Idling a gas engine can be very expensive. Figure about a gallon per hour, a little less as Leo noted, but it's more than just fuel cost. Rule of thumb for cost analysis is that idling a gasser will cost you about $2-$3 an hour in wear and tear on the engine. You won't pay that $2 or $3 up front, but you will over time, in increased repair costs and decreased fuel economy over time, and/or a shortened lifespan of the van.
Every 8 hours of idling is the same as driving 100 miles, both in fuel costs and wear and tear. It's about $5 an hour to idle, with $2.50 to $3 of that in immediate fuel and oils costs, and the rest of it over the long run. But long run or not, you will be paying for it. If you idle roughly 1300 hours per winter (see next paragraph), that's the same as driving 16,250 miles a year while not moving. Over 5 years that's 81,250 miles, and your van's lifespan just got shortened by one year.
If you idle 8 hours a day during the week, and 16 hours a day on the weekends, that's 72 hours in a week, times 4 months (18 weeks), that's 1296 hours. That $3449 in fuel costs alone at $2.70 a gallon. Add another $2592 for the $2 an hour for wear and tear, and a $1200 Espar ($1500 or $1600 if you pay to have it installed, which I recommend unless you've installed them before) starts looking really cheap.
Some will idle less than that, some will idle more. But even if you idle less, down to the point where it's costing you only $1500 each winter to idle, the Espar pays for itself the second year, and starts paying you that $1500 each year thereafter. But for most people the Espar pays for itself the first winter.
If you do a thorough cost analysis of all of the ways to keep warm, the Espar heater is far and away the most cost effective. A generator is great for running electrical appliances and charging batteries, even air conditioners, but running an electric heater with one is not nearly as cost effective as an Espar. The generator will be running at near full load all the time, which burns a lot of gas, and the fan heater won't put out even heat like the forced-airflow of the Espar will.
I wouldn't recommend actually buying it this way at all, but if you look at it, it's actually more cost effective to put the Espar on a 22% credit card and take 4 years to pay it off than it is to idle. It's actually a lot more cost effective. All it takes to pay for the Espar is a couple of good weeks in the van.
The Espar will go through about a gallon and a half of fuel a day. My Sprinter has no bulkhead thus the Espar works to heat the entire van. It goes through about a gallon of fuel every 12-15 hours, depending on how cold it is outside. The van is well insulated. With a bulkhead and only having to heat the front half of the van, it'll use even less fuel.
If you enjoy being cold and sleeping in flight suits, then an Espar probably isn't for you. I did my first winter in expediting without an Espar heater. I'll never do another one without it. It's the most cost effective, most comfortable way to live out here. The Turtle's nose and toes have got to stay warm.
