For most the furnace blower is by far the biggest power draw on the battery. What is worse is absent a cut off designed to stop drawing power beyond that 50% point a furnace that is running during the night can really flatten a battery. Or leave you waking up with no power and a chilly camper.
One can as I have just stick a decent quality battery in the box and figure the two LED interior lights
are unlikely to kill my battery for a lot longer than the water will last. I did add an inverter which will allow me to run some 110 volt items which could lead to drawing down the battery. I plan to add a fantastic fan which also could put a lot more draw on the battery. Vent fan isn't especially high draw but it tends to run for longer periods.
Which brings up the other way to determine battery/solar needs. Find out the draw in amps of the devices you will use that draw power from the battery. Then multiply that draw by the amount of time the device will run in a day.
Repeat that calculation for all your devices. Sum the total will tell you how many amps you will draw in a day from the battery.
3 hours per day @.5 amp = 1.5 amp hours
fantastic fan 4 hours per day @2 amp = 8 amp hours.
laptop 2 hours per day @ 2 amp = 4 amp hours
TV how would I know I don't camp with a TV
Divide battery amp hours by 2 to get your 50% discharge amount. Then divide that by the number of amps you use in a day. This will tell you how long you can camp on battery absent any charging.
Sum is 13.5 amp hours a day.
Battery has 90 amp hour rating divide by 2 equals 45 amp hour capacity
45 divided by 13.5 equals 3.33 days I could run the listed load without any additional charging.
If one has 100 amp hour capacity from pair of big 6 volt true deep cycle (golf cart type) batteries one can go over 7 days with that usage and no additional charge
Charging with solar
, campground power, or tow vehicle will allow you to replace used portion of your stored amp hours. Deep cycle and hybrid starting/deep cycle batteries charge slower than auto starting batteries. They are designed to release power slower and thus take in a charge slower. Making them especially suited to the slow all day charging of solar. Pumping amps in at a high rate from a generator
doesn't work as well for deep cycle as it would to charge your vehicle battery.
Many campers will have a converter to charge the battery when camper is plugged in to campground power. People install a line to charge the camper battery from tow vehicle through the lighting
plug. I have the power line from tow vehicle alternator, a few hours driving can put a full charge back into the battery.
People often do some pretty careful calculations on power usage and solar capacity in order to insure adequate power with a good margin for rainy days with little solar output. Or they do some quick back of the envelope calculations and determine dear wife will want to go someplace civilized well before I will deplete my stored reserve. Or I'll run out of water.
Even adding back in only half of what you use each day doubles the number of days you can boondock camp off the battery. So if a 100 amp solar panel
would replace your normal daily use it would provide for long term camp off grid. If you instead have a 50 amp panel you would get 2x as many days as you would from the battery without any charging.
So if battery alone last 3.33 days as shown above a solar panel that puts back in 50% of daily use (6.75 amp) would give one 6.66 days of camping which I will guess is longer than my wife wants to stay in the national forests these days by at least a day or two.
Bottom line what one puts in the battery box or uses for charging is somewhat individual. For 3 season campers that will use furnace more and be camping where longer stretches of poor solar conditions are possible might opt for a lot more storage and/or a lot more solar capacity. Or a generator
Someone that does 3 and 4 day weekends at rustic campgrounds a few hours from home might be fine with a single battery and nothing, or small solar, or a line to the trailer plug from tow vehicle alternator so they arrive fully charged and get home fully charged.
Some folks always camp with power and the battery mostly just provides lights when they go in the camper at a rest area and keeps the digital electronics powered when traveling.
Costco has often been mentioned as a good place to purchase batteries, I liked the one I purchased from Tractor Supply. I will say I clean the battery, top the fluid with distilled water, make sure to fully charge as checked with a meter once in a while and store for the winter with fluid topped, fully charged and on a battery maintainer. A practice that is the result of not doing it and having expensive battery die after a season or two, freeze or cooked dry are really a good way to get assessed a "learning tax" to replace the battery.