What are solar batteries. An illustration of a solar panel connected to a solar battery mounted on a wall.

Image credit: Adobe

When you receive a quote from a solar panel company, you’ll probably be asked whether you want to include solar batteries in your system – but what are solar batteries? Our article gives you the lowdown on the different types of solar battery available, compares the costs of the leading products on the market and answers the question of whether installing a solar battery is a good investment.

What are solar batteries and how do they work?

A solar battery is a component of your solar photovoltaic (PV) system that stores the excess power you generate. Most of the time, you’ll only use some of the electricity your solar panels produce during the day. Without a solar battery, the unused electricity would either be wasted or exported to the National Grid (if you’re connected). Solar batteries allow you to store this excess electricity so you can use it when the solar panels aren’t operating – usually during the night or in very low sunlight conditions.

Solar batteries are recommended for homeowners planning to live off-grid, as they won’t be able to rely on importing electricity from the National Grid. With a solar battery, off-grid homeowners can power their homes at night, since the excess power generated by their solar panels will have been saved.

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What is depth of discharge (DoD) and why is it important?

Depth of discharge (DoD) is the percentage of a battery’s capacity that can be used on a regular basis relative to its overall capacity. For example, if you have a solar battery that holds 5kWh of electricity and you only discharge, or use, 4kWh, the DoD is 80%. 

It’s important to note that DoD can affect a solar battery’s lifespan, or cyclic life. This is the number of charge and discharge cycles the battery can complete. Battery manufacturers will provide the cyclic life and the rated DoD for each battery. For example, a supplier might state that their solar battery can perform 1,500 cycles at 90% DoD. If a battery is discharged over the DoD, its cyclic life reduces.

What are the types of solar batteries available?

Solar batteries vary in terms of chemical composition and technology. The numerous options available also come at different price points. So it’s vital to research what type of battery fits your budget and whether it will meet your requirements. Provided below is a list of common solar battery types available for residential solar panel systems.

Lead-acid batteries

Lead-acid batteries are the most common solar batteries available and are also one of the cheapest options on the market. As a proven energy-storing technology for over 100 years, they provide high reliability. 

However, due to their lower DoD, lead-acid solar batteries have a shorter lifespan than other options available – on average, five to 10 years. For this reason, most modern installations don’t use lead-acid batteries. However, their cost-effectiveness and high reliability make them a good choice for budget-conscious homeowners.

Lithium-ion batteries

Lithium-ion batteries have become one of the most popular battery types for residential solar PV systems. These batteries offer higher energy density, extended lifespans and a higher DoD. The higher DoD allows for more electricity to be stored and used per cycle before the battery needs to be recharged. 

Lithium-ion batteries also require a smaller space and no regular maintenance, making them suitable even for inconvenient locations such as restricted loft spaces or utility sheds.

Nickel-cadmium batteries

Nickel-cadmium batteries offer high durability and the ability to function in higher temperatures. However, they’re the least popular battery type due to their highly toxic nature, which has resulted in them being banned by several countries. 

Flow batteries

Flow batteries are relatively new, but they have various characteristics that may increase their appeal in the coming years. These batteries offer 100% DoD, a long lifespan of up to 30 years and are fire retardant. However, flow batteries are more expensive. They are also larger than other batteries, to compensate for their lower charge and discharge rates.

How long does a solar battery last?

Solar batteries generally last anywhere between five to 15 years, while solar panels have a lifespan of 20 to 30 years. You’ll therefore need to replace your solar batteries at least once during the lifetime of your solar panel system. 

The lifetime of a solar battery depends on its cyclic life. The size of your battery and your energy usage will be relevant here. For example, suppose your home needs 13kWh to function for a day, and you have a 16kWh solar battery with a 90% DoD and cyclic life of 5,000. Effectively you can use 14.4kWh from your solar battery before it needs to be recharged. Hence, you are looking at a lifespan of around 15 years.

If you use only 10kWh on a particular day, the remaining 4.4kWh of power can be used another day to power your home, effectively increasing the lifespan of your solar battery. Once the solar battery reaches its DoD limit, the charge cycle automatically starts.

It’s important to consider the cost of replacing your solar batteries when deciding whether you want to invest in a solar panel system or whether you want to add solar batteries to your existing solar system.

How much does a solar battery cost?

Solar batteries are not cheap, with costs ranging from £1,700 to over £12,000. The significant variations are due to factors such as the brand, battery capacity, battery type and cyclic life. Another factor affecting cost is whether you want to add solar batteries to an existing solar PV system. If you wish to do so, it could cost you between £500 to £1,200 if you need to change your system’s inverter to one compatible with solar batteries.

We’ve provided the table below to give you a better idea of the brands available and their prices. Some of the prices given are for the battery only (excluding the inverter), while others are for a complete power storage system (inverter included). Also, note that these prices are estimates. They also do not include installation costs.

SupplierCapacityWarrantied cyclesGuarantee periodPrice range
Tesla Powerwall 2.0 (battery + inverter)13.5kWhn/a10 years£9,800–£12,900
Sonnen Batterie Eco (battery + inverter)4kWh–16kWh10,00010 years£4,500–£10,000
Samsung SDI (battery + inverter)3.24kWh6,0005 years for product, 10 years for performance£5,000–£5,500
BYD B-BOX (battery only)2.5kWh–10kWh6,0005 years£1,700–£4,450
LG Chem Resu (battery only)3.3kWh–13.1kWh6,00010 years£2,200–£5,600

Are solar batteries worth it?

Are solar panels worth it? A solar battery installed on the wall of a house connected to an inverter and the consumer unit.

Solar batteries can further reduce your dependence on grid electricity, but they will add to the cost of your solar panels installation. (Image credit: Adobe)

The debate on whether solar batteries are a worthwhile investment is a hot topic, with many experts and homeowners split in their sentiments. Let’s examine this further and try to answer the question. First, if you’re a homeowner looking for a complete off-grid power solution with solar panels, solar batteries are essential. Without solar batteries, your home will not receive power at night as the solar panels will not produce electricity at those times.

For homeowners looking at a hybrid solution of receiving electricity from both the National Grid and solar panels, there are a few factors that will affect whether solar batteries are a good investment. These include your household’s power consumption, the cost of the solar batteries and, more importantly, the energy tariffs for importing and exporting from the National Grid. Another factor to consider is the cost of replacing solar batteries: with their lifespan of five to 15 years, they have roughly half the lifespan of solar panels.

Let’s assume you have a three-bedroom home in London with a 4kW solar PV system and that you’re out each day till 6.00pm. According to the Energy Saving Trust calculator, your solar panels will generate 3,648kWh of electricity per year. Based on Ofgem statistics, your three-bedroom household uses approximately 2,900kWh of electricity per year. Given this information, your solar system will produce an excess of 748kWh of electricity. 

Suppose you export that excess electricity to the National Grid under the government-backed Smart Export Guarantee Scheme (SEG) at a rate of 15p per kWh. In that case, you can earn approximately £112 per year. However, if you store that energy via solar batteries and use it later, you could save £254 per year. This is because you won’t be using the national electricity grid and getting charged 34p per kWh, based on the current electricity rate from October 2022 to April 2023.

Bear in mind, though, that a solar battery system, including a replacement battery set and installation, will cost you around £5,000 to £12,000. Such an investment will result in a net loss of £750 to £5,750 after 25 years, the average solar panel system’s average lifespan. Hence, this reveals that solar batteries are not a good investment for this scenario just yet.

The above calculations are estimates based on assumptions about typical energy usage and tariff rates. We recommend contacting your installer and electricity provider to get a clearer idea of your potential savings using solar batteries.

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