Domestic mains AC frequency sensor

[Robster]

Hi

Please could you think about building in a mains AC frequency sensor into one of your products?  Ideally the Shelly Plug.  There are quite a few posts on the net by people who would like to use this as a measure of reliability for commercial mains supply.   
 

My particular use case is that my house is off grid.  My main power controller unit (SMA “Sunny Island”) automatically subtly changes output voltage frequency (nominally 50Hz in the UK) to signal changes in battery state of charge. It does this to communicate with its own peripherals (eg the inverter) to tell them to ramp down supply or switch off supply when there is too much renewables power.   I’d like to use these changes in AC frequency to trigger “dump” loads (e.g. a water immersion heater) when my solar is providing too much power for my batteries so as not to waste free solar energy. But other people may also be interested in measuring or using AC measurements too. 

[wooly]

how much does it change the frequency ?