The science behind solar PV battery storage

Solar battery storage systems have exploded onto the market and is being touted as the saviour of renewable energy by making solar PV more ‘convenient’, enabling the consumer to maximise the self-consumption of their generated clean energy.

Although battery technology has been in existence for a while in both domestic and commercial buildings, it has predominantly been for off-grid properties and back up purposes. Now, domestic energy storage has gained momentum, especially since the recent cuts in the Government’s Feed-In Tariff. Consumers are now looking for a holistic approach to energy security and effective protection against energy price hikes.

Solar batteries work by storing excess PV generated energy as DC power for later use. In some cases, solar batteries have their own inverter and offer integrated energy conversion. The higher your battery’s capacity, the larger the solar system it can charge.

When you install battery storage as part of your solar pv system, you are able to store excess solar electricity at your home instead of sending it back to the grid. If your pv system is producing more electricity than you need, the excess energy charges the battery for use when your solar panels aren’t producing electricity. You will still send electricity back to the grid but only when your battery is fully charged, and you will still need to draw electricity from the grid when your battery is empty and you are not generating solar energy.

Batteries come in various shapes and sizes, have different attributes and have varying price points! Please read on to discover more about battery storage technology.

Grid tied battery storage

Grid tied batteries are the most common and the cheapest available currently. The battery’s management system requires power from the grid to operate, therefore, in times of power cuts, the battery system also disconnects which means you won’t have power either. Whether this is an issue for you or not will depend on whether your home suffers a lot with power cuts, the typical duration of a power cut and what the impact these power cuts have on your life.
Capacity and power
Capacity is the total amount of electricity that a solar battery can store, measured in kilowatt-hours (kWh). Most home solar batteries are designed to be “stackable,” which means that you can link multiple batteries to get extra capacity.

While capacity tells you how big your battery is, it doesn’t tell you how much electricity a battery can provide at a given moment. To get the full picture, you also need to consider the battery’s power rating which is the amount of electricity that a battery can deliver at one time, measured in kilowatts (kW).

A battery with a high capacity and a low power rating would deliver a low amount of electricity (enough to run a few crucial appliances) for a long time. A battery with low capacity and a high power rating could run your entire home, but only for a few hours.

Depth of discharge
Depending on the chemical composition, some batteries need to retain some charge at all times to prolong its lifespan.

The depth of discharge (DoD) of a battery refers to the amount of a battery’s capacity that can be used. Most manufacturers will specify a maximum DoD for optimal performance. For example, if a 10kWh battery has a DoD of 80 percent, you shouldn’t use more than 8kWh of the battery before recharging it. Luckily, this will be automated for most batteries, so there is no requirement for the homeowner to continually monitor the level of charge.

Round-trip efficiency
Battery storage systems use energy to store energy, and some batteries suffer from power leakage, especially when energy is stored over long periods of time.

A battery’s round-trip efficiency represents the amount of energy that can be used as a percentage of the amount of energy that it took to store it. For example, if you feed ten kWh of electricity into your battery and can only get nine kWh of useful electricity back, the battery has 90 percent round-trip efficiency (9/10 = 90%). Generally speaking, a higher round-trip efficiency means you will get more value from your battery.

Battery life & warranty
Solar batteries are like the battery in your mobile phone – you charge your phone most evenings to use it during the day, and as your phone gets older you will start to notice that the battery isn’t lasting as long as it did when it was new.

For most uses of home battery storage, your battery will “cycle” (charge and drain) daily. The battery’s ability to hold its charge will slowly decrease the more you use it.

Your solar battery will come with a warranty that guarantees a certain number of cycles and/or years of useful life, and because battery performance naturally degrades over time, most manufacturers will also guarantee that the battery keeps a certain amount of its capacity over the course of the warranty.

For example, a battery might be warrantied for 6,000 cycles, or, 10 years at 80 percent of its original capacity. This means that at the end of the warranty, the battery will have lost no more than 20 percent of its original storage capacity.

Many different types of organisation are developing and manufacturing solar battery products, from car companies to university spin-outs.

Whether you choose a battery manufactured by a cutting-edge startup or a manufacturer with a long history depends on your priorities. Evaluating the warranties associated with each product can give you additional guidance as you make your decision.

Grid tied back-up storage

This type of system is again grid tied but the batteries can also provide emergency power back-up in times of power cuts. The back-up version is more expensive because of the requirement to install specialist gateway technology that regulates and manages the provision of emergency power, and in most cases currently, the rewiring and allocation of an emergency ‘circuit’ to isolate the appliances that essentially need to run in the event of a power outage. So, if this significant expense is worth it to you then this may be a viable option.

Off-grid battery storage

Off-grid systems have no connection to the grid at all, usually as a result of living in a remote area or a self-build with the decision to live without mains electricity. Solar power is either used directly, or stored in batteries. Because more power is needed around the winter time, and this is also the period with the shortest days, most people use a back up generator during this period to assist the solar generation.

Not all batteries are created equal

Types of battery

Batteries used in home energy storage typically are made with one of three chemical compositions: lead acid, lithium ion, and saltwater. In most cases, lithium ion batteries are the best option for a solar panel system, though other battery types can be more affordable.

Lead acid

Lead acid batteries are a tested technology that has been used in off-grid energy systems for decades. While they have a relatively short life and lower DoD than other battery types, they are also one of the least expensive options currently on the market in the home energy storage sector. For homeowners who want to go off the grid and need to install lots of energy storage, lead acid can be a good option.

Lithium ion

The majority of new home energy storage technologies, such as the Tesla Powerwall, use some form of lithium ion chemical composition. Lithium ion batteries are lighter and more compact than lead acid batteries. They also have a higher DoD and longer lifespan when compared to lead acid batteries. However, lithium ion batteries are more expensive than their lead acid counterparts.


A relative newcomer in home energy storage is the saltwater battery, for example Aquion. Unlike other home energy storage options, saltwater batteries don’t contain heavy metals, relying instead on saltwater electrolytes. While batteries that use heavy metals, including lead acid and lithium ion batteries, need to be disposed of with special processes, a saltwater battery can be easily recycled.

So whats the difference?

AC vs DC Coupled

Until a few years ago, most solar battery storage systems were DC coupled, meaning that the battery would be charged with the DC power provided directly from solar panels.

Over recent years, battery and inverter technology has moved on significantly and AC coupled battery storage systems are now the norm, allowing home owners to retro-fit battery storage to their existing solar, without the need to change their inverter technology.

DC Coupled

A battery can only store DC energy, and solar panels produce DC energy, therefore it makes sense that the rest of the storage technology should also be in DC to directly charge the battery without any losing any energy through any AC conversion process. This creates a higher system efficiency (less losses), and only requires the one inverter that will do the conversion when the energy is required either from the array or from the battery. But this also means that there can be a single point of failure.

Besides reducing the initial energy losses, a DC coupled battery storage system does not restrict the amount of power charging the battery as it does not get restricted by the capacity of an inverter.

DC coupled solutions also work well with back-up storage systems as the modules can still directly charge the battery even when there is a power cut.

AC Coupled

For AC-coupled systems, the DC power generated by your solar panels is converted to AC by your inverter, to be used by appliances in your home via a consumer board. Any excess energy is used by a bi-directional battery-based inverter that converts the AC to DC to charge the batteries, and reverses this process when energy from the battery is required.

The primary advantage of AC coupling over the traditional system design is that you can add battery backup to an existing grid-tied PV system without changing the existing system’s wiring.

We have done the research so that you don’t have to

What we recommend

The Tesla Powerwall 2 is the most well-known battery currently available, and it’s also currently the best value. The 13.5kWh unit costs roughly £7,000 – fully fitted including VAT, and the known branding and style makes it an appealing and sought-after system.

Why choose the Tesla Powerwall 2?

The Powerwall 2 has a much larger capacity than its other main competitors.
It is AC coupled, which means you won’t have to install a separate charge controller.
A Strong 10-year performance warranty from a reliable brand, ensuring 80% retention after 10 years.
It can be installed both inside and outside, and mounted on the wall or floor.
Its sleek and neat design means that a battery needed be an eyesore.
The Power Flow app allows you to monitor the Powerwall 2.
Capacity can be increased to 10 Powerwall 2’s in one system.
Over the air update technology means that you will never have to worry about the battery keeping up to date with software updates.
Coming soon is the ability to charge the Tesla from the National Grid to make use of time-of-use tariffs.
Back-up gateway technology coming in 2018.

Usable Capacity


Depth of Discharge



90% Round-trip


7kW peak / 5kW continuous

Supported Applications

Solar self-consumption
Time of use load shifting
Off grid


10 Years


Up to 9 Powerwalls

Operating Temperature

-4° to 122°F / -20° C to 50° C


L x W x D: 44″ x 29″ x 5.5″
(1150mm x 755 x 155mm)


264.4lb / 110kg


Floor or wall mounted
Indoor or outdoor


UL and IEC certified
Grid code complaint

Usable Capacity


Depth of Discharge



92% estimated round trip


2kW peak / 5kW continuous


5 Years



Operating Temperature

0°c – 50° C


L x W x D:
(440mm x 410 x 89mm)




Stacked (floor mounted)


Tuv/CE/UN 38.3/TLC

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0800 856 2200

0800 856 2200