Agree, for me with a phev it makes sense as I'm on an old very cheap rate overnight, but when it renews at 40p/12p the go tariff is only worth keeping if I also heat our domestic hot water with electricity.
One way of thinking of the battery is each night you put in say 10kwh if that energy is used at the wheels it makes no difference what mode the car is in, it all gets used up. So the secret is always full to empty and charge every night (or every oppertunity). So precondition before departure, and always sat nav home.
The efficiencies are in NOT running the ice at the wrong time. So not used for heating, avoided during stop start urban, preferred during 50mph plus cruising which the car does automatically by default.
If you find you are coming home on certain journeys with battery left and the ice has run, then you can select e max mode which favours the ev before that specific journey.
For most of my driving is short urban, semi rural, with short urban motorways, 2/3 is electric. and i get 45 to 50 mpg on a long motorway run a couple of times a year at 70mph.
I recon that about 40 to 50p per kwh is the break even point for me given the poor mpg I would get on the short journeys.
And if you are staying in the house, get a proper wall charger. First thing I did when I moved and had off street parking.