Solar PV Battery Storage / Backup

How do solar storage batteries work diagram

How do solar batteries work?

During sunlight hours a Solar PV Battery system will be generating electricity which is used by the property, thereby reducing the electricity imported form the grid, lowering bills.  

However, the light levels during a day will vary, meaning the amount of electricity generated will be variable, or non existent at night. In addition energy usage during the day will vary depending on whether the home is occupied or what appliances are being used. This means that the solar electricity may either be to little or too much to satisfy demand during the day and not able to at all during the evening.

A Solar battery charges when the solar panels are producing more than the house needs and discharge the stored free electricity when the panels are producing less than the house needs. 

They are a temporary store of electricity which as little as possible is exported to the grid during the day and allows the property to utilise the free electricity at night. Backup batteries for solar power are discussed later.

Solaredge storedge tesla powerwall solar battery stoarge usage

How much of a difference will the battery make?

Typical usage of PV energy generated is 40-50%, but with battery storage, usage can become 85% or greater. A well proportioned battery and solar PV system will effectively make a property "off grid" for the majority of the time. 

Battery storage systems have practically no effect on Feed in Tariff incomes as they only act as a temporary store of electricity to be used and metered outside of generation periods. Additionally, as the exported electricity is not metered, but calculated at 50% of generation, this income will remain unaffected.

In addition, AC coupled battery solutions such as the Tesla Powerwall 2, can be charged from the grid. Meaning during the winter when solar generation is low the battery can be topped up at night from economy 7. The property will mainly be using free or low cost electricity the vast majority of the time.

Tesla powerwall 2 solar storage and backup battery performance specifications

Technical Terms

There are several battery systems on the market so it can be difficult to differentiate between them to identify the most suitable product. The below descriptions will help explain what some of the terminology means.

Watts and kWh: A watt is a expression of energy the high the wattage the greater the energy being used. A kilowatt hour kWh is an expression of the amount of energy being used over time. One kWh is 1000w of energy being used constantly for one hour. One kWh is often described as a "unit" of electricity by suppliers.

Charge/Discharge Capacity: This is the amount of energy in watts the battery can charge or discharge at any one time, the higher the wattage the more effective the battery will be at meeting demand.

Storage Capacity (Depth of Discharge): The storage capacity of the battery is often measured in kWh and represents the amount of electricity which can be stored, the higher the value the more effective the battery, with one important caveat!, batteries cannot empty of electricity entirely so what can be stored can not be accessed fully, this is called Depth of Discharge (DOD) Lead acid batteries can only supply 50% of their stored energy, Lithium ion is around 80-90%. 

Charge Cycles: One charge cycle is a battery from fully charged to discharged to fully charged again. The lifespan or a battery is often represented in charge cycles, and a battery will often complete one cycle in a day.

Back-Up: This is the ability of a battery to continue providing power to the property in the event of a power cut. Despite misconceptions, this is not a standard solar battery feature and most systems will shut down in the event of a power cut for safety reasons. It is possible to have a back-up facility with certain products such as the Tesla Powerwall 2, so long as specific requirements laid down by the network operators are satisfied.

Solar Battery Manufacturers

Tesla - Powerwall 2

tesla powerwall 2 ac coupled solar storage battery with backup

Powerwall is Tesla’s revolutionary home battery that stores the sun’s energy and intelligently delivers this clean, reliable electricity when the sun isn’t shining. Combine Powerwall with solar to increase your home’s independence, reduce electricity bills and create a sustainable lifestyle. More:

LG Chem Resu

LG Chem DC coupled solar pv storage battery

LG Chem are a leading global manufacturer of Lithium-ion batteries providing high quality battery storage solutions for self consumption.The RESU Lithium Battery is a compact and lightweight solution. It is designed to allow easy wall-mounted or floor-standing installation for both indoor and outdoor applications. More: