Watts = Amps x Volts
Electrical systems can be confusing. For me, the primary source of this confusion is the differentiation between watts, amps and volts.
To remedy this confusion, I convert measurements of power consumption and storage to Watts and Watt Hours. Watts account for both Amps and Volts, because:
Amps x Volts = Watts
So, if we’re talking Watts, there is far less ambiguity given that both amps and volts are accounted for.
Watts
A watt is a measure of power consumption or production. Our small USB fan pulls 1 watt on low and 2 watts on high. We have a little LED light that pulls 1 watt of power. Our Magic Bullet pulls about 140 watts of power. Our WeBoost signal booster pulls 8 watts of power. All of these are examples of devices that have no built in battery, so they use energy from an external source the entire time they are powered on.
On the energy production side, the energy solar panels collect can also be measured in watts. Our Boulder 100 panel is rated to produce 100 watts, but in reality it generally produces around 65 watts of power in full sun.
Watt Hours
A watt hour is a measure of stored or potential energy. One watt hour of power is the energy required to power a device that pulls one watt for one hour.
Remember our little USB fan that pulls 1 watt on low and 2 watts on high? With one watt hour of power, we could run our fan on low for one hour. On the high setting we could power the fan for half an hour.
What About Amp Hours?
People often speak of their battery setups being something like a “10 amp hour” system. To make sense of what they’re saying, we have to know the voltage of their system. The voltage of their system could be anything. Typically, batteries are 12 volts, but this isn’t always the case.
For sake of argument, let’s say the 10 amp hour system is at 120 volts. This equates to 1200 watt hours of power. If the system were 12 volts, it would only have 120 watt hours of power. This is why speaking in amp hours can be so troublesome. Even though both systems are 100 amp hours, one of them actually has ten times the potential power.
To give this some real life context, let’s say we have a 40 amp hour car battery and a 45 amp hour laptop battery. The car battery is 40 amp hours at 12 volts, resulting in 480 watt hours of power. While the laptop battery is 45 amp hours at 2 volts, resulting in 90 watt hours of power. Even though the laptop has higher amp hours, the actual potential power is far less.
Digital Device Battery Capacity
I also measure the size of my device batteries in watt hours. Most of the time, small device batteries are listed in milli amp hours, but you can find the watt hour equivalent if you look for it. I prefer watt hours to milli amp hours here as well, because the milli amp hour measurement does not account for the voltage of the system, whereas watt hour capacity accounts for both amp hours and voltage of the device’s battery.
Modern cell phones have 5-10 watt hour batteries, tablets 15-40 watt hour batteries, and laptops 50-120 watt hour batteries.
My 2012 MacBook air has a 50 watt hour battery, while Elsa’s 2015 MacBook Pro has a 80 watt hour battery.
Our Solar Setup
Batteries
A kilowatt is 1000 watts. Our lithium battery has a one kilowatt hour capacity, or 1000 watt hours. One of the nice things about lithium batteries is that they can be cycled very low without degrading the battery.
We also have a 400 watt hour absorbent glass mat or AGM battery. AGM capacities are a bit misleading though, because they can only be discharged to about half capacity. When discharged lower than about fifty percent often, AGM batteries will lose charge capacity. So, the effective capacity of our AGM battery is about 200 watt hours, though its rated as 400 watt hours. All that said, we have discharged this battery to twenty or so percent a number of times, and it still works fine.
We chose the Goal Zero batteries because they are simple. All the components of the system are in one box. Solar controller, inverter and battery all in one package. The interface is also super useful on these batteries. The charge percentage is clearly displayed, and they have an input and output reading, which can be toggled through watts, amps and volts. This simple screen has helped me to understand how the system works. Having the input reading is super helpful for positioning our panels too.
Solar Panels
We have two solar panels to charge our batteries. One is rated to collect 100 watts and the other 50 watts. Combined they are rated to produce 150 watts. In reality they typically produce around 100 watts on a sunny day. In our experience the goal zero panels collect about sixty percent of their rated capacity.
Assuming our two panels are producing 100 watts together, we could theoretically charge our 1000 watt hour battery from zero to full in ten hours. 100 watts for ten hours produces 1000 watt hours of power.
Our solar panels are not attached to the Scamp. We decided not to mount them to the trailer because we often park in the shade. The panels have a long extension cable, so they can be moved to wherever the most direct sun is. If we are in an area with lots of trees and shade, we can even take the battery out of the scamp and put it out with the panels to give us more reach. This is particularly helpful in the summer, when we like to park in shady, high elevation areas.
Using Solar Power
Some of our tools and devices have no built in battery. Some examples are our fans, magic bullet blender and WeBoost cell booster. These devices draw power consistently while powered on. The fans pull one or two watts, the magic bullet pulls 150 watts for short bursts, and the WeBoost pulls about eight watts. So, if we leave the WeBoost on for ten hours it will use around 80 watt hours of power, which equates to about eight percent of our 1000 watt hour battery.
Most of our devices have built in batteries of their own. Our BioLite lighting system, computers, iPads and tools all have batteries built in. We make sure to charge all these peripheral batteries when the sun is out and our big batteries are full. This way we aren’t wasting the energy provided by the sun.
How much power do you need?
The two things that require a lot of power are heating and cooling. If you want to run a furnace or AC, you’ll have to have a pretty serious solar setup to accommodate. Other than that, most things can be done with a moderately sized solar kit, as long as the appliances and devices used are efficient.
Our computers are the biggest draw on our batteries. My computer has a 50 watt hour battery. Since my computer is eight years old, the battery lasts about four hours while doing low level tasks like browsing the internet and using word processors. The battery lasts as little as two hours when doing processor intensive tasks like gaming or editing video. So the amount of power used by our computers really depends on the tasks we are using them for. If I wanted to game for eight hours one day, my computer would use around 400 watt hours of power. 50 watt hour battery, lasting two hours per charge, charged four times.
Elsa’s computer is larger and more powerful than mine is, and it’s about five years old. Her battery is about 80 watt hours. When she is editing videos her computer will chew through her battery in under two hours sometimes. So, if it takes two hours to consume her 80 watt hour battery, the computer is using about 40 watts per hour. In an eight hour day of editing she will use around 320 watt hours of power.
If both of us are working on our laptops, we burn through our energy reserves pretty quickly, especially in stretches of cloudy days. This is why we try to offload the majority of our work to our iPads. Our tablets have smaller batteries, are charged directly with DC power via usb and are remarkably efficient.
My iPad mini has a 20 watt hour battery and lasts around eight to ten hours depending on what I’m doing. Phones are similarly efficient, with five to ten watt hour batteries and full day run time.
Collecting Solar Power
Our ability to collect power is a big factor in the spots we choose. Trees, mountains and weather all play a part in how much sun we will receive throughout a day. The amount of power we collect plays a big role in how productive we can be, given that we both use our digital devices for work.
When vetting out new spots, we often use an app called Lumos to see how much sun we will get. It’s an augmented reality app that plots the path of the sun in the sky. We can hold the phone up and see where trees and mountains will interfere with solar collection, and determine how much of the day we will be able to charge.
As the day goes on, we typically move the solar panels every hour or two to keep them in perfect alignment with the sun.
Conclusion
I realize all of this can be massively confusing, the best way to understand how solar power works is to use and collect it yourself. To begin, make a list of all the electronic devices you want to use. If they have a built in battery, write the watt hour capacity next to it. If they continuously draw power, write down the power draw in watts. This will help you understand how much power you will likely need.
When you are looking at a system to buy, convert all measurements into watts and watt hours. This will make the whole process far easier to understand.