Image source: Solar Edge & PV Europe

Solar energy and how to store it have always been natural partners for homeowners interested in reducing their carbon footprint. But the idea of residential solar storage made little sense due to the cost of solar batteries vs. options to export electricity to a local utility provider.

All of that is finally changing. Solar battery costs are coming down. This opened the door for the industry to develop new technologies to provide better on-site solar storage options to residential consumers and turn itself into a booming market.

What is a solar battery?

A solar battery functions in the same manner as any other battery, but that's where the similarity stops. There are four main types of batteries used to store electricity from residential solar power systems. All offer differing results, and all come at different prices.

Lead acid: This is not your average automotive battery. It's large, bulky and heavy, but it's also inexpensive, dependable, and well tested. New technologies are closing in fast, offering longer warranties and better prices, but these probably aren't going anywhere.

Lithium-ion: This family of batteries is already used in more applications than any other. The prismatic types, specifically lithium iron phosphate (LFP) batteries, are commonly used in solar energy storage.

Flow: Flow batteries are a relatively new entrant to the residential solar arena. They contain a water-based solution of zinc-bromide. These are the best batteries for long-term storage because they last for last decades with little maintenance and the internal electrolyte materials can be repurposed.

How do solar panels charge batteries?

Solar panels don't work alone to charge one or more batteries, but the process itself isn't complicated because it only requires the battery, a charge controller, and an inverter. A 12-volt battery can take between five to eight hours to charge.

Charge Controller: Whether or not to include this component typically depends on the size of your solar system. A unit containing a maximum power point tracking (MPPT) circuit maximizes transferred energy. Another plus is automatically disconnecting the solar panel from the battery when fully charged.

Battery: The size and type of the battery you choose will depend on your energy needs. A 12-volt, deep cycle battery is designed to withstand repeated charge/recharge cycles without sustaining any damage to internal plates or electrolytes. The average automotive battery cannot withstand being repeatedly drained and replenished without quickly failing.

Inverter: The inverter is an inexpensive, yet vital component that transforms the electricity stored in the battery so it's usable in your home. Always check the current rating on the inverter, along with the number of electrical sockets it can power.

How is solar energy collected?

Photovoltaic solar panels convert sunlight into energy via a semiconductor material such as silicon. As sunlight strikes each cell on the panel, a portion is absorbed by the semiconductor material. This process releases electrons and allows them to flow, resulting in an electrical current.

Standard panels can convert sunlight into electricity with a top efficiency of around 15 percent. More expensive panels can do the same with about 20 percent efficiency.

What should I look for in a solar battery?

There are several factors to consider when determining which battery will fit your system's storage needs. Low prices are always attractive, but it can easily affect the quality and battery life. Having to finance frequent battery replacements may not be such a good deal in the long run.

Always consider the capacity of a battery or the amount of energy it can store. Voltage is also vital to ensure it at least equals your system's requirements. Battery bank voltage is often determined by the inverter's specifications. A battery's cycle life is probably the most critical consideration. This accounts for the number of charges/recharges the unit can provide before production drops beneath a certain percentage of the full load capacity. This can vary widely from manufacturer to manufacturer on the same size batteries.

How do I know what battery is right for my home?

Never make the mistake of underestimating the size of the battery you need. It can be frustrating to purchase a battery that doesn't give you enough stored electricity on an overcast day or one that isn't large enough to hold additional output on sunny days. Answering these questions can help you choose the right battery for your home:

  • What is your average daily electrical usage in kW Hours?

  • Do you have a backup system? How efficient is it? For example, a net metering agreement or a generator that will kick on automatically.

  • How reliable is the daily sunshine in your area?

  • If poor or unpredictable, about how long would you estimate your system would be able to cooperate without it?

You should also consider how often you use appliances with high power needs.

These questions may seem tedious, but a battery that isn't fully charged or one that can't hold a charge cannot give you what you need or last long. Plus, you can find some of the answers with the help of the solar estimator.

What are the top-selling solar batteries on the market?

When solar battery prices start dropping at a rapid rate, that's good news. There are four leaders in the solar battery race, but the number is growing as rapidly as the industry.

  • Tesla's Powerwall remains the best-known battery for residential systems, making the company a market leader. The Powerwall combines a 13.5 kWh battery with an inverter and control system.

  • SunPower's residential and commercial businesses are geared toward high-end consumers. They produce more efficient solar panels than many competitors, but they plan to produce storage products.

  • SolarEdge produces StorEdge, which will provide backup power. Plans are to produce a product containing an HD inverter combined with EV charging and energy storage could be operated via a smart home's energy hub.

  • Sunrun is promoting its BrightBox energy storage unit and backup power provider in Arizona, New York, California, and Hawaii. The product will also cut peak energy rates when economical.

Unless you plan to go entirely off-grid, purchasing a solar battery will add some extra backup to your system.