I have had a "Jett" Heated Sleeveless Jacket for about 10 years.. It works really well when I have worn it in isolation. This isn't all that often as it doesn't work all that well in combination with my one piece freezer suit, which is what I normally wear. It runs about 4 to 5 hours on the battery pack and I also have a second battery pack for it which I keep charged. I am on my second set of battery packs with the first set lasting about 7 years, before they failed to hold a reasonable charge level.
Whilst these things might sound like a bit of a gimmick / novelty, I think one of these sleeved and hooded heated jackets from AEG / Bunnings would work very well in combination with a set of freezer pants. Particularly on the basis that it has temperature control, whereas a freezer suit doesn't. Depending on the type of observing you are doing you can actually get pretty hot in a one piece freezer suit; particularly if you are using a large aperture scope and continually moving a ladder around and climbing up and down it. On countless occasions I have had to peel back the top half of my freezer suit after a few hours observing and then your top half gets really cold fast. It would be really nice just to be able to turn the top half temperature down a bit, when you start to get hot.
Cheers,
John B