What the hell is a jigawatt? ~ Marty McFly
The Power Audit
The first step in designing an electrical system is to figure out how much power you are going to need. This is done by conducting a power audit, wherein you examine, in excruciating detail, every electrical device you intend to run on a daily basis in order to figure out how much power it consumes. The goal being to reasonably estimate how much power you will consume per day in amp-hours. This will enable you to plan an appropriately sized battery bank and help you determine how much solar capacity you need and the size of the inverter required to convert DC battery power to AC power.
In many cases you can look at the label on a device for the relevant information. However, for others it helps to have Kil-A-Watt or similar device that plugs in between the device you want to measure and the wall plug and measures how much power it consumes. Conducting a power audit also requires thinking about how you will adapt to living with a limited supply of power. A house or apartment with plenty of outlets and the power to feed them means most people don’t think about the impact of turning on an appliance or device, at least as far as whether it will work. I might never have thought about drying my hair while simultaneously cooking a batch of hot wings in the air fryer (especially since I own neither a hair dryer nor an air fryer this point.) While you could conceivably do that in your RV, having a system robust enough to supply two such high draw devices simultaneously would add considerable expense. It’s more practical to simply recognize that you don’t want to run more than one high draw device at a time, which would be a little more difficult for couples or families than a single fella like myself.
Outside factors will obviously also have an impact on your power audit. Different parts of the country receive different amounts of solar throughout the year. If you’re going to be spending time where it gets below freezing for long periods of time, you will need to think about how to prevent your water supply and batteries from freezing and how you are going to heat your living space. I have no plans to regularly visit freezing temperatures, so while I’m going to have heating in my fresh and grey water tanks just in case, I didn’t bother to figure them into my power audit. Additionally, my utility cabinet is small enough that the waste heat generated from the inverter and other devices, combined with the excellent insulation of my locker doors, should keep my batteries well above freezing.
As it turns out I have reasonable power needs. I rather like kicking it in the dark and I typically have no more than one LED light on at a time at night and other than a few computers and an absurd number of monitors, I use very little power other than A/C during the summer months. The same goes for heat. I wear shorts from March through December and only use my furnace December through March. To begin with, I’m not even going to install a heating system in my truck and I’ll be re-purposing the existing heat pump to feed my water heater (it will still have a normal heater in the cab, once I fix it, that is.) If I do install heat at some point, it will be a small diesel heater that doesn’t use a lot of electricity anyway. Radiant floor heating would appeal to me, except I can’t afford to loose a millimeter of headroom that raising the floor would require.
Once you have identified all sources of power consumption, it’s useful to go back through the list and consider how you will actually use them. For example, at present, I brew a pot of coffee every morning and leave the coffee maker on for roughly 2 hours as I make my way through it. I’m not about to give up coffee, but I am planning to pour my freshly brewed pot into a thermos so I can turn off the coffee maker and reduce that 2 hours down to 10-15 minutes. Another example is how many lights I need to power at any given time. The module in my truck has 6 existing halogen light fixtures rated at 30W each. As they’re already 12V, I’m simply going to convert them to LEDs rated at 3W each. Even then, I can’t imaging ever having all six on at one time, which is why I only listed three in my power audit. All of my external lockers are also fitted with dual LED strip lights. The vast majority of the time time, I don’t expect to use these for any meaningful length of time so I didn’t even bother measuring or including them in the audit.
Here then, are the results of my power audit:
AC Loads
Appliances, TV, etc. Anything that plugs into a household wall outlet that doesn’t have a “wall wort” DC adapter (or that doesn’t run continuously all day- see below.)
| Item | Watts | Amps | Minutes / Day | Hours / Day | Amp Hours / Day |
|---|---|---|---|---|---|
| Coffee Maker | 900 | 75 | 15 | 0.25 | 18.75 |
| Air Fryer | 1500 | 125 | 10 | .16 | 20.83 |
| Induction Cook Top | 1800 | 150 | 15 | .25 | 37.5 |
| Monitor | 30 | 2.5 | 480 | 8 | 20 |
| Water heater | 1500 | 125 | 30 | .5 | 62.5 |
| Total | 159.58 Ah |
DC Loads pretending to be AC
Laptops and other things that plug into a wall outlet, but have a wall wort or small box somewhere on the power cord that converts the 110V AC power to DC.
| Item | Output Volts | Output Amps | Watts | DC Amps | Minutes / Day | Hours / Day | Amp Hours / Day |
|---|---|---|---|---|---|---|---|
| Laptop-1 | 18.5 | 4.6 | 85.10 | 7.09 | 600 | 10 | 70.92 |
| Laptop-2 | 16 | 4 | 64 | 5.42 | 30 | .5 | 2.71 |
| Total | 73.63 Ah |
DC Loads
12V devices. LED lights, USB ports, fans, etc.
| Item | Qty | Watts | DC Amps | Minutes / Day | Hours / Day | Amp Hours / Day |
|---|---|---|---|---|---|---|
| LED lights | 3 | 3 | .75 | 120 | 2 | 1.50 |
| MaxxAir fan (high) | 1 | 56 | 4.67 | 120 | 2 | 9.33 |
| MaxxAir fan (low) | 1 | 3 | .25 | 240 | 4 | 1 |
| Water pump | 1 | 96 | 8 | 30 | .5 | 4 |
| Fan | 1 | 9 | .75 | 120 | 2 | 1.5 |
| Exhaust fan | 1 | 3 | .25 | 120 | 2 | .5 |
| Desk lamp | 1 | 3 | .25 | 360 | 6 | 1.5 |
| Reading lamp | 1 | 3 | .25 | 120 | 2 | .5 |
| Total | 19.83 Ah |
Single Charge DC Loads
Phone, tablet, earbuds or other rechargeable devices that are typically charged once or twice a day.
| Item | Battery Size (Wh) | Charges / Day | Amp Hours / Day |
|---|---|---|---|
| iPhone | 11.3 | 1 | 0.90 |
| iPad | 32.4 | 1 | 2.57 |
| Bluetooth speaker | 4400 | .5 | 2.2 |
| Total | 5.67 Ah |
All Day Loads
Things that will run continuously throughout the day or cycle on and off automatically. Refrigerator, A/C unit, water pump, cellular hotspot, etc.
| Item | Qty | Amp Hours / Day |
|---|---|---|
| 24V Refrigerator | 1 | 28 |
| Cell booster | 1 | 16.3 |
| Cell hotspot | 1 | 16 |
| WiFi router | 1 | 15 |
| RaspberryPi computer | 1 | 24 |
| Total | 99.3 Ah |
Adding all that together and plugging into one of the many online power audit calculators gives me the following recommendations:
| Total Usage Per Day | 359 Amp hours |
| Recommend Lithium Battery Bank | 400 Ah |
| Recommended Solar Panel Array | 800 Watts |
| Recommended Inverter | 3000 Watts |
While that gives me a recommended size, I also wanted to build in extra generation and storage capacity, primarily to allow for the addition of a mini-split AC/heat pump. I could have figured that usage into the power audit, but because I basically have no reference for how I would use it since I don’t know how hot or cool my truck will get, or even if I’m for sure going to add it I didn’t feel that was necessary to include. In addition, I also desired some extra capacity for the future without having to worry about retrofitting more solar or battery capacity into the design. Finally, after all that, I had to look at the physical space both of my roof and the utility cabinet where all the electrical gear will be installed. That required selecting specific solar panels and batteries that fit my installation, which then changed the total solar generating and battery bank storage capacities. For example, I have a huge, flat and mightily strong roof and other than leaving enough space for a couple of deck chairs, I wouldn’t have much use for most of that space. Therefore it made sense to me to plop a couple of big solar panels up there. Similarly, two 175 Ah batteries probably weren’t quite enough and three wouldn’t allow me to have a 24V battery bank (at least as far as I understand it, I could be wrong on that), so I doubled that to four batteries. Even though it’s more storage than I probably need, it means I can size the rest of the system components appropriately, whereas if I later went to add extra capacity that might mean having to upgrade the breakers, fuses, wire gauge, etc. Extra battery capacity also means I can weather longer periods of cloudy days and not be as obsessed with optimizing sun exposure.
I then took all those factors and came up with the following design requirements:
| 24V Lithium Battery Bank | 350 Ah* |
| * this is equivalent to 700 Ah @ 12V | |
| Solar Panel Array | 1200 Watts |
| Inverter | 3000 Watts |
In my next post on electrical I’ll dive into the gory details of the system design.