How to Choose Solar Battery Storage

f you are in the process of choosing your own solar system, then you probably heard about solar battery storage. Even though this element is not absolutely essential and might take the biggest bite out of your total installation budget, it can help you save a lot of money in the long run.

Do I need to store solar power?

So how do you know whether you should buy a solar battery storage system or not? To begin with, batteries almost always make sense for off-grid solar. Although solar panels will still be able to generate power, without a battery your energy supply will be intermittent and you won’t be able to power your home with your PV system in cloudy weather or at night. So battery storage is the only way to ensure a stable electricity supply.

You also might want to install a solar system with battery storage if you would like to achieve maximum energy independence. Although on-grid system owners can draw energy from the grid, without a battery they are not able to do so during power outages. Moreover, in case of a blackout, they are required to halt electricity generation by their PV systems for safety reasons. But with a storage backup, they are able to keep their lights on even when the electricity grid is down.

Battery storage is also worth considering if you do not have a one-to-one net metering program in your area, that is, the energy your PV system produces costs the same as the energy you take from the grid. In this case, it would be cheaper to save the power generated by your solar system for later and use it when necessary.

Types of batteries for solar installations

If you decided on solar with battery storage, you will now have to figure out what type of batteries suits you best. There are four main types of batteries for photovoltaic systems: lead-acid, lithium-ion, nickel-based, and flow batteries.

Lead-acid

Lead acid is the most common chemistry for solar batteries on the market. Invented back in the mid-19th century, these batteries were the only technology for pv battery for solar systems until quite recently. Lead-acid batteries are manufactured using sulphuric acid and water.

Their only advantage over other storage types is low cost, which is typically up to 2-3 times lower than of lithium-ion batteries. As for disadvantages, lead-acid batteries are less durable, more space-consuming, and fairly temperature-sensitive.

Lithium-ion

Li-ion batteries, by contrast, are a relatively new and evolving storage technology. They have a wide range of applications, from smartphones to vehicles, and have proven to be a reliable and efficient storage solution.

These batteries are characterized by high energy density, higher depth of discharge, and last up to 10 times longer than lead-acid. However, they are much more expensive than other power storage options for solar systems.

Nickel-cadmium

Although Ni-Cd batteries were invented at the end of the 19th century, they started to be used in solar systems relatively recently. Their primary advantages are high durability and tolerance to extreme temperatures, which makes them suitable for large-scale solar storage systems.

The major problem with Ni-Cd batteries is environmental impact, as cadmium is highly toxic and banned from use in some countries. That’s why they are not commonly used in solar systems.

Flow batteries

Flow batteries are the newest solar storage solution that uses water-based liquid electrolytes flowing between two separate tanks within a battery. They surpass their storage counterparts in several characteristics, the most important of which is a 100% depth of discharge. This means they can be fully discharged with no negative consequences for their lifetime. In addition to this, flow batteries can work at extreme temperatures and don’t require a lot of maintenance.

However, they come with their disadvantages. Flow batteries are larger and more expensive than other solar storage types, hence more suited for utility-scale storage applications.  

To sum this section up, lead-acid and lithium-ion are the best choices for residential pv battery systems. The former is more suitable for those PV system owners who have a limited budget, while the latter is better for those who prioritize efficiency over affordability.

5 things to consider when choosing a battery for solar

Now it is time to look at the most important factors to consider when choosing battery storage for your solar system.

Capacity

Battery storage capacity, also called storage size, refers to the maximum amount of power from your solar system that a battery can store at a given time. In other words, a battery storage capacity shows you how long it can power your home appliances.

Solar power storage manufacturers provide you with two types of capacity ratings: nominal capacity and usable capacity. Nominal capacity is how much power the battery can provide from a fully charged state, while usable capacity shows how much of that power can actually be used.

Batteries for solar panel systems are available in a range of capacities, but a standard battery has a capacity of around 10 kWh. To figure out what storage capacity you need for your PV system you should calculate your average energy consumption.

Power rating

The power rating of a solar battery tells you both what amount of electricity it can deliver and how many appliances it can power at once. It is measured in kilowatts (kW).

Solar batteries are normally labeled with two power ratings: continuous power rating and 5-minute power rating. The continuous power rating of a battery refers to how much energy it can release over a long time interval. This indicator is important for appliances that need an uninterrupted electricity supply, such as refrigerators. A 5-minute power rating shows the amount of energy a battery can put out for a short period to power appliances that need short bursts of power, like a blow dryer.

Lifespan

You can evaluate battery storage lifespan with three metrics: expected years of operation, throughput, and cycle life.

Batteries for solar systems normally last between 5 and 15 years. Most lithium-ion batteries are manufactured with a minimum life expectancy of around 10 years but last around 15 years in practice. Lead-acid batteries live around 5-7 years.

Energy throughput stands for the total amount of power a battery storage can deliver over its lifetime. Cycle life means the number of times it can be fully charged and discharged over the life course. Although both indicators may help choose solar battery storage for a PV system, cycle life does not take into account capacity degradation over time. This means a battery will be able to deliver much less power during one cycle at the end of its lifespan than at the beginning.

Roundtrip efficiency

Roundtrip storage efficiency shows how much electricity is lost with each charge and discharge of a battery. The higher the roundtrip efficiency, the less energy produced by the PV system it loses during operation. Lead-acid batteries typically have around 75% roundtrip efficiency, while for li-ion batteries this can reach up to 96%.

Cost

Battery storage is normally the most expensive part of a solar system. Today most batteries for residential use cost in the range of $8,500 to $10,000 not including installation. You should also pay attention to the battery storage cost per kWh, which ranges from $400 per kWh to $750 per kWh. It will tell you how much you will have to pay for each kWh of energy from your PV system stored in a battery.

Final thoughts

A solar system with battery storage involves higher upfront costs than a system with no backup. But if chosen wisely, energy storage can provide you with reliable 24/7 power which can be used any time independently from the grid.

On top of that, going solar with battery storage reduces your carbon footprint, as CO2 emissions from grid electricity production account for about 25% of all greenhouse gas emissions.

‍