Solar Batteries (The Ultimate Guidebook)

Store your solar energy in solar batteries!

Every energy revolution contributed to mankind’s development, from the use of wood in the Stone Age to charcoal and oil in the 19th century.

We are now leaving the era of electricity.

Electricity is defined as a flow of charged particles – electrons.

Unlike other fuels naturally available on Earth, electricity has to be produced from primary resources (charcoal, oil, wind, solar). Currently, the production of electricity originates massively from non-renewable sources (64%):

  • Coal (38%)
  • Gas (23%)
  • Oil (3%)

Other production sources are from:

  • Nuclear (10%)
  • Hydro (16.2%)
  • Wind (4.8%)
  • Solar (2.1%)
  • Biofuels (2.4%)
  • Other renewable sources (0.5%)
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Since the 70’s global electricity production has been multiplied by 5 and is still growing at a rate of 4% per year.

During the last 20 years, wind and solar technologies showed the fastest growth in terms of electricity production.  

Energy storage is essential for on-demand use, or for mobile applications. Storing natural sources of energy like charcoal, oil, or gas in a tank is easy.

On the other hand, storing a secondary source of energy like electricity (electrons) is a technical challenge. We cannot put electrons in a container, obviously!

Electricity in Nature

For medium to long-term storage, scientists and researchers came out with chemical storage of the electrons called the battery.

Many different types of batteries were developed for all types of use. You will find one for the smallest electronic gadget (such as those stored in solar power banks) to the heavy-duty electricity storage.

In this article, we will give an in-depth look at the different types of batteries available to store solar energy: the solar battery.

What Is A Solar Battery?

A solar battery is a battery that is designed to store electricity produced by solar panels.  

solar batteries
A solar battery for street lights

The battery is an essential component of your solar system, let’s see why.

Basic Working Principle Of A Rechargable Battery

Every battery is made of the four main components below:

  • Two electrodes: Anode (-) and Cathode (+)
  • A porous membrane that separates the electrodes
  • An electrolyte

The nature of those components will vary depending on the battery technology.

Anodes and cathodes are usually made of metal. They are connected by a wire and immersed in the electrolyte.

An electrolyte is a liquid that contains charged particles called ions, the membrane is porous to ions.

Let’s remember our chemistry lessons, with oxidation and reduction reactions:

During discharge, an oxidation reaction occurs at the anode that generates electrons.

Meanwhile, at the other electrode (cathode), a reduction reaction using the electrons is happening. A flow of electrons (electricity) is therefore created between the two electrodes. This is the output of the battery.

The solar battery maintains its electrical neutrality at all times thanks to an exchange of ions in the electrolyte.

solar battery
Basic working principle of a battery (example of Li-ion)

During charging the opposite reaction occurs. Oxidation at the cathode and at the same time reduction at the anode. The flow of electrons is in the opposite direction compared to discharge mode.

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Solar Energy – A Variable Source Of Energy

The photoelectric effect described by A. Einstein in 1905 is the birth date of our modern solar panels.

A solar panel converts visible light (photons) into electricity (electrons). Thanks to scientists, we now have highly efficient and reliable solar panels.

solar panels
An array of solar panel

Our sun is a natural source of energy, therefore it is intermittent. You will see its intensity fluctuating during the day. Consequently, your solar electricity production will always change.

You will certainly prefer to use electricity at all times, so you need to store your production. The most versatile and reliable way to store solar energy is through a battery.

This is the solar battery.

Why You Should Couple Batteries To Your Solar Panels

In real-life conditions our energy consumption is predictable. Consumption peaks occur in the morning around 7 am when we wake up and in the evening around 7 pm.

household load
Household load and power generated by solar panels

Unfortunately, those peak consumptions are far from the peak production of your solar panels. The biggest production is when the sun shines bright at noon.

This has resulted in the wide usage of batteries.

Coupled with solar panels they allow us to use solar energy at any time and in all conditions.

Your battery capacity should be sized according to your needs. It should also match the production of your solar panels.

Let’s see what the different battery technologies available on the market are.

10 Of The Most Useful Battery Terms

If you look at the technical terms of a commercial battery you might find yourself completely lost! Don’t worry, below you’ll find a top 10 list of the most relevant ones.


Rechargeable batteries store energy in the form of chemical energy.

Energy is defined as a certain power supplied during a certain time.

Energy is measured in kWh. For example, 1 kWh of energy is equal to 1 kW of power supplied all the time during 1hour.

Energy = Power * time

On your electricity bill, the energy company charges the kWh consumed monthly. For example, 0.2559 USD per kWh in California.


The battery supplies electrical power. Power is measured in Watt (W). Every load connected to your battery will request a certain power to operate at its nominal capacity.

Power is defined as Current (A) * Voltage (V)


The voltage of your battery will fluctuate from its nominal value depending on the state of charge of your battery, the instant load, the temperature. The fluctuation is generally around 20% from the nominal value.

During charge, for example, a 12 V battery will reach 14.4 V, down to 10 V when fully discharged.

You will also have instant voltage fluctuation if you connect a heavy load to your battery. 


Electric current is measured in Ampere. This is the flow of electrons (electricity) that your battery can release (discharge) or recover (storage). This value is closely related to the power of your battery.

Charging and Discharging

Charging is the storage of electricity in the battery.

Discharging is the supply of electricity by the battery.


Your battery stores a limited amount of energy defined as its capacity.

The battery capacity is expressed in kWh but you can also find the capacity in Ah (Ampere hour) or Amps.

The capacity of your battery is influenced by several factors:

  • Temperature
  • State of charge
  • Electrical load connected

A high discharge rate will drastically reduce your battery capacity.

Let’s look at the example below:

You might see this type of specification for your battery capacity:

12 V nominal voltage

100 Ah during 20h at 25°C

It means that your 12 V battery can supply a total of 100 Ah over 20 hours.

In terms of energy it is equivalent to 1.2 kWh over 20 hours: 100 Ah*12 V= 1200 Wh

The important figure here is 20 hours. It means that the 1.2 kWh is supplied straight during 20 hours (no peak load, no cut off).

So that would be equivalent to 60 W of load at any time during 20 hours: 60 W*20 hours = 1.2 kWh.

In the end, if you exceed on average this value of 60 W over the course of 20 hours, you will discharge your battery quicker and you will reduce your total capacity.


Batteries work only with Direct Current (DC) for charging and discharging.


All our daily appliances use alternative current (AC).

DOD (Depth of discharge)

To improve the battery life duration, manufacturers do not recommend discharging the battery fully. The advised DOD might vary depending on the type of battery, for lead-acid: 50% and for lithium: 80%.

Cycle Life

The cycle life is the number of times you could fully discharge and charge your battery until its rated capacity falls below 70%.

What Are The Different Kinds Of Solar Batteries?

The 2 main battery technologies available as solar batteries are:

  • The lead acid battery
  • The lithium-ion battery

They both share the same working principle. However, they differ in their active material.

One of the most important features you’d want to look at is energy density. This is the energy your battery can store per kilo (kWh/kg). The higher this value the better.

Both technologies are available in different energy storage capacities. The nominal voltage of a lead-acid battery is typically 12 V. On the other hand, for a lithium-ion battery, it can be 12 V, 24 V, or 48 V.

Let’s take a closer look at the different technologies and see what would be the best for a solar battery.

Lead Acid Battery

Our oldest rechargeable battery technology was developed more than 150 years ago by French scientist Gaston Planté. Do you know that the first car to reach 100 km/h was an electric car with lead-acid batteries built in 1899?

lead acid battery
Schematic representation of a lead-acid battery (wet type)

The energy density of a lead-acid battery is low, on average 35 Wh/Kg. This is mainly due to the heavyweight of lead electrodes.

Below is a quick look at the main components of a lead-acid battery:

Anode (-)Lead (Pb)
Cathode (+)Lead (Pb)
ElectrolyteSulfuric acid (H2SO4)
Membrane/SeparatorDepends on the technology (fiberglass mat, rubber…)

Deep Cycle Battery

Lead-acid batteries were primarily used to crank cars. They produce high current pulses and will not undergo deep discharge.

On the contrary, a solar battery requests a steady discharge current flow over time with a deeper depth of discharge (DOD).

To increase the depth of discharge deep cycle batteries use thicker lead electrodes and separators.

Deep cycle lead-acid batteries are divided into 2 groups:

  • Wet or flooded battery (refillable)
  • Valve regulated lead acid battery (VRLA)

Wet/Flooded Lead Acid Battery

Wet batteries have a liquid electrolyte that you need to monitor to ensure that the electrodes are fully immersed.

wet/flooded lead acid battery
A wet battery (flooded type) with distilled water for electrolyte leveling

They need to be refilled with distilled water and well ventilated as they could release hydrogen if overcharged.

Properly maintained wet batteries exhibit a slightly longer useful life compared to VRLA batteries. However, it is not recommended to purchase this type of battery.

Wet batteries are not user-friendly.

They require safety precautions during maintenance and operation and have a longer recharge time.

On the market, wet batteries are being replaced by the VRLA type.

Valve Regulated Lead Acid Battery (VRLA)

VRLA batteries are sealed and maintenance-free batteries.

They are available under the name of Gel and AGM batteries that highlights some of their technological features. They are available in two forms – AGM and Gel. Let’s take a look at some of their features:

AGM battery

Absorbent Glass Mat (AGM) is the material that composes the battery separator of an AGM battery. In this type of lead-acid battery, the electrolyte is absorbed on a fiberglass mat. Therefore, they are sealed and maintenance-free batteries.

A VRLA battery – AGM type. This battery is maintenance-free.

Are AGM Batteries Safe?

This type of battery is safe to operate and maintenance-free as they don’t need to be refilled. They are sealed to prevent any emission of gas.

How Long Do AGM Batteries Last?

The total life cycle of AGM batteries is strongly dependent on the depth of discharge (DOD). The AGM battery life could vary between 200 and 1’000 cycles.  Usually, the constructor recommends a 50% DOD, allowing a total of 500 to 700 cycles.

Are AGM Batteries A Good Choice As Solar Batteries?

AGM batteries are a good choice as solar batteries for low-budget systems. They could undergo deep discharge and are more durable than car batteries.

You might want to take a look at our comparison chart to choose the best technology that will fit your needs and your budget.

Gel Batteries

In this battery, silica is added to the sulfuric acid electrolyte to make it a gel.

Gel batteries are lead-acid batteries with a semi-solid (gel) electrolyte.

A VRLA battery – Gel type. Sealed and maintenance-free.

Are Gel Batteries Safe?

They are user-friendly and therefore safe to operate.

They have sealed batteries with no liquid electrolyte. Gel batteries are leakproof and will not emit toxic fumes while charging.

How Long Do Gel Batteries Last?

Gel batteries have a similar lifespan to AGM batteries.

Their total life cycle is greatly influenced by the depth of discharge. Constructors always recommend a 50% DOD to reach 700 cycles of service max.

Are Gel Batteries A Good Choice As Solar Batteries?

Looking for a low-budget technology but one that is still efficient?

Gel batteries are a good choice. However, be aware that they will perform well for 2 years, then drop greatly in performance thereafter.

Have a look at our comparison chart to see what technology is the most robust and cost-effective.

The Lithium Battery

Lithium-ion batteries are rechargeable. They were developed in the ’80s and made available in the early ’90s. Therefore, this green technology is still quite new and benefits from constant improvement.

The energy density of a commercial lithium-ion battery is on average 120 Wh/kg.

Below is a quick look at the nature of the main components of a lithium battery

Anode (-)Carbon
Cathode (+)Metal oxide
ElectrolyteLithium salt
MembranePorous to Lithium-Ion

Currently, there are 6 types of Lithium-ion batteries. However, we will focus only on the Lithium Iron Phosphate battery (LFP). This is the most widely available on the market.

solar batteries
Lithium Iron Phosphate (LFP) battery

Let’s have a look at the LFP batteries as solar batteries:

Lithium Iron Phosphate batteries are rechargeable lithium-ion type. Unlike lead-acid batteries which are one unique system, lithium batteries are composed of an assembly of smaller-sized cells of cylindrical shape.

The industrial standard is the 18650 lithium cell.

lithium battery cell
18650 Lithium battery cell – 18 mm *65 mm

The number of cells can be adjusted to reach the required voltage and energy capacity of the battery pack.

Are Lithium Iron Phosphate Batteries Safe?

Thanks to multiple safety components Lithium Iron Phosphate batteries are safe to use.

Lithium metal in its pure form is highly flammable when in contact with air or water.

Therefore, lithium batteries have multiple safety components. Each unique battery cell is carefully sealed in an air-tight cylindrical format.

The cell assembly that composes the lithium-ion battery pack is monitored by a Battery Management System (BMS).

The BMS ensures a balanced state of charge in every battery cell at all times. It also prevents overcharging and discharging.

How Long Do Lithium Iron Phosphate Batteries Last?

LFP batteries have a rated lifetime of 2,000 to 4,500 cycles (or 10 years) at 80% depth of discharge.

Why Are Lithium Iron Phosphate Batteries So Expensive?

This type of battery is more expensive at first than deep cycle lead-acid batteries because their technology is more recent.

The lithium battery is only 20 years old, compared to 150 years old for lead-acid batteries.

However, their market share is growing rapidly thanks to electric vehicles. Consequently, their price is dropping year after year.

Is There A Better Solar Battery Than Lithium?

Lithium batteries like the LFP (Lithium Iron Phosphate) are currently the best solar batteries available on the market.

I listed below some of their main advantages:

  • Light weight
  • High storage capacity
  • Tolerant to rapid and intermittent charging
  • Can supply high current load
  • Low self discharging rates
  • Full discharge is possible

They still have a higher initial cost, but in the end, they demonstrate robustness and reliability in the context of variable solar charging.

Furthermore, you will find them cheaper to use in the long term compared to lead-acid types.

How Much Do Solar Batteries Cost?

The cost of a battery varies depending on its capacity and technology. Therefore, we can give a price estimation per usable kWh:

We are taking into account their usable capacity.

For lead-acid batteries, manufacturers recommend discharging them at 50% of their maximum capacity and for lithium batteries 80%.

On average for the same available energy, a lithium battery is 2.5 times more expensive than a deep cycle lead-acid battery.

The good news is that the lifetime of a lithium battery is 3 times the one of a deep cycle.

In the end, over its lifetime (2000 cycles), your lithium battery will cost less than a deep cycle battery.

 Cost per usable capacityOverall lifetime cost (2000 cycles)
Lithium Iron Phosphate battery (80% DOD)565 USD/kWh565 USD/kWh
Deep cycle battery (Gel, AGM) (50% DOD)367 USD/kWh1065 USD/kWh

What Kind Of Battery Is Best For A Solar System?

Lithium iron phosphate batteries are far more fitted for solar charging than deep cycle lead-acid batteries (Flooded/Wet type, AGM, Gel).

The main reason is that lead-acid batteries require a much longer charging time, up to 16 hours. Most of the time, this is not achievable with solar energy.

For example, lots of tropical countries have only 12 hours of sun per day.

Conversely, Lithium batteries are tolerant to fast and variable charging. For example, at noon when the solar panels are at their peak production, your lithium battery will easily absorb high current in a fast-charging mode. Whereas the deep cycle lead-acid battery will be limited by its technology.

In terms of pricing, you will find lithium batteries more expensive at first. But look over their whole lifetime, they will be cost-effective, at least 20% cheaper than the lead-acid types.

You can take a look at our comparative chart below that summarizes the main features of each technology:

Lithium iron phosphateWet typeAGMGel
Energy density (Wh/Kg)120 Wh/Kg45 Wh/Kg35 Wh/Kg35 Wh/Kg
Average life cycle 80% DOD2000-4000250-350200-300200-300
Cost per usable kWh648 USD500 USD555 USD555 USD
Self-discharging5% 10% 5% 5%
Rapid charging2 h 8-16h 8-16h 8-16h

Note that if your battery has reached its rated life cycle, you can still use it, but its capacity will decline further down to zero.

It will take more time for a lithium battery to reach zero capacity compared to lead-acid batteries.

Is It Worth Getting Solar Batteries?

Get a solar battery to store your solar panel production during the day and use it any time!

Our sun is a natural source of energy, therefore it is variable. Its production output varies during the day and in the case of climatic events.

The peak production of the sun is at 12 noon, however, our peak electricity consumption occurs in the morning around 7 am and in the evening at 8 pm.

Our consumption is therefore disconnected from the production time. You now see the necessity to store solar energy to fully exploit the potential of your system.

If you travel in a camper van equipped with solar panels, you will certainly need electricity for the light in the evening. Consequently, you will store the electricity produced by your monocrystalline solar panel during the day to release it at night.

solar panels
Street lighting is one of the best applications of solar batteries

How Much Is A Solar Battery For A House?

A 2 kWh battery with a 2kW power peak for domestic use costs around 1,500 USD. Above all, it comes with a 10-year warranty (or 4,500 cycles) at 70% of the rated capacity.

This storage capacity will be enough to run the whole domestic appliance for 1 day. You can associate multiple batteries to increase the overall capacity of your system.

You will need to purchase an inverter to convert DC current into AC used by your appliance.

Over the last ten years, the cost of lithium batteries for domestic use dramatically dropped, meaning that the technology is now accessible to all and is replacing the fuel generator in off-grid systems.

Can You Use Car Batteries For Solar Power?

If you are on a limited budget you can use your car batteries as solar batteries. Even second-hand car batteries will still operate under low power load (lights).

A 12 V car battery

Car batteries are 12V lead-acid batteries that can supply high currents for a limited time.

Coupling solar panels and car batteries will work. However, your system will not be optimized as car batteries are not designed for deep discharge or intermittent charging. In addition, the life duration of your battery will be low.

How much money can you make with a solar battery?

In 2022, combine net metering and electricity surplus compensation to make money from your solar battery.

With the global energy price surge, you could save and earn thousands of dollars with your solar battery coupled to solar panels. Let’s see how with a system installed in Los Angeles, California.

Electricity bills are always paid regarding the number of kWh used in a month. In Los Angeles, as of April 2022, you will pay on average 0.3$/kWh (depending on the rate plan).

So, if your system coupling solar battery and solar panel has a lower cost per kWh, you earn!

Let’s first calculate the LCOE (Levelized Cost Of Energy) of your system. LCOE is expressed in USD/kWh and takes into account the global production of your system during its lifetime: 25 years for the solar panels and 10 years for the batteries and electrical systems.

For 5kw solar panel, the LCOE is 0.04USD/kWh (average price inc. installation and tax rebates in California)

LCOE Solar battery (Alpha ESS Smile 5): 0.16USD/kWh

Total estimate LCOE: 0.2USD/kWh

In the end, your solar battery system will cost less than 0.3USD/kWh, therefore it is already competitive with utility prices in California.

Net Metering & Net Surplus Compensation Program

Follow the two steps below to reduce your electricity bill and earn with solar battery:

Step One:  

Go for net-metering. With this scheme, every kWh that you supply to the utility company is credited to your account. You can then use utility electricity for free until your account is back to zero.

You don’t earn money with net-metering rather you save money (a lot!).

Step Two:

To earn money, go for the Net Surplus Compensation program. PGE (Pacific Gas and Electricity Company) can pay your surplus at 0.047 USD/kWh (rate of May 2022).

For example, if your net metering account is already credited with worth 3-4 days of your domestic consumption, you can opt to sell the rest of your electricity. Therefore, your solar battery becomes profitable.

Can you build your own solar battery – DIY ?

With the advent of Lithium batteries, even the non-specialist can build his own solar battery. A lithium Solar battery is made of few components:

  • Lithium cell modules
  • Battery Management System (BMS)
  • Battery balancer
  • Wires and connectors

All those parts can be purchased online individually. The advantages are:

  • Cheaper than a ready-made solar battery
  • Can customize each part
  • Select better quality components

As an example, a DIY solar battery with 3kWh capacity will cost you:

484 USD for the battery cell + 134 USD BMS + 35 USD balancer + 35 USD electrical connectors = Total 690 USD

For the same capacity that’s around 30% cheaper than buying a ready-made low-cost lithium battery from Amazon.

Five Of The Best Solar Batteries On The Market

When looking for a solar battery you might want to focus on 4 main parameters:

  • Usable battery capacity or depth of discharge
  • Durability (the number of cycles)
  • Adaptability to solar energy charging
  • Price

We have selected for you the 5 best solar batteries on the market:

Best Solar Battery For Heavy Domestic Use

Tesla Powerwall

The Tesla Power wall

  • Huge 13.5 kWh storage capacity
  • Plug-and-play design with a built-in inverter to deliver Alternative Current (AC)
  • Enough energy for 3 days of total autonomy
  • Longest warranty: 10 years at 70% initial capacity
  • Lowest cost per kWh over its lifetime

Best GEL Battery For On-Board And Domestic Use

solar battery
Renogy Gel Battery 12 V, 100 Ah

Renogy Gel battery Deep cycle, 12 V, 100 Ah

  • 0.6 kWh of usable capacity
  • Maintenance free
  • Can supply high discharge current
  • Has the lowest self discharging rate (3% month)
  • Can operate at temperature below zero degrees

Best AGM Battery For On-Board And Domestic Use

Renogy AGM battery, 12 V, 100 Ah

Renogy AGM battery Deep cycle, 12 V, 100 Ah

  • 0.6 kWh of usable capacity
  • Maintenance free
  • Extended life duration: 750 cycles at 50% DOD
  • Lowest self discharging rate (3% months)
  • Safe to operate

Best Lithium Iron Phosphate Battery For Domestic Use

Renogy 48 V, 50 Ah high durability Lithium battery

Renogy Smart Lithium Iron Phosphate Battery, 48V, 50Ah

  • 2 kWh of usable energy
  • Extreme life duration: 4’500 cycles or 10 years
  • Support fast charging/discharging up to 50A
  • High tolerance to intermittent solar charging
  • Associate multiple batteries to increase the capacity of your system

Best Lithium Iron Phosphate Battery For On-Board Use

Renogy LFP battery, 12 V, 50 Ah

Renogy Lithium Iron Phosphate Battery, 12 V, 50 Ah

  • 0.8 kWh of usable energy
  • Long life duration: 2’000 cycles
  • Support fast charging/discharging up to 50 A
  • High tolerance to intermittent solar charging
  • Associate multiple batteries to increase the capacity of your system
  • Light weight for in-board use

Final Thoughts

There is a solar battery for all needs and every budget. The AGM and Gel lead-acid batteries are well adapted to low-budget and small-capacity systems.

On the other hand, lithium iron phosphate batteries are the best when it comes to storing solar energy. They are perfect for domestic and in-board use (boat, RV…).

Romain Metaye, PhD

Romain Metaye, PhD

Dr Metaye has a Ph.D. in chemistry from Ecole Polytechnique, France. He is a renewable energy expert with more than 11 years of experience within the research world. During his career, he supervised more than 150 projects on clean energy. Off-grid smart systems, solar energy, battery and the hydrogen economy are among his specialties.

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