Are Solar Batteries Worth It?

A study by Which? revealed that eight in ten solar panel owners owned or would consider purchasing solar batteries. Fuelled by the limitations on feeding excess electricity back to the grid (Feed-In Tarriff) from your solar pv system, solar batteries have grown increasingly popular. Homeowners are now storing their excess electricity production in home energy storage batteries for when they require it, rather than relying on the grid when the solar system isn’t producing sufficient electricity.

What is a solar battery?

Installation of a solar battery allows for storage of excess energy from your solar pv system. This enables homeowners to utilise this stored energy for when you require it, such as when the sun isn’t shining, instead of it otherwise going to waste.

Solar batteries can power any device connected to the mains in your home. Just as the energy from your solar pv system powers your appliances, a solar battery will power appliances on-demand in your house.

Typically, solar pv panels are most efficient during the middle of the day, coincidently when a house is usually empty and energy consumption is at its lowest. With a home energy battery, instead of this energy going to waste, the excess energy is stored in a solar home energy battery of your choosing.

How much does it cost to install solar batteries?

The cost of solar batteries varies depending on several factors, such as the size and brand of the battery. Calculating the cost that a solar battery can save you can be difficult as various factors need to factor into the calculation, including:

  • Cost of the battery & installation

The cost of the battery and installation is an important factor as this will determine how long the payback period will be.

  • Storage capacity of the battery

If the home energy battery has a higher storage capacity, the larger the amount of potential energy that can be stored and therefore saved from going to waste.

  • Excess electricity generation

Amount of excess energy your solar pv would typically generate that would go to waste without a home energy battery.

  • When that excess energy will be used

If used during the evening, you'd typically be paying for higher peak rates for grid electricity.

Types of solar battery

There are several types of solar batteries available, and depending on several factors such as budget, it may help determine which type of solar battery is right for you. The four main types of solar batteries used in residential solar systems are lead-acid, lithium-ion, nickel-based or flow batteries.

Lead Acid Battery

Lead-acid batteries are typically a cheaper alternative for those looking to installer a home solar battery. Lead-acid batteries have been around for centuries and have been utilised in countless home solar systems. The common traits for lead-acid batteries include:

  • Cheaper energy storage solution
  • High reliability
  • Shorter lifespan

These properties mean that while lead-acid batteries aren’t the most common in modern installations, they are often a good alternative for those seeking a cheaper energy storage solution. They are also a very reliable option, having been used for centuries. They’re slightly more limited in terms of their storage location, with the battery having to be installed upright. A lower depth of charge also means that they have a slightly shorter lifespan than other batteries, averaging between 5-10 years.

 

Lithium-Ion Battery

The most popular type of home solar battery is a lithium-ion battery, found in Tesla’s Powerwall battery & the LG Chem RESU lithium-ion battery, amongst others. These batteries have the common properties of a lithium-ion battery, including:

  • Higher battery energy density
  • Long lifespan
  • Higher depth of discharge

Their properties mean that the lithium-ion batteries can be stored in a smaller space while holding a larger amount of power. They also require no regular maintenance, so once installed it can be left. This also means that it can be installed in areas that aren’t easily accessible as they don’t need to be accessed for regular maintenance. Also, with lithium-ion batteries having a higher depth of charge, more of the energy is stored within the battery before it has to be recharged.

 

Nickel Cadmium Battery

Nickel-cadmium batteries aren’t the most popular type of battery used in home solar batteries, falling behind lead-acid and lithium-ion batteries. The main benefit of these types of batteries is their durability and ability to operate at higher temperatures. Nickel-cadmium batteries are also extremely toxic, so they’re banned in several countries.

 

Flow Batteries

Flow batteries are a relatively new technology however possess several characterises that may see their popularity grow in the coming years. At the moment, flow batteries have several limitations that currently reduce their popularity, including:

  • More expensive
  • Low charge and discharge rates when compared to other composites of battery
  • They’re typically larger in size to help offset their lower charge/discharge rates

Despite this, the technology has several benefits, including:

  • 100% depth of discharge without impacting the battery health
  • Longest lifespan of any alternative battery of up to 30 years
  • Fire retardant, so no risk of thermal runaway as you have with other batteries

Consideration for a solar battery

When considering a solar battery there are several variables that can determine whether the investment is worthwhile. These include:

  • Price

Ensuring that you pay the right price is important, as this will determine the payback period from the further utilisation of your solar pv that is stored in the solar battery. This, coupled with the lifespan of the battery, will determine whether a solar battery is a worthwhile financial investment.

  • Capacity

Ensuring that the capacity of your solar battery is adequate to capture the energy that would otherwise go to waste is important. Higher capacity generally means a higher cost, so ensuring that the correct capacity battery is installed for your requirements is important.

  • Voltage

The voltage of the solar battery should be taken into consideration when deciding upon a suitable battery. If the voltage of the solar battery falls below that of the required levels for running a household or office, power from the grid will also be required to make up the shortfall.

  • Cycle Life

The cycle life of a battery determines how many times a battery can be charged and discharged before the capacity drops below a certain level. The shorter the cycle life, the less efficient the battery becomes overtime at capturing the excess solar pv production as the capacity of the battery will be lower.

Retrofitting Solar Batteries

In most cases, installing solar batteries at the same time as the installation of your solar pv panels is the most efficient, especially when considering cost-saving. When retrofitting solar panels, AC and DC coupling need to be considered and the impact of these set-ups.

 

To understand the process, understanding the currents used in various parts of the household is important:

Solar PV Current – DC Current (Direct Current)

Household Appliances – AC Current (Alternating Current)

Battery Storage – DC Current (Direct Current)

 

When power from the battery is required to power a household appliance, the AC charge in the battery is converted to a DC current for use in your home or business. For customers that already have PV panels fitted with a DC current, an AC coupling that incorporates either a hybrid or battery inverter is preferred as it minimises the rewiring that is required both on and off the roof.

AC coupled systems require two inverters, one located between the panels and consumer unit & another between the consumer unit and battery unit. Due to the power passing through multiple inverters, AC coupled systems are less efficient than DC-coupled systems.