With a hybrid you have to average fuel consumption over the whole year. Sometimes excellent in summer, sometimes less impressive in winter or when driven hard or at high motorway speeds. But IMO even at its worst its still better than many ,if not most , conventional ICE cars under similar conditions. . Personally I would still value its silence,ease of driving and excellent torque even if there were no fuel advantage.
Even full EV cars dont deliver their claimed range in cold weather or at high speeds.
I noticed that whenever the ICE runs ,even if slowly and not under much load, it uses petrol at the rate of at least 5 litres/100 km ( 56 mpg) . From a cold start in cold weather it can be a long time before the car is warm enough to stop the ICE and use EV mode so there is not much opportunity to improve upon 56 mpg and worse.
This is partly due to extracting cabin heat from the ICE and needing to generate more electrical power for extra fans, lights etc. But a big factor is that the HV battery is less efficent when its cold so it takes longer to charge it enough to benefit from EV mode. Some Hybrid manufacturers chose to experiment with complex, expensive and heavy heating and cooling specifically for the HV battery to improve its efficency slightly, and maybe also engine compartment shutters as suggested by John Ratsey.
I think Honda concluded that extra weight and power consumption of these heaters largely offset any fuel efficiency gains and only gave a marginal improvement in limited circumstances. So the Jazz relies on cabin heat to regulate the HV battery. If the cabin is cool enough or warm enough for passenger comfort its also adequate for the HV battery .This does have a small fan to circulate cabin air but no dedicated heating elements or air conditioning. But using heated seats rather than extracting heat from an ICE thats not yet reachedits full operating temperature ,can be more fuel efficient for a while .
