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The Best Solar Batteries Available For Your Solar PV System

Tesla Powerwall 2

When buying a new solar panel system for your home or company, you should always evaluate your solar battery options dedicated for your solar PV (photovoltaic) system and for the energy needs of your household or company.

You need to know what budget you have for your solar battery system, how long do you want for the battery to last and how much power it can provide. You can use a free budget template to help figure out some of the details.

Knowing the energy demand of your home or company will help you choose among the different types and models of solar batteries available on the market today.

How to Choose Among the Different Solar Storage Options

Usually, you will use solar batteries for your solar panel system only if you live off-grid or you simply don’t want a grid-tie solar PV system because you want to store the energy produced by your panels and you also want to have electricity in case of a blackout in the grid.

The battery bank will store energy produced by the panels and needs to be able to provide power for at least two consecutive days.

The solar company that will install the solar PV system on the rooftop of your house or company, can provide you with a solar battery that could store the energy produced by your solar panel system.

However, if you want to choose the solar battery by your own, you should always check for the battery capacity and its power ratings, the depth of discharge (DoD), the round-trip efficiency, the manufacturer, and always check the warranty.

Grid-tie or Off-grid

If you live in the city or close to it and you have access to the grid, you can opt for a grid-tie solar system, which will use both the energy provided by the grid and the clean energy produced by your PV panels.

Using a grid-tie solar PV system will allow you to lower your energy bills by using the energy produced by your panels and less energy from the grid, but in the case of a blackout you will remain without power even of you have solar panels installed on your rooftop.

To avoid the lack of power when a blackout occurs, you need a solar PV system that can store energy in solar batteries.

The battery system will provide energy during a blackout, during the day and night, and even in the days when the Sun is not shining, but no longer than one or two days (depends on the size of the battery bank and the energy consumption of your household).

An off-grid solar PV system will work perfectly with a solar battery bank.
However, without being connected to a utility company, you have no energy bills to pay, and also no backup from the grid.

This is the reason why you need to know everything about solar batteries because if you choose being well-informed you will get the best battery bank for your solar PV system and your energy needs.

Battery Capacity and Power Ratings

The battery capacity shows the total amount of electricity that can be stored by the battery and is measured in kilowatt-hours (kWh).

Solar batteries created for home use are usually designed to be “stackable”, which means that the user can create a battery bank (by connecting multiple solar batteries) to store the energy produced by its solar PV system.

A solar battery with a high storing capacity and a low power rating will deliver a lower amount of electricity (which is still enough to run a small number of appliances inside the house) for a long time, while a solar battery with low storing capacity, and a higher power rating can run your entire home, but only for a short period of time (only a couple of hours).

Depth of discharge (DoD)

The depth of discharge (DoD) refers to the amount of energy that has been used from the battery capacity.
Almost all manufacturers will specify a maximum DoD to ensure an optimal performance of the battery.

It is not wise to completely discharge a solar battery by ignoring its maximum DoD because this way, the life of the battery will be severely reduced.

Most solar batteries will retain some charge all the time due to their chemical composition, but even so, it is advisable not to consume 100% of your battery charge.

Example: a battery with a capacity of 10 kWh may have a DoD of 90%, which means that you can use only 9 kWh of its capacity before recharging it.

Round-trip efficiency

A battery’s round-trip efficiency refers to the amount of energy that can be used from the total capacity of the battery (the percentage of useful energy).

Let’s say that we have a 10 kWh battery, but we can use only 8 kWh of energy from its total capacity, and this means that the battery has a round-trip efficiency of only 80%, because we can use only 80% of the energy stored in the battery.

A higher round-trip efficiency of the battery means that you will get more energy from the battery itself, which means that you will get more economic value from that battery.

Battery life and the warranty provided by the manufacturer

If you use your solar battery every day, the battery will cycle daily (charge and drain), which in time will reduce its capacity to store energy (decreased efficiency).

All solar batteries have a warranty provided by the manufacturer, which will guarantee a certain number of cycles and/or years of useful life.

The manufacturer will also guarantee a certain amount of battery capacity that will be ensured over the course of the warranty.

To answer the question “How long I can use a solar battery? you need to know the name of the manufacturer and the percent of battery capacity that is lost over time.

Example: a solar battery that is guaranteed for 5,000 cycles, or 10 years of usage at 70% of the original capacity, refers to the fact that after a decade of usage, the battery will lose only 30% of its initial capacity to store energy.

How to Calculate Your Needs

When living off-grid, you need to be very careful with the energy consumption of your house because the energy is not provided by a utility company, but is produced by you.

When choosing a solar panel system that will be the only source of energy for your off-grid home, you need to be aware what are your energy needs to choose the proper solar PV system and a battery bank that is big enough to store the energy produced by the panels.

To calculate the energy demand of your home, you need to know what appliances and electronics will be used in the house.

We first need to calculate the daily energy consumption of your house.

To do this you need to take all the appliances, electronic devices and light bulbs used in and around the house and calculate the energy consumption per hour for each device.

To do this in the most accurate way possible, it is not enough to read the data on the back of the device or on the device itself.

To know the real load produced by a device that is connected to an outlet you can use a power meter to see the real consumption per hour for each device.

When you add all the consumption values for each device used in the house, you can find the daily consumption by multiplying the total consumption per hour of all the devices with the number of hours of use per each device during a 24 hour period.

Calculating daily consumption – Example

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Let’s say that you decide to move off-grid and live in a tiny house, a cabin, a yurt, etc., and you plan to use there a chest freezer, a mini fridge, one laptop, a phone, maybe a TV and a few light bulbs (LED light bulbs).

The total consumption per day in this case would be as follows:

  • the chest freezer around 480 watt hours per day;
  • the mini fridge around 660 watt hours per day;
  • the laptop around 200 watt hours (during 3 hours of usage) per day;
  • the phone around 10 watt hours per day;
  • a TV used about two hours a day would consume around 200 watt hours per day;
  • the LED light bulbs around 240 watt hours per day.

All these devices would consume roughly 1,790 watt hours, which equals 1.79 kWh (kilowatt-hour).

After knowing your daily energy consumption, you need to know how many days of backup power (stored in your batteries) you would need.

Some places are good for solar power, while other places can be better suited for wind turbines and not very much for harvesting solar energy.

As an average, you can choose a 3 day reserve, but if you live in an area with many overcast days during a month, you should choose more than 3 days.

To know how much reserve power we need in this case, we will multiply the daily consumption of 1.8 kWh with the number of reserve days.
If we rely on a 3 day reserve, we get 5.4 kWh, or 5,400 watt hours.

Ohm’s Law Can Help

By using Ohm’s law, we convert watt hours to amp hours, so for a consumption of 5,400 watt hours divided by 12 volts (the voltage of our battery bank), we get 450 amp hours.

However, because of the way the batteries work, you can’t entirely discharge any battery without harming its health, so let’s say that our batteries have a DoD of only 50%.

By using only half of the energy stored in the battery bank, we will need 450 amp hours multiplied by 2, which equals 900 amp hours for a 3 day reserve.

Best Solar Batteries Available Today

To start talking about the best battery for your solar PV system or even for a wind power system, you need to understand that car batteries are not the best option here.

Contrary to the general belief, the flooded lead-acid batteries that are used in cars are not the best option to store the renewable energy produced by your solar panels, they are actually some of the worst options to rely on, especially when living off-grid.

Flooded lead-acid batteries used in cars are quite popular among people living off-grid just because they are cheap, and easily available.

Car batteries are not very suited for off-grid living because they provide high bursts of electricity for only a short period of time and this happens because they are designed to provide high power in short time to start the engine of a vehicle.

A solar energy storage system requires a battery that provides low current for a long period of time.

Lead Acid Batteries

Flooded lead-acid batteries are designed using smaller (shorter) electrodes, while a solar battery (a deep cycle battery) uses longer electrode plates.

The lead-acid battery is an established technology that consists of: lead-acid batteries, AGM (Absorbent Glass Mat) batteries and gel batteries.

AGM and gel batteries require no maintenance, while flooded lead-acid batteries require maintenance.

In terms of price, the deep cycle flooded lead-acid batteries are the most affordable (between $70 and $80 per kilowatt hour).

AGM and gel batteries have similar prices, but they are way more expensive than flooded lead-acid batteries (between $250 and $330 per kilowatt hour).

In terms of performance, gel batteries are pretty similar to AGM batteries, but they usually have a longer lifespan.

Lithium Ion Batteries

Batteries using the lithium-ion technology are long lasting, robust and durable energy storage devices.

They are lighter and smaller than lead acid batteries of equivalent capacity, which means that are more practical.

Nowadays, lithium-ion batteries are becoming cheaper because their price is dropping by 8 percent per year.

For solar energy systems, the main types of lithium-ion batteries are lithium cobalt oxide and lithium ferro phosphate batteries.

Today, the cheapest lithium ion option is provided by the Tesla Powerwall that has a price of $505 per kilowatt hour.

Saltwater Batteries

Saltwater batteries or sodium ion batteries are very exciting due to their performance and price.

This type of batteries are also one of the safest batteries available today because they can handle heat very well and they are also completely recyclable.

They have a high number of charge cycles and they can be discharged deeply without affecting their lifespan.

Saltwater batteries are very stable batteries that require no maintenance.

Sodium ion batteries are cheaper than lithium ion batteries (between $400 and $500 per kilowatt hour.

Saltwater batteries are the cost-effective option compared to lead acid batteries because they have a high number of charge/discharge cycles and a higher DoD.

Sodium ion batteries have also some drawbacks related to the fact that they are slightly heavier than lead acid batteries and way heavier than lithium ion batteries.

Top of the best solar batteries for solar PV systems

  • Lithium Ion batteries are the best option due to their longevity.
  • Sodium Ion batteries (Saltwater batteries) due to the large number of cycles and high DoD.
  • Deep Cycle Gel batteries and Deep Cycle AGM batteries.

Conclusion

Choosing the best battery pack for your solar PV system can help you reduce your energy bills and the carbon footprint of tour home close to zero.

Article written by:

I am a writer and reporter for the clean energy sector, I cover climate change issues, new clean technologies, sustainability and green cars. Danny Ovy

1 Comment

  1. Jesse Ford

    Thanks for mentioning that you’ll use solar batteries for a system if you live off-grid and want to store energy produced by the panels. My brother is thinking about looking into solar batteries next month because he’s producing a lot of power and doesn’t want to send that energy back to the grid. I think it’s a good investment to hire a reputable company that has the equipment he needs in case he wants to add it to his solar arrangement.

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