These are load control devices for off-grid power systems with AC coupled renewable generation sources and battery inverters which produce a variable frequency supply such as Victron Multiplus and Quattro and the SMA Sunny Island range both current and legacy models. SMA refer to this as Frequency shift power control and Victron refer to it as PV inverter support -assistant required in the VE software.

They are used to switch on additional loads to take advantage of surplus renewable generation which cannot be stored. They respond to a rise in the system frequency caused by battery inverters signalling a high state of battery charge, or by low load on a generator with frequency droop.

Each switch will control one appliance and is wired into the cable supplying the appliance. They require no user intervention and will automatically switch the appliance on when the system frequency is high and switch it off when the frequency is low. The frequency setpoint can be adjusted during installation.

They incorporate a number of features to ensure stable operation of an islanded power system:

• Once the frequency goes above the switch-on setting there is a delay before the appliance is connected; the higher the frequency is above the setting, the shorter the delay.
• The switch-off setting is a lower frequency than the switch-on setting to ensure stability.
• Once the frequency goes below the switch-off setting there is a delay before the appliance is disconnected; the lower the frequency is beneath the setting, the shorter the delay.

Can control loads up to 13A (3kW at 230V; 1.5kW at 120V). The most appropriate loads are heaters (water heaters, storage heaters, space heaters). Appliances with pumps are not appropriate due to the potential for rapid switching of loads.