The house is fitted with 16 solar photo-voltaic panels providing 4kW maximum output. The system is grid-connected and was installed to make the most of the 'Feed-In Tariff' scheme. Under the scheme, every unit of power generated on the roof pays us a cash sum, this is regardless of whether the power is exported onto the grid or used within the property.
This gives an incentive to find the best way to use the power, rather than exporting. Free electricity AND getting paid to produce it !
The pic shows a piece of kit designed and built in Jon's Workshop, the Solar Saver
The unit constantly monitors mains conditions and determies the magnitude and direction of electrical power flow - in from the grid, or out to the grid.
Put simply, when the solar panels are delivering more power than is being used in the house, there will be a power flow from the house to the grid.
If the water in the hot water tank is below the thermostat setting, the Solar Saver diverts surplus power from being exported and dumps it into the hot water instead - free hot water !!
Power flow is measured from sampling mains voltage, and multiplying this by the output of a current transformer clamped on a meter tail where the power company cable enters the house. Power flow direction is indicated by the phase relationship between voltage and current. The Solar Saver samples both voltage and current waveforms, sampling each around 60 times per cycle. These time domain samples are converted to the frequency domain using a Fast Fourier Transform. The FFT output gives a complete data set for all power and phase angle calculation to follow.