PowerSwitch

For those who like doing a bit of DIY there is a useful tool available. A standard UPS has a 230v to LV DC to charge its small internal battery and then an inverter to produce 230v again. So you can buy one with worn out batteries very cheaply and at whatever power output you choose. About 1/2 or 1 ...

There are several devices being sold now to variably control the power to a 3 kW immersion heater element depending on how much power is being exported. The only way that I can imagine these achieve that is by using a triac switch. It will work but will generate an odd current waveform on the supply...