News & Insights
SSWUK Battery Review 2021

Energy Storage Systems (ESS) have fallen significantly in cost, and can greatly increase your self-consumption of solar energy. Rising electricity prices have meant solar batteries for domestic households have become more and more popular – but which battery is the best right now?

Unfortunately, the question of “which battery is best?” is not necessarily that simple. Whilst the Tesla Powerwall 2 may offer good value storage capacity in terms of £/kWh – in order to be cost-effective, it is imperative that you select the battery which best matches your individual system and requirements.

Below, we have outlined what we regard as the positives and negatives of a few of the most accomplished domestic battery systems currently available in the UK.


An important factor to consider is the size of the battery you choose. Too large a capacity, you might install more storage than you can fill. Too small, you might end up needing to install further capacity (at a higher price than installing a larger battery initially). The size of the battery that you require will largely depend on two factors, the size of your PV system and your electrical usage. Larger systems can store more energy but have an increased cost associated with them thus sizing your system correctly is vital.

It is not just size that matters either. There are now an abundance of different battery manufactures and technologies available. As well as this, it is important that you take into account manufacturer warranty, £/kWh and in some cases the aesthetics of the model. Selecting the appropriate solar battery requires careful consideration, while we have tried to be objective, we realise that we are not comparing apples with apples in most circumstances. This confuses things slightly, however, it also aptly reflects the dilemma the customer faces when trying to decide!

AC or DC Coupled Battery

What is the difference and why does it matter

Alternating current (AC) and Direct current (DC) are the two ways in which electrical power is transmitted, AC current is the current found in your home socket and throughout the national grid. DC current is what is produced by PV panels.

The battery can either have a built in inverter or use a hybrid inverter and is dependant on where the battery is located within the system. An AC coupled battery is charged through the distribution board where as a DC coupled battery is charged directly by the solar panels through the hybrid inverter. One key difference in the functionality of each battery is that in a power outage a DC coupled battery will typically not be able to supply power to your home, however it is possible with an AC coupled battery and a backup gateway.

DC Coupled Batteries

A DC coupled battery requires the installation of a hybrid inverter. The inverter will supply energy to the home and where excess generation occurs, power is supplied to the battery, enabling the energy to be used later when solar generation reduces. DC coupled battery manufactures include: LG, BYD and Solax.

  • A DC coupled system is typically more efficient than an AC coupled system due to the fact that the electricity is only converted from DC-AC once.
  • Allows centralised monitoring of the system, as PV and battery power flow through a single component.
  • Hybrid inverters typically cost more than their standard counterparts.
  • The battery is unable to supply backup power to the property in the event of a power outage due to G99 regulations.
  • In the event that the inverter breaks the battery will be unusable, until the problem is resolved.
  • In the future, electricity trading/grid balancing services are set to be more common, but this is not currently possible with a DC coupled battery.
AC Coupled Battery

An AC coupled battery is installed into the distribution board and comprises of a battery and a battery Inverter/charger. Unlike a DC coupled system the battery can be charged either from the solar inverter or directly from the grid. AC coupled battery manufactures include: Tesla, Sonnen and Emphase

  • Easier and cheaper to install for existing solar PV systems.
  • The ability to discharge from both the battery and the PV simultaneously allows for a higher peak demand to be met.
  • In the event of a power outage some AC system such as the Tesla Powerwall 2 allows the stored energy in the battery to be supplied to the property.
  • Allows for electricity trading/grid balancing in the future.
  • Lower overall efficiency compared to DC coupled batteries.
  • Battery export rating counts as export capacity, thus a DNO application will be required in some circumstances.

Tesla Powerwall 2 AC

Marketing photo of Tesla Powerwall 2

The Tesla Powerwall 2 is by far the most publicised battery out there, and it’s also currently the best value for £/kWh for an AC coupled system. The known branding and style makes it an appealing and sought-after system.


  • 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 (and potentially expensive) hybrid inverter.
  • 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.
  • User friendly app allows you to monitor the Powerwall 2, along with your solar PV, or Tesla Vehicle.
  • A Tesla Powerwall system is easily scalable, and capacity can be increased to 10 Powerwall 2’s in one system.
  • Access to preferential electricity tariffs such as the Octopus / Tesla plan which could allow you to benefit from flat import/ export rates as low as 8p/kWh for qualifying customers.


  • A large base capacity of 13.5kWh may not suit clients with smaller systems.

Technical Specs

Usable Capacity: 13.5kWh

Lifetime charge cycles:  Unlimited

Efficiency: 90% round-trip efficiency

Max continuous power: 5kW

Operating Temperature: -20°C to 50°C

Dimensions: L x W x D: 115cm x 75.5cm x 15.5cm

Weight: 125 kg

Performance Warranty

The Tesla Powerwall 2’s Performance Warranty for self-consumption and backup use ensures performance of no lower than 80% energy retention after 10 years, following initial installation, with unlimited charge cycles if connected to solar. For any other usage the same warranty applies but with 37.8MWh of aggregate throughput.

SonnenBatterie AC

Marketing image of the sonnenBatterie

Sonnen’s energy storage system (ESS), the sonnenBatterie, has a good track record in Germany, where the ‘Sonnen Community’ scheme allows a network of batteries to interact and trade power between each other. The system is scalable in 2.5kWh blocks.


  • The sonnenBatterie has an easily expandable capacity, and is scalable up to 15kWh, or 6 batteries in total.
  • Like the Tesla, it is AC coupled with an inbuilt charge controller.
  • The sonnenBatterie is a small and stylish unit.
  • With the ‘sonnenApp’, you’ll be able to check your energy usage and production to monitor, analyse and control the supply and demand of your household.


  • Very high £/kWh. If you require larger capacity, scaling up in 2.5kWh blocks will prove more expensive than opting for a larger battery system in the first place.
  • It can only be mounted indoors.
  • Lower guaranteed capacity (70%) after the warranty period compared with Tesla (80%).

Technical Specs

Usable Capacity: 2.5kWh

Lifetime charge cycles:  10,000

Efficiency: 91% round trip efficiency

Max continuous power: 1.3kW

Operating Temperature: 5°C to 30°C ambient

temperature range

Dimensions: height x width x depth: 139  x 67 x 23 (cm)

Weight: 58kg

Performance Warranty

Storage capacity will be not less than 70% of the labelled storing capacity over the duration of the warranty period. The warranty period for the Performance Warranty terminates upon the earlier of (i) 123 months after purchase of the Covered Product, or (ii) 120 months after installation of the Covered Product by an Authorized Dealer, or (iii) 10,000 full (100%) charging cycles of the Battery Pack.

Enphase AC Battery

Enphase AC Battery marketing photo

The Emphase storage system is a small and simple AC coupled battery system which is expandable.


  • The Enphase battery has an easily expandable capacity and simple installation process.
  • It is a small and compact unit which looks simplistic and stylish.
  • It has some neat software which is user friendly and accessible via their app.


  • A high price per kWh of storage when compared with its competitors.
  • Lower life time charge cycles guaranteed than some of its competitors

Technical Specs

Usable Capacity: 1.2kWh

Lifetime charge cycles:  >7300

Efficiency: 96% round-trip efficiency

Max continuous power: 0.26kW

Operating Temperature: -20°C to 45°C (ambient

operating temperature)

Dimensions (of box): width x height x depth: 39 x 32.5 x 22 (cm)

Weight: 25kg

AC Coupled Batteries

Tesla Powerwall 2SonnenBatterieEmphase
Depth of Discharge (kWh)
Max Continuous Power(kW)51.50.26
Approx. installed Cost (£)790045002417
Warranty10 years @ 80%10-years @ 70%10 years @ 70%
Operating Temp (°C)-20 to 505 to 40-20 to 45
Dimensions L/W/D (cm)115/75.5/15.5139/67/2339/32.5/22
Weight (kg)1255525
Wall/Floor MountBothBothWall
Scalable10 units8 units4+ units

All prices stated will vary for each job and is only intended as a rough estimate


LG Chem RESU marketing photo

The LG Chem RESU is a good quality product from a proven manufacturer. The 6.5kWh LG Chem RESU offers the clearest appeal out of its 3 available sizes when looking for a smaller battery than a Tesla Powerwall 2.


  • It is available in a variety of sizes (3.3kWh, 6.5kWh, 10kWh), with the 6.5kWh unit offering a good value mid-range choice.
  • It can be mounted inside or outside.
  • Using a RESU Plus (expansion kit) up to 3 batteries can be cross-connected to increase the usable capacity (however, if you plan to expand your capacity, a 13.5kWh Tesla Powerwall 2 may be a better selection).
  • As a DC battery, it will not effect export capacity restrictions.


  • It will require the installation of a hybrid inverter which complicates the installation procedure, increasing both component and installation costs.
  • Warranty covers for 60% retention after 10 years, 10-20% less than competitors.

Technical Specs

Usable Capacity: 5.9kWh

Lifetime charge cycles:  6,000

Efficiency: 95% round-trip efficiency

Max continuous power: 4.2kW

Operating Temperature: -10°C to 45°C ambient operating temperature

Dimensions: width x height x depth: 45.2 x 65.4 x 12 (cm)

Weight: 57 kg

Performance Warranty

The Performance Warranty for the LG Chem RESU is valid for 10 years from the initial installation date, retaining at least 60% of Nominal Energy for either 10 years after the date of the initial installation or for a minimum Energy throughput of 16.1MWh.


Photo of wall mounted BYD B-Box Battery

BYD are a long standing and reliable Chinese manufacturer of electric vehicles and rechargeable batteries.The system can be wall mounted, or scaled by adding up to 3 additional 2.5kWh battery storage units to a B-Box battery cabinet. Up to eight 10kWh battery cabinets can be combined in parallel to create 80kWh of storage.


  • BYD provide a strong warranty, ensuring 80% capacity retention after 10 years (for 1 cycle per day).
  • The B-Box currently provides the best value for small storage capacity, which may prove more cost-effective than larger batteries like the Tesla if better matched to your consumption, despite a higher £/kWh.
  • Easily scalable in 2.5kWh units.


  • Can only be mounted indoors.

Technical Specs

Usable Capacity: 2.4kWh

Lifetime charge cycles:  >6,000

Efficiency: >95.5%

Max continuous power: 2.56kW

Operating Temperature: -0°C – 55°C

Dimensions (of box): width x height x depth: 60 x 60 x 60 (cm)

Solax DC Battery

The Solax storage system is available in 4.5kWh and 6.3kWh sizes that can be installed in series with up to 3 units in a single phase and 4 units in 3-phase system. This allows for a maximum storage capacity of 18.9kWh and 25.2kWh respectively. Solax batteries have to be paired with a hybrid inverter to be added to the system, which may increase costs if retrofitting.


  • Flexible capacity design and can be scaled to suit a variety of consumers.
  • Cheap £/kWh storage solution.


  • Not the most eye catching design.
  • No back up power in the event of a grid shutdown.
  • Low life time charge cycles (6000).
  • Lower round trip efficiency compared to other DC coupled batteries.

Technical Specs

Usable Capacity: 90% of stated

Lifetime charge cycles:  6,000

Efficiency: >92.7%

Max continuous power: 3.68kW

Operating Temperature: 0°C – 55°C

Dimensions (of box): width x height x depth: 47.6 x 46.4 x 18 (cm) (Varies depending on battery/invert size)

Weight: 23kg

Performance Warranty

10 year warranty or 6000 cycles promising retaining 60% of the original capacity.

Pylon Tech DC Battery

Pylon Tech are a Chinese battery manufacturer who to date have installed 1.5GWh of battery storage globally. They currently offer a US2000 2.4kWh battery unit which is scalable up to 14.4kWh.


  • Small starting capacity allows the unit to fit a variety of consumers. Particularly small domestic properties.
  • Compatible with a variety of hybrid inverter manufacturers.
  • Low £/kWh cost.


  • Not as ascetically pleasing as other designs
  • Warrant promising 70% retention after 6000 cycles or 60% after 10 years which is lower than some other battery manufacturers.

Technical Specs

Usable Capacity: 2.16

Lifetime charge cycles:  6,000

Efficiency: Not Stated

Max continuous power: 5kW

Operating Temperature: -20°C – 60°C

Dimensions (of box): width x height x depth: 44.2 x 41 x 89 (cm)

Weight: 24kg

Performance Warranty

After 6000 cycles the battery will retain 70% of the stated capacity or after 10 years the battery will retain 60% of the stated retention.

DC Coupled Batteries

LG Chem RESUBYD B-BoxSolaxPylon Tech
Depth of Discharge (kWh)
Max Continuous Power(kW)
Approx. installed Cost (£)6691440041203845
Warranty10 years @ 60%10-years @ 80%10 years @ 90%10 years @ 60%
Operating Temp (°C)-10 to 450 to 55-20 to 45-20 to 60
Dimensions L/W/D (cm)42.5/65.4/12100/60/6047.6/46.4/1842.2/41/8.9
Weight (kg)52882324
Wall/Floor MountBothGroundBothBoth
Scalable3 units8 units of 42 units -1 phase
4 units- 3 phase
6 units

All prices stated will vary for each job and is only intended as a rough estimate.
1- 3.6kW inverter with 6.5kWh storage.
2- 3.6kW inverter with 5kWh storage.
3- 3.6kW inverter with 4.5kWh storage.
4- 3.6kW inverter with 4.8kWh storage.

We hope the information presented here will give you a better understanding of the various types of battery storage systems currently on the market. If you are still unsure the team at SSWUK, would be happy to talk through your requirements and recommend a system for you.