Most people who install energy storage do so for the resiliency benefit: they're looking specifically for backup power in the event of an emergency. That particular ability makes storage an enticing purchase and an increasingly frequent pairing with solar. But the value of resiliency depends on how long you expect–or need–to run your home, leading to one main question: how long do solar batteries last?
How long a solar battery lasts depends on how big the battery is, how much electricity you use, and how quickly you can recharge the battery.
The typical solar battery stores between 10 and 20 kilowatt-hours (kWh) of electricity, while the average home uses about 30 kWh per day.
When you pair a battery with solar, you can recharge the battery as soon as the sun comes up in the morning, effectively allowing for indefinite backup.
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Short answer: it depends!
Several different factors influence how long a solar battery will last, all of which we'll cover below. But the calculation for how long a battery will last depends on three main factors: 1) how much electricity you store in the battery, 2) how much electricity you use, and 3) how quickly your battery can be recharged.
Given the variation in storage products and system sizes on the market today, it's hard to generalize. But under typical circumstances, if you install solar and an average battery, you can expect the battery will power your essential loads–think lights, refrigerator, wifi, chargers–for a couple of days. If the sun is shining and topping off your battery's charge, you can keep those devices powered indefinitely. The larger the battery, the more you can power; the sunnier it is, the more likely you are to be able to power appliances for longer.
There are two main components to understanding how large a battery is: stored capacity and power. Stored capacity characterizes how much electricity the battery can hold at once and is expressed in kilowatt-hours (kWh). Most home battery systems store between 10 and 20 kWh of electricity, though many are expandable so that you can add extra capacity by buying more batteries. Power is a mark of how much electricity your battery can discharge at once, expressed in kilowatts (kW). Most batteries provide between 5 and 8 kW of power output.
If you think of a battery like a bathtub, stored capacity is akin to the size of the tub (i.e., how much water can you keep in the tank), while power is akin to the size of the pipe from the drain (i.e., how much water can you move out of the tub at once). The bigger the tub, the more water you can fit in; the wider the drain, the faster you can deplete it.
Batteries work the same way: the larger your stored capacity, the longer you can run the appliances in your home. The larger the power, the more devices you can power at once. But, as discussed further in the next section, the more appliances you run at once, the quicker your stored capacity runs out.
Different appliances in your home require different amounts of power to use and run for different amounts of time. For instance, a standard CFL light bulb requires 12 Watts (W) of power, a microwave oven might use 1,000 W, and a central AC unit might use 5,000 W (Note: 1 kilowatt, or kW, equals 1,000 W).
To determine how much electricity each one of those appliances will use, multiply the power rating (in W) by the amount of time you plan to use it (in hours). In other words, keeping ten lights on for ten hours would use 1,200 Watt-hours (Wh) (12 W * 10 bulbs * 10 hours), while running a microwave for 2 minutes would only use 33 Wh (1,000 W * 1/30th of an hour). So, despite requiring more power, using a microwave to zap some food will likely use less electricity than keeping your lights on throughout the day.
For context, the average American home uses about 30 kWh of electricity each day. So if you have a standard battery with around 10 to 20 kWh of stored capacity, the electricity stored in your battery would only be able to power half of the typical home for a whole day or the entire consumption for half a day. If you use more devices, the stored capacity will be depleted faster. If you reduce your consumption, the stored capacity will last longer.
If you have standalone storage and the grid goes down, then it's as soon as you deplete your battery. Your backup power source is gone until the grid is restored. If you pair a battery with solar, however, your solar panels can replenish your battery's charge when the sun is shining. Additionally, when you pair solar with storage, you can power your home straight from the solar panels during the day and only draw on your stored energy reserves after the sun goes down, preserving your storage capacity.
Effectively, this means that when you pair solar with storage, you can use that solar battery to back up your home indefinitely, so long as the sun keeps shining.
Whether you're looking for solar or solar-plus-storage for your home or business, EnergySage can help. When you sign up for a free account on EnergySage, we'll gather multiple quotes (up to seven!) from reputable solar companies in your area. Our Energy Advisors can help walk you through the pros and cons of different solar and storage options, and our knowledgeable installer partners will help size your battery system to ensure you have all the backup you need.