Eletrical System

Hey everyone,

I recently got a 2019 Promaster 3500 Ext, and I’m diving into some DIY projects. However, I’m a bit lost when it comes to designing the electrical system because I don’t really understand voltage stuff :sweat_smile:.

I’m looking for some guidance on setting up my electrical components, especially considering the appliances I want to power:

  • Dometic Penguin II Low Profile Rooftop Air Conditioner
  • Fridge
  • Water heater
  • Maxxair fan
  • Diesel heater
  • Water pump
  • Projector
  • Blender
  • Hair straightener
  • Induction cooktop
  • 3x LED strips

Any suggestions on what kind of system I should go for would be greatly appreciated!


That’s quite a list… I hope you’re not planning on running all that on battery power…


"Count your blesssings, for they are many." ~ Unknown

I need suggestions. Idk :woman_shrugging:
I am not gonna use all the products at once surely but what would be other options?


With your “wants”, I’d want a generator, or constant access to shore power.

"Count your blesssings, for they are many." ~ Unknown

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:

  1. 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).

  2. 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.

  3. 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.

Hey Melis, appreciate your explaination.
So I am thinking to get propane stove instead of induction and I reduced the list and listed the system as below. Please let me know if I am missing anything
Thank you!

  • Dometic Penguin II Low Profile Rooftop Air Conditioner
  • Fridge
  • Water heater
  • Maxxair fan
  • Water pump
  • 3x LED strips

And the system I am looking to buy is

Renogy 800W 12V General Off-Grid Solar Kit Solar Panel: 4*200W 12V Rigid Solar Panel, Charge

Controller: Rover 60A MPPT W/ LCD & B2 Module,

3000W 12V Pure Sine Wave Inverter Charger W/ LCD…

2x 12V/24V/48V 200Ah Core Series Deep Cycle Lithiu…

I am not sure do I still need DC-DC charger?
Please let me know if I have to add up anything else.

Thank you

Anilafcc14, it helps a lot to have fewer high amperage appliances on your list, and 800W of solar panels and 400Ah of LiFePO4 batteries is a very capable system. I would suggest calculating your electrical usage on a typical day in the van and comparing it to how much you expect to be able to recharge your batteries from your solar panels. Then do the same for a “worst-case” scenario where you are using more power than usual, and you are in a long stretch of rainy days. It’s a balancing act between usage and recharging, so even if your net loss each day is 100 Ah because you used that much more than you could recharge, a fully charged 400Ah system would last 4 days before it is completely depleted.

Also consider where you will be traveling, since some locations have more sunny days each year than others; some are more forested; and some are nearer the equator.

So depending on your daily usage and recharging ability, you might be able to do well without a DC-DC charger. I would suggest getting a battery monitor so that you can see at any time how much capacity is left in your battery bank, and recharge when needed.

Also factor in wiring, connectors, wire loom, fuses, fuse holders, 12V fuse blocks, circuit breakers (if your design has any), and tools for building the electrical system. 60A of charging power requires pretty thick cables, which are expensive, bulky, and don’t bend around sharp corners easily. Consult a wire gauge chart like this one:

and make sure you have the right thickness of wire - preferably one size thicker for safety - and that your connections are tight, to help prevent electrical fires.