Obviously, the group size matters for the starting battery, but does it for a house battery, I.e. do they need to match?
What matters for the starting battery is cranking amps. The group size is the battery's footprint and dimensions.
The starting battery and house battery do not need to match at all insofar as its footprint. The house battery can be whatever size you have room for. You will (or should) have the two isolated from each other using a battery isolator. The only thing you don't want to do is have the cranking battery and the house battery be two different types, or battery chemistries. You don't want a traditional wet cell (including maintenance-free) cranking battery and then have gel or AGM batteries for your house banks. Wet, Gel and AGM batteries each require different charging voltages and different charging regimens, so you cannot properly charge two different battery types with one alternator or charging source.
I will be installing a 1500-watt inverter to run a microwave and to keep items such as my laptop charged, as well as possibly a 22" HDTV. I'm OK with having the van running when using the microwave; for lesser loads, how much battery capacity should I have?
How much battery capacity you need is directly dependent on how many amp hours you will draw from the battery banks between full recharges. A guide to start with is a 24 hour period. How many amps will you draw in 24 hours?
AMPS x VOLTS = WATTS
Conversely, WATTS divided by VOLTS = AMPS
How many amps does the TV draw, at 12 volts? You'll be running things through an inverter, so whatever the draw is at 120 volts, you have to multiply that by 11 (10, plus 10% for the inverter loss). So, if on the back of your TV it says it draws 25 Watts at 120 volts, that's 120 volts / 25 = .2083 amps, but at 120 volts. To get it to 12 volts you have to multiply .2083 x 10 for 2.083, times 10% for the inverter loss gives you 2.293 amps.
If you watch the TV for 5 hours, that's 5 x 2.293 = 11.465 amp hours used by the TV during that 24 hour period. Then round up to at least 12, maybe 15, because you always use more than you think.
You need to do that for everything you will be running off the inverter. Also figure in every 12 volt load you have, like the 3 amps for the Fantastic Fan.
The laptop can be tricky, because it might say the input draw is 4.94 amps @ 19 volts. Well, 19 volts times 4.94 amps = 93.86 Watts. So, 93.86 Watts divided by 12 volts = 7.82 amps, times 10% is 8.60 amps from the battery bank to charge the laptop. Multiply that times how many hours you'll be charging it. If the laptop is fully charged and you operate it while plugged in to the inverter, figure out how many Watts or amps the laptop draws while plugged in.
There are no shortcuts to this. You need to be aware and to KNOW what your requirements are, and KNOW them as they change over time with you adding or subtracting the appliances and amp hours you consume.
Once you know how many amp hours you'll draw in a 24 hour period, double that figure. That will be the absolute minimum amp hour capacity of the battery bank. The reason is, you don't want to discharge the battery bank more than 50%. The more and more often you drop below 50%, the shorter the lifespan of the batteries will be.
If you want to run a microwave, you'll want to run the engine for sure, and even at that you'll really need at least a 200 amp hour capacity battery bank. Otherwise the high draw of the microwave will just put too much of a stress on the inverter and the batteries, unless you have high dollar true deep cycle batteries.
When you are not using the inverter, turn it off, because it'll draw 1-1.5 amps even with nothing plugged into it.
Another thing you will need is a battery monitor, like the Xantrex LinkLITE monitor. It keeps track of all amps in and all amps out and lets you know at a glance how much power you have left in the batteries. It's like a gas gauge for the battery bank. If you don't have a battery monitor, you'll be living by the words to live by above. You'll be killing your batteries. You will always discharge deeper than you estimate, and you will always undercharge them when recharging. Chronically discharging too deeply and undercharging them results in you calling your batteries pieces of crap, causing you to go out and buy bigger batteries or more expensive AGM batteries so they'll last a little longer while you mercilessly kill them.
