I won't waste my money on battery powered heaters in clothes. That's because I understand the electrical power requirements, which just can't be gotten around (d*mn those laws of physics!).
In my opinion, unless you combine GOOD insulating clothing in the first place, at best battery powered clothing heaters provide short lived comfort. All you'll do otherwise is waste the heat produced away to the envirnment.
The best generator of heat is your own body. Proper insulation may even require "venting" excess heat by unzipping jackets, etc. The body at rest radiates about 100 Watts worth of heat. If you're physically active, this is higher.
I suppose if you have circulation problems, heated clothing may help. Likewise if you're not physically active in cold weather for some reason, such as hunters in a stand or something.
Even so, battery powered heaters should only AUGMENT the body's radiated heat under the use of decent insulating clothing...because you can't comfortably carry around a battery source large enough to afford to waste the heat produced.
Wattage for DC heaters is simple to calculate: Power (Watts) = Voltage (Volts) X Current (Amps).
Play with Ohms Law and the power equation and you also get Power = Current squared X Resistance (Ohms).
Battery capacity if typically expressed in Amp-Hours (Ah). If you have a battery rated at 12,000 mAh, it's theoretically capable of providing 1 Amp of current for 12 hours, or 12 Amps for one hour. (Capacity is usually determined at lower current loads: higher current loads typically not 100% ratioable and result in less time.)
So, if you assume a power in Watts for an electrical heater and know the voltage of the battery source, you can figure out the current draw and therefore battery life. A quick google found typical jacket liner wattage to be 75 to 100 Watts. Assuming the lower power rating of 75 Watts on a 12 Volt battery, that's a current draw of 6.25 Amps at full power.
A 3.0 Ah battery won't last a half hour at that setting.
If you assume a power setting of 12 Watts on a 12 Volt battery, that's 1 Amp. A 3.0 Ah battery would last 3 hours.
At 6 Watts...6 hours. 3 Watts? 12 hours.
So, insulation is absolutely critical.