Again, if you check the booklet, that information is provided as well.. Depending on what bike you have and what year, the available amps you have on your charging system is spelled out.

Example, mines an FL family EFI bike. I know that stock, I have 39.4 amps available at 1500 rpm, I have 45.2 available at 3000 rpm... The bike uses about 28 amps under normal load. That means I have about 10.8/17.6 left to use.

My jacket liner draws 6 amps , gloves draw 2.20... That's 8.2 vs the 17.6 while riding, so I know I'm safely well under an overdraw threshold.

And that's all more than likely very conservative; I can run two jackets, a pants liner, and two pairs of gloves with no issues; though I wouldn't do that if the bike was just idling there for an extended time. Also take into account what lighting you're running, upgrading from halogens to LEDs cuts the bikes draw way down, little things like that affect what you have available to use for accessories.

 

Curious, because I have no experience with the battery operated stuff - will it last for 8-10 hours per day, for a few days?

 

I've never needed the high setting, Low is usually plenty.

High 100% - 135°F 2 hours
Medium 66 - 115°F 4 hours
Low 33% - 90°F 6 hours

 

The instructions that came with my first Gerbings gear said not to plug into the bike electrical system until the engine was already started. It will run down the battery quick if you do...especially 2 sets of gear.