How to choose the best battery storage for a solar energy system

When comparing different solar battery storage systems, it can be difficult to determine which characteristics and technical specifications matter most. The battery storage is so new that you probably don’t know anybody with a battery who you can ask about their experience.

There are certain specifications when evaluating your solar battery options, such as how long the solar battery will last. And how much power it can provide. Below, learn about all of the criteria, as well as the different types of solar batteries.

How to compare your solar storage options

As you consider your solar-plus-storage options, you’ll come across a lot of complicated product specifications.

Capacity & power

Battery size or capacity measure in kWh (kilowatt-hours). This is the total amount of energy a battery can store. However, the usable capacity may differ from the total battery capacity. Generally, the usable capacity is limited to 80 to 90% of the total battery capacity to increase the lifespan and avoid a potential battery shutdown at very low charge levels. For example, a lithium battery with 10kWh total capacity may have its usable capacity limited to 80% to increase its lifespan.

While capacity tells you how big your battery is, it doesn’t tell you how much electricity a battery can provide at a given moment. To get the full picture, you also need to consider the battery’s power rating. In the context of solar batteries, a power rating is the amount of electricity that a battery can deliver at one time.

A battery with a high capacity and a low power rating would deliver a low amount of electricity (enough to run a few crucial appliances) for a long time. A battery with low capacity and a high power rating could run your entire home, but only for a few hours.

Depth of discharge (DoD)

Most solar battery storage need to retain some charge at all times due to their chemical composition. If you use 100 percent of a battery’s charge, its useful life will be significantly shortened.

The depth of discharge (DoD) of a battery refers to the amount of a battery’s capacity that has been used. Most manufacturers will specify a maximum DoD for optimal performance. For example, if a 10 kWh battery 90% DoD, you shouldn’t use more than 9 kWh of the battery. Generally speaking, a higher DoD means you will be able to utilize more of your battery’s capacity.

A battery’s round-trip efficiency represents the amount of energy that can be used as a percentage of the amount of energy that it took to store it. For example, if you feed five kWh of electricity into your battery and can only get four kWh of useful electricity back, the battery has 80 percent round-trip efficiency (4 kWh / 5 kWh = 80%). Generally speaking, a higher round-trip efficiency means you will get more economic value out of your battery.

Battery life & warranty

Battery lifetimes are measured with three different metrics: expected years of operation, expected throughput and expected cycles. A battery’s expected throughput and cycles are like a car’s mileage warranty. Throughput lets you compare how much electricity you’ll be able to move through your battery over its lifetime. Cycles measure how many times you can charge and discharge a battery.

To convert a battery’s expected or warranted throughput into an expected lifespan, divide the throughput (expressed in kWh) by the usable capacity of the battery to estimate how many full cycles you’ll get from your battery, and divide that number of full cycles by the number of days in the year: a 20,000 kWh throughput warranty on a 10 kWh battery means 2,000 expected cycles, or a cycle per day for 5.5 years.

Your solar battery will have a warranty that guarantees a certain number of cycles and/or years of useful life. Because battery performance naturally degrades over time, most manufacturers will also guarantee that the battery keeps a certain amount of its capacity over the course of the warranty. Therefore, the simple answer to the question “how long will my solar battery last?” is that it depends on the brand of battery you buy and and how much capacity it will lose over time.

How to decide which battery specs matter for your needs

There are a number of different potential decision criteria and comparison points to make when evaluating your energy storage options.

If you want to power more of your home at once, look for a solar battery with a high power rating
If you want to be able to power a more energy-intensive appliance (like a sump pump), look for a battery with a high instantaneous power rating
If you want to run your home with your solar battery for a longer amount of time, look for a battery with a higher usable capacity
If you want to get the most out of every kilowatt-hour of electricity you put into your battery, look for batteries with a higher roundtrip efficiency
If you are space constrained and want to get the most amount of storage out of the least amount of space, look for lithium-ion nickel manganese cobalt (NMC) solar batteries
If you want a battery with the longest lifetime that you can cycle the most amount of times, look for lithium iron phosphate (LFP) batteries
If you want a battery with the absolute highest safety rating possible (don’t worry, they’re all safe!), look to LFP solar batteries

What is the best solar battery?

It’s hard to say. The best battery from the size of your home, and all the way you want to get from energy storage. If you have a large home, you’ll want to look for a high-capacity battery. So that it can keep pumping out electricity for hours on end. If you’re price-conscious and care more about optimizing your solar energy system, a smaller battery may be the best choice.

With this in mind, selecting the best battery is not straightforward. Because all battery systems need to be judged on a range of criteria including:

Usable storage capacity (kWh)
Output power ratings – Continous and peak
Efficiency – charge & discharge losses
Cost – Battery upfront cost
Cycle life – capacity loss over time
Inverter and system compatibility
Operating temperature range
Manufacturers warranty conditions

This might sound overwhelming, but luckily we have done the hard work. By performing our own comprehensive testing and gathering feedback. They are from many experienced solar professionals to determine the best battery for each application.