Working out your optimum battery storage capacity
Having some idea of the excess power you have available that could be going to the grid each day tends to be an assumption unless you have been metering the daily export over a year or so, we have to make some assumptions.
Is the house all electric? This can affect the household power consumption, especially if someone is at home all day. Do you already have a power diverter to send excess power to an immersion heater? If so, you may not have much excess power to spare for the batteries, depends on the size of PV and how much hot water you use during the day, etc.
The other main issue is the size of your Solar PV system. I have reproduced a generic chart indicating anticipated output from a 4kWp solar array across the year. You can see that the Winter generation is dramatically different to Summer. Maximum Summer generation alone should not be used in the calculation of in the battery size, otherwise you could have a battery that only gets charged properly for 3 months of the year. Oversizing the battery is a false economy, unless of course you are looking to ‘time shift’ power by charging on Off Peak electricity overnight. Oversizing tends to be a common mistake for one-size-fits-all batteries such as the 14kWh Tesla.
Generally for a 4kwp system, even if your daytime use is low, the best economic compromise is to have 10kWh of storage installed. Most of the time, the battery will never get fully charged, but the capacity is available for sunny summer days to capture a good amount. We find that most households on mains gas usually do not use more than 8kWh overnight as lighting and appliances are now really low energy.
There are many battery backup and Solar Storage systems on the market, we have been checking them out to save you the hassle!
The units on our web page can offer both battery storage, backup option in case of power cut and self-use optimisation to allow you to use stored PV generation at night. Not all units out on the market can do all of this.
Victron Energy Storage System (ESS)
The Victron ESS – 
This system offers versatility on inverter saze and battery size and type that we can install. This enables us to use the Victron system to fit any installation type you may require, from 800W right through to 75kW 3 phase. All we need is to match it to your Solar PV size to ensure the maximum effectiveness throughout the year. A small PV system will never charge a big ‘one-size-fits-all’ battery for example, your system needs to be tailored. See above for an outline of the calculation process.
If you click through to the Victron ESS page, you will see the outline costs for 5 different packages depending on your premises size and installed PV system.
SoFar Solar battery storage
The newest addition to our range is the budget SoFar Solar battery storage. It offers a great performance using Pylontech Lithium Ion battery modules, enabling a system that can be tailored for your storage requirements. The Sofar inverter unit contains all the processor requirements to calculate the power that can be stored in the
batteries. With sensors fitted on your PV output and your grid connection, the exact amount of excess power that would be exported to the grid is calculated.
See our SoFar Solar page for more information and price list. The installed costs are one of the best in the industry, coming in far less expensive than Tesla (nearly 50%) and many others on the market. The SoFar Solar offers online monitoring and the option of Off Peak charging AND backup power for those emergency times when you get a grid fail.
Tesla Powerwall 2
We are approved to install the Tesla Powerwall 2.
With a one size 13.5kWh of storage, the Powerwall 2 we install contains Tesla Lithium cells, a self contained inverter plus an external Backup unit.
The Powerwall 2 can be installed inside or outside a building and will be connected to your Grid distribution board and local internet to enable you to access the Tesla Portal to monitor and control your storage options.
See the Tesla Powerwall 2 Page for more information.
