In a vague kind of way, you can think of current (which is measured in amperes, also simply called “amps”) as the rate of flow of electricity, and voltage as the amount of force or pressure behind that flow. Kind of like a stream or a river flowing.
The basic rule is that the number of amps an appliance uses, multiplied by the volts that the appliance requires, equals the number of watts of power that it consumes per hour. If you multiply watts times the number of hours the appliance is running, you get the total amount of power the appliance used up during that time, measured in watt-hours. Abbreviations: amperes = A, volts = V, watts = W , watt hours = Wh.
Sometimes it is easier to think in terms of amps and amp-hours (abbreviated Ah) instead of watts and watt-hours. A lot of 12V batteries are labeled in amp-hours, and you could build a 12V battery bank out of multiple 100 Ah batteries.
To determine how much battery power you will need, you will need to do a calculation of your electrical load. As an example, suppose I use just three appliances: an induction cooktop, a fridge, and a water heater. Using example numbers, the number of watt hours they consume in one day is:
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Induction cooktop, plugs into the wall, so it requires 120V. Uses 1800 W when set to high heat. If used to cook for 1 hour each day, it uses 1800 Wh each day. But because the induction cooktop plugs into an inverter, and the inverter needs some power just to operate, let’s assume the actual draw from the 12V battery bank is 1.25 times 1800 Wh, or 2250 Wh. (An inverter is a device that converts 12V DC, say from a battery, to 120V AC, which common household appliances with 2 or 3 prongs on the plug can use).
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12V fridge, cycles on and off as the interior temperature fluctuates around the temperature setting, but uses 0.66 A on average. Run it 24 hours/day, so it uses 0.66 A * 24 hours = 16 Ah. Since it is a 12V appliance, total power usage in one day is 16 Ah * 12V = 192 Wh.
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Water heater: suppose we run this 120V water heater every day to initially heat 3 liters of water to near-boiling temperature, then maintain that temperature for 5 hours, at which time we turn it off for the rest of the day. The initial heating takes 45 minutes and uses 6.7 A. This operation uses 6.7 A * 120V = 804 W. Since it takes 45 minutes, it uses 804 W * 3/4 hours = 603 Wh. Then the water heater uses 30 W to maintain the water at near-boiling temperature for 5 hours; this uses an additional 30 W * 5 hours = 150 Wh. So the total operation uses 603 Wh + 150 Wh = 753 Wh. But since this water heater plugs into the inverter, which itself uses some electricity, we would actually consume 753 * 1.25 = 941 Wh from the 12V battery bank.
So these 3 appliances use a total of 2250 Wh + 192 Wh + 941 Wh = 3383 Wh every day.
If I had a 12V LiFePO4 battery bank that has 3383 Wh capacity, then this daily usage would completely drain my fully charged battery bank every day. So I would have to fully recharge it every day.
For recharging, one way is to get a DC-DC charger. The wires coming from this device connect to the starter battery in your vehicle on the one hand, and to your battery bank on the other. So when it is recharging your battery bank, electricity runs from your vehicle alternator to the starter battery, to the DC-DC charger, to your battery bank.
Suppose I have a DC-DC charger that charges at 60 A and 14.4 V. Then this charger supplies 60 A * 14.4 V = 864 W. So to replenish 3383 Wh each day, I would have to run this charger for 3383 Wh / 864 W = 3.9 hours each day. Since the DC-DC charger runs only when the vehicle engine is running, I would have to drive at least 3.9 hours each day, with the DC-DC charger running the whole time, to replenish my battery bank fully. (Note: for LiFePO4 batteries, the DC-DC charge schedule is more complicated than the constant amperage and constant voltage assumed in this example, so this example calculation is only an approximation).
Running more appliances each day would require driving longer, if the DC-DC charger is the sole method of charging the battery bank. Charging with solar panels on the roof while driving or in camp can help, especially if there are a lot of solar panels, but cloudy or rainy weather and parking in the shade at camp would reduce the power output of those solar panels.
Your list has some very power-hungry appliances: air conditioner, blender, induction cooktop, hair straightener. You would need a very large battery bank, compared to most van builds, and a powerful charging system to run it all. You would probably need a high output alternator that can provide a lot of charging power without overheating. If your DC-DC charger and solar panels do not provide enough charging power for your needs, then you would have to rely on a generator or shore power to make up the difference.
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