When I first started car camping and doing extended road-trips, the question of power needs came to the fore. Which naturally led to lots of excited research into what kind of batteries to buy, how to choose and setup solar arrays, what sorts of fuses and transformers I needed to get and where to fit them into the extended circuit.

But deep into this research, I hit a deadlock – my own limitation on how electricity works. This post is an attempt make sense of this complex field of electricity, with its confusing array of variables, and many interlocking concepts.

In this post we will not even get circuit diagrams or any of the planning tools out there. Instead, we take a step back into the Annie of it all (sorry, couldn’t help a reference to Community #AndAMovie), and examine some simple theoretical concepts that will help us to think of our power system in a cohesive, integrated manner.


To fully grasp the concepts in this post, it would be helpful for the reader to have an understanding of the basic concepts of Current, Voltage and Wattage. For a simple introduction, check out the “Before we begin” section of my writeup on calculating battery capacity when selecting a battery.

The Glue – Power Capacity in Kilowatt-hours (kWh)

Some of the confusion surrounding my forays into understanding how all the electrical devices would have to size up against each other was primarily for 2 reasons

  1. There was always a whole mess of interlocking variables to work with, and I couldn’t really place how the causes-and-effects worked
  2. From a practical perspective, all the devices I needed to use were of different amps and voltages, while batteries were usually denoted in amp-hours

Because of this confusing way in which the different variables tended to interact, I needed a glue variable.

What I realised was that in setting up an off-grid system, the most crucial and important centerpiece of the system is the battery. And so I settled on this glue: Kilowatt-hours, or kWh.

Kilowatt-hours (kWh)

Kilowatt-hours (kWh) is a a measure of capacity. While we measure water in litres or gallons, we can measure the amount of electricity using Kilowatt-hours (kWh).

Technically speaking, because when we measure electricity use, we are actually measuring energy, we can choose the unit of Joule (J) as our measure of capacity. In fact, each kWh is equivalent to 3,600,000 Joules. The problem is that in practice, using Joules as a measure offers several complications, including the fact that most businesses do not measure energy consumption in that way. They do so in Kilowatt-hours (kWh).

But why do we measure energy in terms of time?

See, kWh is actually code for the amount of Kilowatts consumed every hour. And this really tripped me up, because why would anyone want to measure power capacity in terms of time?

A probably inadequate reason is that Watt (W) is a measure of the flow of energy per second. If we want to get a quantity out of it, we have to measure it in terms of how much flow happens per unit time. It’s kind of confusing. But for our purposes, we just need to know how to calculate Kilowatt-hours (kWh) for it to be a useful starting point in our forays into electricity.


The equation to arrive at Kilowatt-hours (kWh) is straightforward:

For the power consumer: kWh = A X V X h / 1000
For the battery:        kWh = Ah X V / 1000

kWh = Kilowatt-hours
A = Amperes
V = Volts
h = hours of use
Ah = Rated Amp-hours

For a deeper understanding about how the battery calculations work, you can check out my other post about it.

Power capacity planner: Lights-on

Shameless plug: For those who want to skip the equationing, check out this off-grid power capacity planner I built: LightsOn. It takes what I have detailed out below, and automated most of the equation-handling away.

Spreading out the Tendrils

So we know that Kilowatt-hours (kWh) is the anchor for our off-grid system. It tells us how much energy we can store in our battery setup, and how long we can keep our devices running for. From this, everything else follows.

Time to charge or discharge

The time to charge or discharge your battery or batteries also matters. This is because this will tell us

  • How long our devices can last with the energy we have stored up.
  • How long we might need to sit at a power socket (maybe in our accommodation or cafe?) to charge the battery.

But how do we determine this? THe easy way is to

  1. Get the kWh capacity of the battery
  2. Get the kWh usage of the device we need to power, per day
  3. Divide the former by latter

For example, if we have

  1. A battery with 10 kWh,
  2. A phone that needs 2 kWh a day,
  3. We know that the battery will last us 10 / 2 = 5 days (if we are only using the battery for the device)

Tips for Amperage (A)

Amperage or amps is a measure of current. The essential thing to understand about it is that devices drawing amps can have a rating less than the power source. This is because amps are a pull factor – meaning that the device pulls the current out of the power source or battery, and only consumes what they need.

So just check that the rated Amps of the devices that will be used together at the same time are less than the battery’s or power source.

Important notice about Voltage (V)

Voltage however needs to be equal. A higher voltage will fry your device, while lower voltages will not provide enough electric potential for use, leading to performance penalties.

Voltage transformers

This brings us to the use of voltage transformers. These step up or step down the voltage from the power source or battery, into the correct voltage required by your device.

The essential thing to know here are

  • What are the input and output voltages, and if they match the power source and device respectively
  • The power loss due to the step up or down, read the manual


Different power sources and device use different types of current: Alternative Current (AC) or Direct Current (DC). To use both of these kind of devices, you need a AC/DC transformer.

You don’t always need solar power

This really depends on your travel style, how large your battery is, and whether you have reliable access to alternative sources of power. Some of these alternatives could be your car engine, or even cafes and intermittent places of accommodation.