Calculating your Electrical Needs
Simplified analogy of physical properties of DC electricity
Warning: This post may hurt your brain a little bit. No pain, no gain.
If you plan to use electric devices on your travels, you’re going to need to include an electrical system in your build. As remote workers, our electrical system is quite possibly the most important aspect of our build after designing a bed that ensures a good night of sleep. You may be considering a basic electrical system that taps the existing engine battery (and using your cigarette lighter), or adding a portable power station, or going all in on an advanced DIY build. Regardless, before you design your system and shop for components, having a grasp on common terminology and understanding your battery needs is critical.
Let’s discuss some terminology that will guide you on your electrifying journey. The diagram above attempts to illustrate the principles of electricity using water as an analogy. The top diagram is illustrating electricity flowing through a wire, and the bottom diagram is using water in a pipe to illustrate the principles of electricity. As you read the defined terms below, refer to the diagram above for clarity (we hope). Admittedly, this stuff can be mind-bending.
Terminology:
Alternating Current (AC): Current that flows in alternating directions periodically through a wire. Devices that require a two or three-prong cord are wired in AC.
Direct Current (DC): Current only flows in one direction. Cars are wired in DC. The diagram above is a DC diagram, as the current only goes one direction.
Amperes (Amps): This is the measure of current in a system. The current is analogous to the flow rate of water in a pipe, which is defined by how quickly a specific volume of water is moving through a pipe. So, in a way, Amps are measuring the speed of electricity in a system measured in “volume”.
Resistor (Ohms): A resistor is a passive electrical component that is used to reduce current. The level of resistance is measured in Ohms - the higher the Ohms, the more resistance it adds to the system. In our water diagram, both the spigot and the bottleneck in the pipe act as resistors, as they reduce current.
Voltage (Volts): Voltage is measured in volts. In our water analogy, Voltage is the amount of pressure in the system (which should be considered a constant in our example), so increasing the voltage (pressure) will also increase the current (amps).
Wattage (Watts): This is the product of volts x amps, which is defined as P (power) in your system. Again, using our water analogy then - watts is equal to the pressure of the water multiplied by its flow rate. So increasing the volts or amps, increases the watts, and decreasing the volts or amps decreases the watts.
Amp Hour (AH): Not to be confused with the measure of amps above, Amp Hour is a theoretical measure that defines the capable charge of a battery by stating how many continuous amps could be drawn from a battery in one hour. This isn’t a practical measure for DIY van build needs, but what is of value is higher numbers means more stored power, while lower numbers means less stored power.
Watt Hour (WH): This is a measure of power over time (an hour) which is another way of stating capacity of a battery. A battery that is 12v and 100Ah (amp hours) is converted to watt-hours by multiplying the two. So 12V x 100AH = 1200WH. The larger the WH, the higher the capacity of the battery.
Hopefully the definitions above help you to understand some of the basic terminology used in the world of batteries and electricity. Still confused - read through the terminology a few times! (Still confused? Don’t worry - what you really need to know will be explained below). With these terms in mind, it’s time to turn our attention to understanding how much battery power you’ll need in your custom build.
What size battery do I need?
The more WH (Watt Hours) or AH (Amp Hours) a battery is spec’d at, the more energy it can store, allowing you to either run more energy hungry devices (such as an electric water heater) or run less hungry devices (such as charging your phone) for longer periods of time. A “hungry device” runs at higher Amps (and Watts). You’ll be able to determine which devices are the hungriest when examining the device spec sheet. To understand your anticipated power consumption, you’ll need to do some load calculations on the items you intend to use in your build.
This step will require you to do some research, as you’ll need to understand the Amp rating on every item you plan to run off of your power system. You’ll find it useful to use a load calculator to understand your battery needs. Feel free to download the calculator below by clicking the button below and going to File / Download to save the file on your computer. (Clicking takes you to a Google sheet to view and download. Nothing is downloaded to your computer unless you chose to do so)
AC/DC Converter - Note the Input & Output voltage
If you’re unclear whether a device is 12v DC or 120v AC, the spec sheet will supply this information. Generally, the dead give away is whether the device has a two or three-pronged brick-like plug attached to it, as pictured to the left (above on mobile devices) Note: some 12v DC devices will come with a converter plug so you can use your device at home or where ever 120v outlets are installed. As an example, our 12v DC wifi router came with a converter so that it can be used at home or at a business. It is more efficient to hard wire a 12v device into the 12v outputs of your battery than to use the 12v converter’s two or three-pronged cord. The converter will cause electrical loss due to inefficiencies which is released as heat, so your 12v device will run at slightly higher rates. Electricity in your build is a valuable commodity, so make sure to wire everything in the most efficient way possible.
If we look at the image on the left again (above on mobile devices), we can see that the converter notes it’s input and output voltage. This particular converter can be plugged into either a 120v or 240v AC outlet (input) and will deliver 12v power through the cord to the device (output). If you already own devices that you want to include in your build, looking at the output of any plug converter will tell you the voltage of that device.
We’re going to show you two examples below using the Load Calculator. The first one is for a 12v Direct Current (DC) device and the second is for a 120v Alternating Current (AC) device that we intend to use in our build. For the 12v DC device, our example is of a water pump. When we look at the online spec sheet for the water pump (many devices have the spec on a sticker on the device or on the converter as we noted above), we’ll see that the pump runs at 6.5 amps. We’ll add this value to the Amps @ 12v column.
Water Pump Spec above from the manufacturer
Add your 12v DC Devices to the top of the calculator
After you’ve noted the Amp draw (amps @ 12v column), you’ll need to specify how many hours per day the device will be drawing (consuming power) from your yet to be purchased battery. This is not an exact science, as a few factors apply here. A lot of devices will pull similar amps the entire time they are in use - such as charging your phone. A refrigerator on the other hand has a '“Duty Cycle” when it is cooling itself. It draws near it’s peak amps during the Duty Cycle, but continues to run at lower amps for the rest of the time it’s on - our fridge is on 24/7. A good rule of thumb for duty cycle devices is to input 8 hours, suggesting they are working at peak draw for a third of the day. Always assume your devices will be using power more frequently, rather than less frequently, to avoid underestimating your load.
We can see that once you’ve added your hours per day usage (0.5 hours), the calculator will tell you how many Amp Hours the device will use per day (3.25 hours). Continue to add your 12v DC devices to this section of the calculator. Feel free to add more horizontal rows as necessary so that you have enough rows to include all your 12v devices. Some 12v devices we are using in our build: roof fan, water pump, LED lights, composting toilet fan, and router.
Add your 120v Devices to the bottom of the calculator
Water Heater amperage Spec above from the manufacturer
Now it’s time to total your 120v AC devices. Enter them in the same fashion as you did for 12v devices by inputting the Amp draw of each device in the amps @ 120v column (We entered 12 amps for our hot water heater, as noted in the spec above). The calculator will multiply this column by 13 to give you an amps @ 12v value. In a perfect world, we would multiply 12v x 10 to get our 120v calculation, but inverters that convert the 12v DC power of a battery to 120v AC are inefficient, so it’s better to multiply by 13 to calculate for the loss. In our example, our hot water heater runs at 156 amps @ 12v.
Add values for the amount of hours per day that the device will be in use in the Hours Per Day column. In our example, we only turn our hot water on for approximately 45 minutes per day to get warm water for dishes or a shower - thus we noted .75 hours in the calculator cell. If we opted to keep the hot water on all day, we would have entered 8 hours (1248 Amp Hours!), as the hot water heater is one of those tricky Duty Cycle devices.
The watts column isn’t used in calculating Amp Hours, but is important for another reason. Your inverter (a device that will change your 12v DC battery power to 120v AC) will be rated for a total number of watts (Ours is rated for 2000 watts with a surge protection of up to 3500w). It is essential for you to understand how many watts you anticipate drawing from your 120v and 12v source, as they will have caps based on the equipment you buy. For example, our hot water heater runs at 1440w and our induction cook top runs at a max of 1800w. With our 2000w inverter, we can only use one of these devices at a time to not exceed our 2000w inverter, as running them both at the same time would draw 3240w. This means that we don’t heat water and cook at the same time. We have gotten into a routine that helps to avoid running these items synchronously to avoid drawing more power than our inverter is capable of delivering. Adding the Watts to your calculator will give you a sense of what items could trip or even damage your inverter if turned on at the same time.
Once you’ve added all of your 12v Direct Current and 120v Alternating Current devices to the load calculator, you can look at the total at the bottom. This number represents the best estimate for your daily power consumption. In a perfect world, you could just buy the battery with a capacity that most closely matches your load calculator total. Unfortunately, things are never that simple. The sun isn’t always shining, so you may want power reserves if you plan to recharge your power station with solar panels. Additionally, you should try not to deplete Lithium batteries below 15% capacity to increase their lifespan and AGM batteries should never be more than 50% depleted.
Beyond solar charging your batteries, it is also possible to charge batteries via your vehicle’s alternator and/or shore power. When using all-in-one power stations, you are going to want to refer to the manufacturer’s literature or FAQ for technical literature on how to do this.
Alternators charge your batteries at various rates, as they are also tasked with keeping the electrical system in your car running. They produce more electricity when the engine is revving and less when it’s idling. Some nomads add another alternator to their vehicle as a dedicated device to charge their batteries.
Shore power is a term used to describe outlets that are available at slips/docks, or in our case - campgrounds and homes. These outlets come in various amperage and will have different connectors for each to keep systems from connecting to outlets that could damage them. Sometimes we stay at campgrounds specifically to charge our batteries in cases where there has been little sunshine or we aren’t driving long distances to take advantage of alternator charging.
Ok. You are now armed with the knowledge to go battery shopping. We’d suggest shopping for a combination of batteries that will at least double (but triple is better) your load calculation total. Batteries are expensive, so you will need to weigh a few factors when shopping - such as cost, weight, size, and capacity. We personally recommend using lithium batteries over AGM. Check out this very in-depth article on why Lithium batteries are cheaper per watt and are better performers in nearly all circumstances.
Happy shopping!
Thinking of using a Portable Power Station for your DIY custom van build? Here’s how we did it.
***As a disclosure - John is trained as an architect and is in no way qualified to give electrical advice to others. In this article, we are sharing our DIY knowledge and experience with our own system. Do your due diligence to follow all safety standards and protocols. If those who are more qualified than us should find erroneous definitions or facts, we welcome corrections in the comments for the betterment of all. We will update the article as errors or better practices are discovered.
