How much battery do you really need?

home battery energy usage

One of the most common puzzles to a solar installation project with energy storage seems simple, but is quite complex to define: how much battery do we really need?

The reason the answer isn’t as straightforward as pointing out a specific product or listing a kilowatt number is that it depends heavily on what you want the system to do. Think of it like the gas tank on a car: the amount of time the vehicle will run on a tank of gas depends on how many miles you want to drive. You can fill up a quarter of the tank and it will get you around town for a week, but if you’re planning a road trip, the same amount of gas will be gone in an hour or so.

An energy storage solution is the same way. The same amount of battery can last overnight or for a week or more, depending on what you are running with it.

So, the true answer to the question of how much storage you need is, what are you using it for, and how long do you need it to last?

The average home uses 32 kW of energy per day. Using that metric, a system needs at least that much battery storage in order to power all of the electrical appliances in a home for 24 hours. More battery storage is needed to last longer or to make it through a string of cloudy days with no sun input.

However, if the system isn’t designed to fully power the home every day, the answer changes. In an emergency event, the use of multi-level reticulation and load shifting prioritizes critical loads and allows conservation of the available energy.

Generally, refrigeration, security and sometimes heating systems are deemed as top-tier critical functions. Other uses, such as lighting, appliances, and air conditioning are classified as less critical in descending order. The system can then shut off the less vital systems as needed and extend the life of the battery. Using this approach, a home can power critical functions for eight to ten days with a battery as small as 10 kW.

The best way to plan an efficient solar array that will truly meet the needs of the homeowner is to ask in-depth questions before you begin.

  • What appliances are critical to the needs of the individual?
  • How much sunlight is available on average?
  • How long does the system need to function independent of the grid?

A combination of totaling the kWh of each tier of energy use and having an in-depth conversation about what will run and when gives an installer a pretty clear picture of how much energy storage a system needs.

Glenn Jakins is the CEO of Humless.

— Solar Builder magazine


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