I am a fan of daytime running lights (DRL) and am dismayed that the US version of the V does not have a component that will enable this feature (the Canadian version of the V does have DRL). Obviously one could just turn on the headlamps but that would reduce the life of the halogen lamps and also reduce the fuel economy due to power demand. Are these measurable costs? I understand that the V5 has LED headlamps .. it may be that LEDs would solve both of these issues due to very long life and low power draw. I wonder if the 2013 models will incorporate (either or both) LED and DRL.
(I use my headlights anytime the car is ready, but I value safety higher than energy) Above 40 something*, the Prius engine is already running, so there is a an easily measurable cost for the lights being on. (1 gallon of gasoline per hour is 39 kWatts, your lights consume about 200 Watts) Below 40 something*, The engine will run more often with headlights on as the HV Battery will be drained faster, this will be a much larger, but harder to quantify cost. * the exact cutoff speed varies by model, but is mostly in the 41 to 45 MPH range.
Like Jimbo below, I elected to pay these costs, and left the lights on nearly full time since acquiring a car with auto-off lights in 1997 (until swapping my 2010 Prius for a 2012, below). Bulb replacement costs have been miniscule. I remember only two replacements (the '97 is still in the household, and is easy to change myself), but could be forgetting some. I do like the DRLs in the 2012 liftback, and am now using them. Hopefully they will migrate to the other models shortly. But after fifteen years of not touching the light switch on any regular basis, I'm having to relearn to turn on the real lights as dusk approaches. The 39kW at the gas tank drops to about 13-14kW at the ICE shaft. From a base of 60 mpg, that would produce a drop of just under 1 mpg. Because of the very nonlinear nature of the mpg scale, the apparent mpg loss shrinks sharply at lower base mpgs. Put another way, 200W of lighting should increase gas consumption very roughly 0.015 gallon/hour. An ordinary commuter blowing down the highway at 80 mph, burning 2 gal/hr, will be very hard pressed to even detect it, as their MPG drops from 40.0 to 39.7. But a hypermiler burning just 0.3 to 0.5 gal/hr in crawling city traffic probably will see it, as their MPG drops from 75 to just 73 or 71. Leaning towards the hypermiler end of the spectrum, I quickly made use of my new DRLs.
