From what I have read, it heats the top immersion heater first, and once the thermostat has turned off, it then starts on the bottom immersion heater, and it only gives out what is spare solar, but not sure what it would do with a 300 watt load instead of a 3000 watt load, if the towel rail was 3 kW then I would say likely no problem, but at 300 watts, I don't know how it would work.
The iboost was designed for the old system, where you got paid what they thought you had exported, rather than what you have actually exported.
I fitted one in error, a simple time switch would have been better, but before solar was installed I did not really understand how it worked.
The off-peak is 8.5p/kWh and the export payment is 15p/kWh, so better to use off-peak than solar to heat the immersion, unless lucky enough to still be on the old system, personally the iboost+ was not cheap, and I would not want to damage it, I would expect they would stop making them soon, as no longer saving money.
As to if cheaper using an Ideal vogue gas heater, we opentherm or solar, I don't know, I note since fitting a 27-inch immersion heater it is using around 14 kWh per week, my on/off boiler uses around 30 kWh, but your modern boiler one assumes is far better, once I was indecisive, now I am not so sure.
I note the Eddi-2.1 which is simular to the iboost+ but with a few more functions states
Most purely resistive heaters can be used with eddi, examples include immersion heaters, storage heaters,
convection heaters and underfloor heating mats. Refer to the Application and Examples section for wiring
diagrams using different heater types. Whichever heater type is used, the following criteria must always be
observed:
• One or two purely resistive loads may be connected (Note: only one is live at any one time)
• Maximum rating of each load is 3.68kW
• Minimum heater rating is 150W
• Resistive loads only
• The loads neutral conductor must be connected to heater output N terminal
• No electronic controls to be connected to Heater terminals; mechanical thermostats only
• No timers to be connected to Heater terminals, including mechanical timers
I note the minimum rating, so maybe you do need iboost to tell you what their minimum rating is?
