Battery Back Up Introduction

There are 3 main considerations when designing a battery back up system

  1. Is the system On or Off Grid?
  2. DC coupled system or an AC coupled system?
  3. Does the system need to work in a power cut?

There are also secondary considerations like, budget, brand , Lead Acid or lithium and size etc we will come back to these at the end but lets first look at the first 3.

  1. Is the System Grid Tied or Off Grid.
    A grid tied system is less complex and the easiest solution. Off grid needs more careful planning and a full understanding of the exact power needs. They are more common than one might think as every boat is an off grid system.  If we are off grid we may not need a 240v ac network. But if we do we may use a system like the Sunny Island that manages the batteries, creates the grid and can also switch generators on when the batteries are too low.
  2. DC or AC coupled. The recent increase in interest for batteries is from owners with PV systems or installing them now. Feedin tariff is a consideration of which system is best. Whenever we charge a battery and then use that electric we have is termed a round trip efficiency of approx. 90%. This means we will loose 10% of our generated power.  For this reason AC coupled systems are the most prevalent as this loss takes place after the generation meter.
    In this situation our flow is    DC – AC – DC – AC
    A DC Coupled system would be DC – DC – AC   shorter and less loss, but all the losses are before the generation meter.
  3. Does the system need to work in a power cut?
    To work fully in a power cut we need a big infrastructure. First we have to have what is called a G59 relay. This is a changeover switch that disconnects the house form the Grid. If you didn’t do this you would be powering the grid. Then we need a device to generate a grid for us. Most inverters merely copy the grid where as the device we need has to create and then maintain the 240v 50hz grid.  These are quite expensive and come in sized outputs depending on how much energy you will use. 3kw are the basic ones, once we get upto 10kw and larger these become specialist.
    Lots of the AC coupled systems are offering a half way house where they will provide one circuit. This is permanently wired through the battery management controller and can be used for a lighting circuit or fridge freezer etc.

Other considerations

Battery Type Lithium v Lead Acid.
These both work differently Lead Acid discharges to 50% and Lithium to 85%, But apart from this they can both do the job equally well. There is a trend towards Lithium as it is less bulky. Electric cars are using Lithium and there seems to have been a recent price drop as this technology advances.


There are a number of manufacturers available.  The Controller and battery manufacturers are generally different companies. Some listed below

Victron are an older well established company many years in Boats and off Grid.
SMA One of the market Leaders in Inverters and off grid systems
Sofar – Large player with a close relationship with Pylon batteries

Growatt – Established company

BYD – Established company also makes batteries

NeDap -




All the below are well established companies








We have purposely listed the brands before the budget.

Its our experience most people will want a system to help increase self consumption in which case budget will have to be viable to warrant the spend. Its less frequent the primary concern will be continuity of supply in which case there is a needed spend to achieve a reliable system.

Therefore most people will want a grid coupled AC system like the SoFar , Growatt & BYD which offers good performance, limited off grid use and can be charged from the grid as well as PV.  These also offer scalable system size. Ie you can start with 1-2 batteries and scale up to 8. A Typical 2.4kw system cost approx. @£2400 – 20kw £8700


When designing for continuity of supply you would be more suited to a Victron, SMA Sunny Island type system with LG,, Mercedes or Tesla type battery – Could also be Lead Acid. When we did the Tesla training course one line stands out – Never sell this system on getting you money back – it is a lifestyle choice to have a Tesla battery system to minimise the effect of power outages. In America they have lots of power cuts every month of the year. A Typical 10kw system cost £10,000+ -20kw £16,000+



When considering battery back up it is very important  to install some monitoring. Preferably before installation to get an idea of your power usage and what size system will be best. We have some software to suggest this but it is always best to measure. This is another good idea why a scalable system is good so you can expand as needed.


Economy 7

You can also use batteries with Economy 7. Charge the batteries on economy rate so you don’t pay the higher tariff in the peak hours.


Current VAT laws state that if a battery system is sold on its own we must charge 20% VAT on the supply and installation. However if a battery system is sold and with a PV system then it becomes 5% rated for VAT

Our current prices for a retrofit SoFar system with Pylon Tech Lithium batteries are below. These inc 20% vat. If installed with a PV system the reduced rate vat will apply eg:

2,4kw system £2400 sold on its own, sold with a pv system then £2100.


2.4kw = £2,400

4.8kw = £3,300

7.2kw = £4,200

9.6kw = £5,100

12kw = £6,000

14.4 kw= £6,900

16.8kw = £7,800

19.2kw = £8,700

Latest Prices

PV System

4kw SunPower E20 327 watt panels £7500

4Kw PV with 2.4kw Battery Back up £6950

4kw Pv system From £4950

2.4kw Battery System £2400

4.8kw Battery System £3300

7.2kw Battery System £4200

9.6kw Battery System £5100