It may seem overwhelming at first, but all we really need are
The primary goal is how to figure out how much power you will require on a daily basis; this is called the daily power requirement. We will work through simple examples step by step and end up with watt-hours (Wh, see below).1)
Watts are only half the story. We need to know how long the you will run the doodad on average each day.
NOTE: some loads, even if plugged in 24/7, cycle on and off. A good example is a compressor refrigerator. A common rule of thumb is that a fridge will run the compressor ~1/3 of the time. So you would use 8 hours instead of 24 hours for that load. (24 / 3 = 8).3)
It may be easiest to do this in a spreadsheet where you can see all the loads at once, and twiddle numbers to see how it affects your daily power requirements. Here's a simple example:
The numbers in green were calculated automagically by the spreadsheet.
Aside: if you haven't made or used a spreadsheet before, here is a beginner's intro on youtube. Many computers come with a spreadsheet program (like Excel) and there are free ones like LibreOffice Calc and free online ones like Google Sheets, etc. Google Sheets was used for the screenshots in this article.
The more accurate your estimate the easier it will be to build a system that meets your needs on your budget.
Individual loads in the van will be either
Loads run off inverter will draw more power because of inverter losses4). The inverter specifications will say exactly what this the efficiency rate is, but for our purposes we will assume 90% (i.e. 0.9).
To get a more accurate estimate when can enter this efficiency rating into the spreadsheet, divide loads into DC/AC, and let the spreadsheet do the work for us:
The number is yellow is the inverter efficiency. I typed that in one spot so the spreadsheet could refer to it as often as it needs to.
A couple observations:
Sometimes a load doesn't use much energy (Wh) because runtimes are short, but still consumes a lot of power (W). An example might be an electric coffeemaker.
It might run for only 5 minutes a day (1200w / 60 minutes in an hour * 5 minutes) = 100Wh. But during that run it is pulling ~118A (1200w / 0.85 inverter losses / 12v). So the entire system (batteries, wiring, inverter) has to be sized to that 118A load thar runs only 5 minutes a day. Do you love that Keurig enough to buy extra batteries, fatter wire, and a bigger inverter to run it?
Now that we assessed our daily power and max current requirements we can think about