There’s an old joke about a solar-powered flashlight – it only works when the sun is out. If you’re looking at a solar setup, you’ll probably want to use it at night – so you’ll need a battery to store power for when the sun isn’t shining. While battery technology isn’t advancing as quickly as solar panels themselves, advancements still show up every few years. Right now, the first choice when you’re selecting a battery is between lead-acid and lithium ion.
Lead-acid Deep-cycle batteries are an okay choice for small or temporary installations. They’re commonly used for powering electrical systems on boats or recreational vehicles, so there’s plenty of 12V wiring and appliances available. Trouble is, they’re not meant for continuous usage, so you’ll end up replacing them in a few years. Older lead-acid batteries are referred to as “flooded” since they require water to be added to the electrolyte. Flooded batteries need to be deployed outside so that explosive hydrogen gas won’t build up in your home. Modern deep-cycle batteries are sealed and don’t require the maintenance that Flooded batteries need. Regular automotive batteries aren’t useful for solar installations, since they’re damaged by continuous deep discharge.
Lithium ion batteries are a more modern alternative – like the kind of battery you’ll find in your laptop, cell phone or Tesla Powerwall. They’re superior to lead-acid batteries in every aspect except price. Lithium batteries charge and store energy 15% more efficiently, store twice as much power and have a lifespan up to five times that of high-quality lead-acid batteries. These advantages come with a cost: Lithium batteries can cost two to three times as much as the equivalently sized lead-acid battery.
Of course, the best value battery is the one that’s the right size from your home. A battery with not enough storage capacity will need to be replaced more quickly – either because it reaches the end of its life more quickly or because it needs to be replaced with a higher capacity model. A battery that’s too large for one’s needs is more money up front. But one can make a good estimate by determining your peak load (the maximum wattage of all appliances in your home), the time you plan to run each appliance per day and how many days of power you’d like to have in “reserve” (hopefully more than the most cloudy days you’ll see in a row).