The Brief

Beco were approached to design a hybrid of wind and solar power system complemented with a battery bank.  The result was off-grid, energy independence for this home in Somerset.


The first step was to check that a grid connection was not feasible or prohibitively expensive.

Usually the critical amount is between £15,000 and £20,000 depending upon the electrical demand of the end user.

System design

Having ruled out a grid connection,  the elected design was a SMA “Sunny Island” system that allows for the renewable energy inputs to be connected to the system via the conventional 230V ac output.

This has the advantage of not requiring the renewable energy inputs to be connected to the battery – allowing them to be sited where the most convenient location of the source is. This design feature also allowed the user to claim the Feed-in-Tariff (FiT) from the PV system as any conventional system would.

To maximise the potential from the renewable sources of the system a ‘load shedding’ system that not only disconnects non-essential loads when battery capacity drops and renewable sources are scarce but also diverts to ‘dump’ excess power (to heat hot water or electric space heating) when the batteries are fully charged and the renewable sources are generating excess power.

For emergency use or service battery charging, a diesel/petrol generator can be easily used just by connecting a single plug and starting the generator.

Sizing the system

Running an off-grid power supply is an operator unique experience! It does involve participation and understanding of electricity and how it is consumed. It would be almost impossible to design and build a system that everybody would be able to use – so it is critical to know what the expected user demand will be.

An alternative method is to design a system which will generate and deliver a certain amount of energy every day. The system described here comprised of 3kWp Solar PV, a 3kW Wind Turbine and a battery bank with a capacity of over 20kWh daily @ 50% of the batteries capacity.