buying electricity during peak, expensive hours (such as in a region on TOU rates), you can spend more time pulling from your battery and potentially avoid the high-cost block entirely. Especially if you're charging your battery during the day with solar panels, beefing up capacity is a great way to ensure you're getting maximum savings on electricity from a storage system.
The big caveat: Home batteries are (often) stackable
Battery capacity is very important, but perhaps more importantly, a single battery's capacity isn't necessarily the capacity you'll install. For example, you can install multiple Tesla Powerwall batteries together to create a much larger system than an individual battery. If physical space is an issue for you, that's when battery capacities in a single product will be more important. For homes with large electric bills, you'll almost always have to install a stacked battery system to store enough energy.
Individual battery capacity only matters to a certain extent, but it can certainly be an important factor. Below, we've included a table of the largest battery from each of the most common home battery brands, ranked from highest capacity to lowest:
Biggest home batteries: Battery model capacity ranked
As we noted above, the capacity of an individual battery doesn't always mean much for battery-to-battery comparisons. For example, while the Tesla Powerwall is one of the smallest batteries on this list, it is far and away one of the most popular products on the market. Finding a home battery with the functionality and performance you want is important because most products can be combined to create larger-capacity battery banks.
No matter what battery you have your eye on, the best way to know you're getting the best deal for a high-quality installation is to compare multiple quotes. On the EnergySage Marketplace, installers compete for your business, so you know you're getting a great price for a solar-plus-storage system. Sign up today for free to get started, or begin with an instant estimate of your savings potential.
