How to choose the best battery for a solar energy system

There are different types of solar batteries for home use available on the market today. They have different specifications, and to choose a proper solution for your needs, you have to compare them. The main types of batteries used in solar-plus-storage systems are lead-acid, lithium-ion, and salt water.

How to Select Optimal Batteries for Your Solar Panels

While choosing solar batteries, one has to take into consideration a number of parameters like the amount of energy one can get from the battery or the battery’s longevity. In this post, we discuss every factor to be considered when selecting a storage system and compare various kinds of solar batteries.

Which specifications to compare

When you start to choose a battery for a solar generating system, you will find many technical parameters. The most essential of them are power and capacity, DoD, round trip efficiency, warranty period, and producer. 

Power and capacity

Battery’s capacity shows how much electrical power can be stored in a battery. This value is commonly expressed in kilowatt hours. The majority of batteries for residential use are separate modules that can be stacked together to expand the capacity of the solar+storage systems.

In other words, capacity is a battery’s size. However, this parameter does not show the amount of power that can be withdrawn from the battery at once. So, you also have to take into account power rating, which is how much energy can be provided by a battery momentarily. This parameter is expressed in kW.

High-power and low-capacity batteries can power the whole house during several hours, while low-power and high-capacity batteries are able to provide little energy (sufficient to power only several devices) during long periods.

Depth-of-discharge

The majority of batteries have to be kept charged at some level all the time. Full discharging of the battery decreases its lifespan considerably.

DoD means how much electricity has been withdrawn from the battery. Many battery makers indicate the maximal possible depth-of-discharge for their products. For instance, the battery with a capacity of 10 kilowatt-hours and 90% depth-of-discharge allows using up to 9 kilowatt-hours before it needs to be charged. The higher depth-of-discharge is, the more capacity of a battery can be used.

Round trip efficiency

This parameter shows how much of the electricity fed into the battery can be utilized (expressed as percentage). For instance, if the battery has been charged with 5 kilowatt-hours of power and can provide 4 kilowatt-hours of power to be used, its round trip efficiency is 80%.

Warranty

In the majority of residential applications, solar batteries get charged and discharged every day. With the course of time, a battery loses some of its ability to keep charge and gets discharged faster. If a battery has a 5,000-cycle or 10-year warranty at 70% of its initial capacity, this implies that by the end of this period the product will lose up to 30% of its energy storing ability.

All solar batteries are warrantied for a specified quantity of charge-discharge cycles or duration of service life. Due to gradual performance degradation, battery makers commonly warranty how much of a battery’s capacity will be kept by the end of its guaranteed lifespan. So, the useful life of your battery is specified by the manufacturer and the amount of original capacity that will be lost gradually.

Producer

Solar batteries are developed and manufactured by various firms including technology start-ups and car makers. Technology start-ups commonly provide next-generation technologies, but have short track records. As for car makers, they may be existing for many years and have proven longevity of the batteries produced. However, the technologies they use are usually less innovative.

It’s up to you to decide if to buy batteries made by advanced start-ups or well-established producers. While making a choice, evaluate the warranty provided for each particular battery.

How lasting is a solar battery?

The question can be answered in two different ways. One approach is by determining the period of time when a battery can keep the house powered. As a rule, a 100%-charged solar battery can provide the household with energy during the night when a PV array doesn’t generate any electricity. In order to calculate this more exactly, some more factors should be taken into consideration. They include the battery power & capacity, your electricity consumption, as well as if your solar system is grid-tied.

For example, an average American family consumes about 30kWh of electricity daily. The capacity of most standard solar batteries is around 10 kilowatt-hours. Theoretically, in order to power the house by the batteries alone for 24 hours, such a family will have to install 3 such batteries.

In practice, it doesn’t work that simple. PV modules produce electricity in the daytime, which is usually sufficient for six or seven hours. At the same time, very few battery types can perform at their maximal capacity. Most of them cannot be discharged by more than 90 percent, which means that a 10-kilowatt-hour battery can provide only 9 kilowatt-hours of useful energy. 

Thus, in a solar-plus-storage solution, a single or a couple of batteries will be enough to run the home at night when a photovoltaic system does not work. In case you don’t use any solar generating systems, at least three batteries will be needed to deliver enough electricity for the household during the whole day and night. Besides, if your goal is disconnecting from the power grid, take into account a back-up power source sufficient for several days of possible dull weather.

Useful life of solar batteries

An average life of a battery is 5-15 years, which means that solar batteries require replacing minimum one time during 25- or 30-year life of a solar array. But modern PV modules have become more lasting during the latest years, so batteries are likely to offer longer life in the nearest future, too.

One more factor the service life of your battery depends on is the way you maintain it. The ambient temperatures impact battery performance. In order to prolong the battery’s lifespan, you should protect it from extreme cold and heat. The temperature increase over 90 degrees Fahrenheit leads to battery overheating, which requires charge reducing. On the contrary, temperature fall under 30 degrees Fahrenheit requires higher voltage for the battery to get fully charged. Lots of battery makers, such as Tesla, offer an opportunity to moderate temperatures as a solution for this issue. But if your battery is not provided with this option, you have to think of alternative ways to cope with the challenge (for example, an underground box or enclosure). If you maintain your battery properly, it can significantly increase its useful lifespan.

Which solar batteries are the best?

Most solar batteries have one of the following chemistries: lithium-ion, lead-acid, or salt water. Li-ion is the most expensive type of batteries, but it is the optimal choice for most PV solutions.

Lead-acid

This tech has been utilized in off-the-grid energy generating solutions for dozens of years. This is the cheapest battery type available today, but the lifespan of such batts is shorter and depth-of-discharge is lower compared to other storage options. Lead-acid technology can suit those who plan to disconnect from the electrical grid by installing many batteries.

Li-ion

Most modern batteries for residential application are based on Li-ion compositions. Such batteries last longer, their depth-of-discharge is higher, and they are more lightweight and smaller in comparison with their lead-acid rivals. But Li-ion storage solutions are priced higher than other battery options.

Salt water

This is the newest technology used in solar batteries. Such a battery consists of salt water electrolyte instead of a heavy metal and is easy to recycle (unlike its counterparts requiring special disposal). The drawback of this tech is that it hasn’t been as tested as the other battery types. Aquion – the manufacturer of salt water batteries for residential sector – went bankrupt three years ago.