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How to Choose the Best Solar Battery? (Defining Key Parameters of Solar Batteries)

While installing a solar system, one of the key decisions to make is the type of battery to pair your system with. The market is flooded with different types and sizes of batteries, and this might overwhelm solar shoppers. But fret not!

Thankfully, there are some metrics designed to help you choose the best solar battery for your system. And that’s what we are discussing in our today’s article. 

How to choose the best solar battery?

Batteries are constructed with different technologies employed, materials used, and ratings. Understanding them helps us narrow down our solar battery options. Below, we have covered all that is important to choosing a solar battery. 

1. Size and capacity rating

Our first determinant while choosing a solar battery is its size and capacity rating. The capacity rating of a battery is a measure of the amount of energy it can store. Typically expressed as kilowatt-hours (kWh), amp-hours (Ah) or milliampere-hours (mAh), it tells us how long the battery will be able to power a device before it needs to be recharged.

A battery with a higher capacity rating will be able to power a device for a longer period of time than a battery with a lower capacity rating, assuming that the device is using the same amount of power. In general, the capacity rating of a battery is an important factor to consider when choosing a battery for your solar system. You will need to assess your needs and goals before ordering a battery.

Most often, batteries display two types of capacity ratings: total and usable. The latter refers to the amount of electricity stored that you can actually use. Batteries come with different usable capacities, but you can achieve your desired rating by connecting multiple batteries.

2. Power rating

The power rating of a battery refers to the maximum amount of electrical power that the battery can deliver at one time. It is typically expressed in kilowatts (kW) or amperes (Amps) and usually has two categories.

  • One is the peak power rating—that is, the maximum power a battery can provide for a short time.
  • Another is called continuous power rating, which measures the amount of power the battery can offer continuously.
Next Earth Solar Tip: Most solar batteries available in the market come with a continuous power rating of 5 kW, and a peak power rating of 7 kW. You should aim for a power rating of at least 5 kW.

The power rating of a battery is an important factor to consider when choosing a battery for your system. For example, if you have appliances that require a high level of power to operate, such as air conditioners, you will need a battery with a higher power rating. In general, the higher the battery’s power rating, the more quickly it will be able to power a device. Conversely, since it gives a quick burst of power, a higher power rating doesn’t mean that the battery will stay charged for longer.

Doesn’t the capacity rating and power rating appear the same? No, they are not the same. Think of it like this: a battery with a high power rating and low capacity can keep your appliances powered for a short period of time. But a battery with a low power rating and high capacity will not struggle to keep your appliances running for longer periods.

3. Safety rating

The safety rating of a battery means the measures in place to prevent the battery from overheating, thermal runaway (catching fire), or exploding. When choosing a solar battery, it is especially important to consider the safety ratings, as solar batteries are often kept indoors and can pose a greater risk of overheating or catching fire if they are not properly designed, installed, and handled.

For example, NiCd batteries use Cadmium, a highly toxic metal that poses a risk to the environment if not properly disposed of. Similarly, some battery chemistries are more prone to thermal runaway than others. However, batteries available for solar shoppers in the US are safety-certified, but some types of batteries are safer than others, even above the government-mandated requirements.

Also, while considering the safety ratings of the battery, it is also important to follow proper safety precautions when using and storing the battery. This includes handling the battery carefully, following the manufacturer’s instructions for use, and storing the battery in a cool, dry place away from flammable materials.

4. Depth of discharge

The depth of discharge (DoD) of a battery refers to the extent to which a battery can be safely discharged. For example, if a battery has a capacity of 100 amp-hours and you use 50 amp-hours of electricity from it, the battery has been discharged to a depth of 50%. Today, most batteries come with a DoD higher than 60%. Flow batteries have been touted as the ones with 100% depth of discharge—you can safely empty the battery to the last bit.

For solar power systems, the ideal depth of discharge can vary depending on the specific battery technology being used, your budget, and the desired lifespan of the battery. If you go for lead-acid batteries, they come with a 60% of depth of discharge. Lithium-ion batteries, conversely, have an 85% of DoD, but they are more expensive than lead-acid batteries.

If you frequently discharge a battery below the recommended DoD, it will affect the performance and lifespan of the battery.

5. Roundtrip efficiency

Roundtrip efficiency is another important many often overlook. It refers to the percentage of energy that can be recovered from the battery when it is discharged, compared to the amount of energy originally stored in the battery. For example, if a battery has a roundtrip efficiency of 70%, then when it is fully discharged, it will be able to store 70% of the energy that it originally contained. The battery itself consumes the rest 30%.

Some batteries might appear affordable, boasting high capacity ratings. But what if the battery uses half the power for powering itself? That means you will install twice the size of the battery bank you need for your system.

The ideal roundtrip efficiency for a solar power system will be above 80%, allowing you to get the most out of the battery and minimize energy losses. However, it is important to note that batteries with higher roundtrip efficiencies may also be more expensive.

6. Lifespan and warranty of the battery

For solar systems, these two are important indicators. Energy storage is expensive and cannot be replaced every two years. Thanks to the improvement in battery technology and the use of different chemistries, solar batteries can come up with up to 15 years of lifespan. Some batteries, such as flow batteries, have an estimated lifespan of up to 30 years—but that’s an emerging technology.

Lead-acid batteries last for around 2-7 years, depending on how you maintain and use them. Lithium-ion batteries are designed to last for 15 years, with the standard warranty period being ten years. Aim for at least ten years of lifespan, or 10,000 cycles*, whichever comes first.

*Battery cycles refer to the number of times the battery can be charged and discharged. 

7. Cost of the battery

Lastly, the cost of the battery. Based on our discussion above, we all would like to buy batteries with high capacity ratings, high power ratings, better roundtrip efficiency, a higher DoD, and longer lifespans.

But the buck stops (literally) at the cost of the battery. Lithium-ion is considered best for pairing with solar systems but is also expensive. Some of them, such as Tesla Powerwall, can cost $12,000 per piece. On the other hand, you can design a lead-acid battery bank of comparable size for around $5,000. What’s the trade-off here? You will have to sacrifice your battery’s longevity, efficiency, DoD, and so on.

But do you know that you can reduce the cost of your solar battery by 30%? Yes, batteries make it to the list of eligible expenses for claiming the Federal Solar Income Tax Credit! That means you can install a battery worth $12,000 for $8,400.

That was all about how to choose the best solar battery. Happy shopping!

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