Whats the wisest way to Use the EV switch?

Assuming the weather wams up above 32 deg F then the reason you might want to leave some charge in your battery when parked is that you can still use EV to avoid the gas mileage hit of warmup. When your gas engine is burning up gasoline your car needs to be moving. The faster you can get your car coasting while keeping it running gently the better. If you drain your battery than you've committed yourself to a long period of getting zero mpg. The only exception to this rule is if you know that once you start your car you are not going to be stopped at any traffic lights for the next 3 minutes.

 

The EV switch could get you past the aggravating situation of having your engine idle senselessly. If it's below freezing for many hours then of course the EV switch is useless to help your morning warmups because your EV request will get rejected. If you park your Prius during mid day than even if it's below freezing often the EV mode will work for startups. For the mid 30's or higher than it becomes very useful.

However, if you drain your batter too low than you'll never get to know the benefits of EV to aid your morning starts because the EV mode will always get rejected.

 

I basically follow everything you do. In addition I sometimes use EV to assist acceleration from a standstill up to around 15 mph. Then I pulse the gas to kick out of EV mode and continue with the gentle acceleration. My general goal is to always avoid getting my display to ever show below 10 mpg. From my experience if I am frequently unsuccessful in avoiding single digit displays during acceleration I will end up averaging in the 40 mpg range. If I am successful I'm well on my way to the 50's.

I used to think the best approach to maximizing mileage is to get long periods of 99.9 mpg. But what I find is that the biggest factor is to minimize the amount of time you spend in low mpg. In the mathematics of average gas mileage it only takes a tiny smidgen of single digit mpg gas mileage to wipeout the benefits of stretch of 99.9 mpg gas mileage.

 

For me using ev at night is best. Couple of extra reasons that don't apply for everyone: It is cold here so ev has very limited current capability in the mornings, except in summer. Often too cold for EV first thing in the morning. Also though I would like to delay start up, have some coasting, and lights to wait at there is one hill that is a bit steep for ev and the engine usually starts up on that hill if I try to delay startup. So for me it is way better to use ev at the end of trips.

Also at my end of trip, engine will normally be off anyways. The EV mostly just keeps it from starting up to keep warm, and fuel use for more heat would be a waste when about to shut down.