Power Consumption

  • by Hillary

Power consumption refers to all the DC and AC loads that are powered by the house battery. Accurate knowledge of what these items are and how much energy they will consume in an average day is required to properly size your system’s house battery (and from the battery size comes the required solar panel production). Let’s start with an example to show how this is done.

Contents

Sizing for DC Loads

Let’s say you have a 200 Ah battery that is connected to four LED puck lights that draw a total of 1 A of current. Since it is recommended with AGM batteries that you do not discharge below 50% of the total capacity, we can calculate 50% of 200 Ah is 100 Ah. To determine how long it would take to deplete the battery down by 100 Ah, just make the following calculation: (100 Ah) / (1 A) = 100 hours. Therefore, you could run the four puck lights for 100 hours before your battery would be discharged to 50% (which you shouldn’t go below if you can avoid it).

Knowing how long you can run the puck lights is not as useful as knowing how much battery you will consume during typical usage. The calculation is very similar, but let’s go through it again just for completeness.

The first thing you need to know (or reasonably estimate) is how long you will use the puck lights for each day. This can be a tricky thing to estimate, especially if you have never lived in a van before. Everyone’s needs will be different and, of course, will vary from day to day. But let’s assume that you use the lights during each meal (assume 1 hr/meal) and then for 2 hours at night before going to bed. One thing I recommend when calculating usage is to err on the side of overestimating usage. In reality we don’t use our lights 5 hours/day on average, but it is better to be oversized on your battery than undersized.

So, if we use our lights for 5 hours per day, and the lights draw a total of 1 A, then the battery usage will be (1 A) × (5 h) = 5 Ah per day. That means that the minimum battery size to avoid going below the 50% SOC for AGM batteries would be 10 Ah, assuming that you have properly sized your solar system array to recharge your battery each day.

Now, you probably aren’t going to run just four puck lights inside of your campervan, so the actual battery sizing calculations will be a bit more involved than the simple example I have just given. In order to get a meaningful estimate you will need to determine the current draw of every electrical item you plan to install in your van. You will also need to come up with an educated guess of how long each appliance will be running each day. Furthermore, if you plan to buy an inverter to run AC appliances there is an extra step involved in the calculations. This is covered in the next section.

Sizing for AC Loads

There will probably be some items that you want to use in your van that use alternating current (AC) power. AC is the type of electrical power that is delivered through the electrical grid to homes and businesses. It is what most appliances in your home run off.

Sizing the Inverter

AC runs at 120V, while most van power systems are 12V DC. In order to convert from 12V DC to 120V AC a device called an inverter is required. Just like the house battery, the inverter must be sized for the AC items that you are planning to run inside of your campervan. To do this, find the power rating (generally expressed in Watts or kiloWatts) of each of your AC items that you plan to use. Next, ask yourself if you will be using any of them at the same time (for example, we use our electric kettle and coffeemaker at the same time to brew our coffee and tea in the morning). If so, add the power rating of those items together. Whatever item (or items if they are running concurrently) has the highest power rating determines the size requirement for the inverter. The maximum power of the inverter must be higher than the power rating of this device or devices.

Energy Usage Calculations for AC

After sizing the inverter, it is also necessary to determine how much of the battery each of these appliances will use on an average day, just as we did as an example earlier for the DC puck lights.

An AC appliance that draws 1 A from the inverter will require many more amps from the 12V battery. If the inverter was 100% efficient in converting from 12V DC to 120V AC, the conversion would be pretty simple: just multiply the amps of the AC appliance by (120 V) / (12 V) = 10. However, inverters are not 100% efficient. A high quality inverter might be 90-95% efficient in converting from DC to AC. To compensate for this, the amps of the AC appliance must be multiplied by (120 V) / (12 V) / 0.9 = 11.

So let’s do another quick example. Assume we want to run our Instant Pot in the van. A typical meal cooked in the Instant Pot takes about 20 minutes including heat up time. Looking up the amp rating online, our Instant Pot is listed as 10 A max. It will not use 10 A during the entire cooking process, but to be conservative let’s just assume it will. First, we convert from AC current to DC current:

IDC = ((120 VAC) / (12 VDC) / 0.9) × IAC

IDC = 11 × (10 A) = 110 A

Now that we have the DC current drawn from the battery, the procedure is the same as we did with DC loads. Multiply the DC current by the time it will run to get the energy consumed in Ah. This is (110 A) × (⅓ h) = 36.7 Ah. Therefore, to run an Instant Pot for 20 minutes each day, the minimum battery size is found by dividing by the maximum depth of discharge for the battery type:

  • AGM: (36.7 Ah) / (50%) = 73 Ah
  • LFP: (36.7 Ah) / (80%) = 46 Ah

So now we have covered an example of how to calculate consumed Ah for a DC load and an AC load. In order to size the battery for your electrical system, this number needs to be calculated for all of the DC and AC loads that you plan on using in your campervan.

Everyone’s needs are different, but to give you an idea of what we included in our calculations, below are lists of DC and AC components that we use regularly in Dolly. In reality, you can probably leave out smaller items to simplify things. I didn’t include Hillary’s Fitbit charger when I did my sizing calculations.

DC Loads

  • Refrigerator
  • Vent Fan
  • Lights
  • Water Pump
  • Cellphones
  • Laptop
  • Portable Jump Starter
  • Fitbit and Smart Watch Chargers
  • Portable Air Compressor

AC Loads

  • Blender
  • Instant Pot
  • Coffeemaker
  • Coffee Grinder
  • Electric Kettle
  • Electric Grill
  • Electric Toothbrush Charger
  • DSLR Battery Charger
  • Portable Bluetooth Speaker

Next Steps

Earlier in this article I mentioned that I would be writing a detailed guide on sizing of the components in your electrical system. This includes both the amount of Watts you should have for your solar panel setup and the capacity of your battery (or batteries). Stay tuned! But in the meantime, if you have any questions do not hesitate to reach out to us. You can find our social media links at the bottom of the website.