I have a 2019 Lexus ES 300h. I have read that using the EV (all electric mode) can reduce your mpg. Not sure how, but am wondering if it runs battery power down which results in the gas engine having to take more of the workload. Is that correct or is it another reason?

 

That's right, the battery pack is there to assist the engine and to provide power for electrical systems. It doesn't have much capacity to run in EV mode for long. The new ES300h has a redesigned transmission that allows the traction motor to run without the engine at higher speeds but that also means depleting the battery pack faster.

 

You'll want to use EV mode in scenarios where you don't need the ICE to start unnecessarily. Like when moving cars around in the morning. Since the engine will shut off a minute or so later, no need to incur the fuel consumption and wear and tear starting an ice does when you're only driving the car for a very brief moment.

 

If you run down the battery charge in EV mode, that discharged battery becomes little more than a heavy lump that has to be dragged around by the internal combustion engine, meaning the engine has to do more work as it drives the car and charges the battery, without assistance from the electric motor.

If, however, you leave it in regular hybrid mode, the engine tries to keep the battery charged up so that there is some charge available to drive the electric motor to assist the engine when needed.