I have 2 inverters, an 800 watt MSW and a 400 watt PSW. I try not to use them, but when I do, the PSW is charging my Makita Li-ion batteries, and powers my 140 watt soldering gun. The 800 watt MSW powers my Angle grinder.
12v TV, water pump, fans, fridge, lights, Stereo. laptop power supply.
I used to have a 12v mattress heating pad i was fond of, but it only lasted 3 winters, and this winter I went without it. On low one needed 22 amp hours per 8 hours, on high it needed 55 AH plus, over 8 hours, so Significant battery power was required.
While i have 9 ciggy plugs distributed around my interior and wired with 10awg. I largely do not use them.
The exception is for a voltage controller used as a speed control for 2 of my fans, and also to power a USB recharging source.
http://www.amazon.com/PowerLine-Uni...3&sr=8-5&keywords=powerline+voltage+converter
http://www.amazon.com/Certified-App...id=1432520885&sr=8-3&keywords=usb+car+charger
These low wattage items present little issue for a Ciggy plug receptacle, but when regularly asked to pass 5 amps and above they will develop issues and start wasting power as heat.
Gotsmart your in cup water heater element, it likely would heat faster if you could bypass the ciggy plug. When my mattress heating pad still worked, its ciggy plug caused a lot of voltage drop. when I eliminated it in place of a 45 amp Anderson powerpole, it got so much more voltage it heated up much faster.