[GUIDE] Off-Grid Power Estimate

Note: This is a technical subject with a lot of information, and a fair bit of math. It’s never too early to plan for your electrical needs, but you may not have all of the information to get all the way through this exercise. Don’t give up! Just put it on hold until you have the information you need. Do it in a few passes, revise it as you go, and don’t worry if it’s a work in progress!

When we talk about being off-grid, we are referring to the time spent without the need for certain amenities. These amenities include but are not limited to sources of electricity, water, fuel (for vehicles, generators, etc), food, cell/satellite service and waste management. During your travels, you will find yourself in situations of varying “off-grid-ness” where you have access to some, but not all of the above. You may also want to live off-grid, even when you are somewhere where you do have access.

For the purposes of this post, we’re specifically talking about being in off-grid situations where you are without access to a source of electricity (the electrical grid) and want to be able to use electrical appliances and devices that require a charge.

When putting together an off-grid electrical system, it can be difficult to know what size and types of components you will need. After this post you’ll be able to estimate the capacity of the electrical system you need to power your off-grid adventures. We’ve created a worksheet that you can use to calculate the sizes you will need based on what you’re powering. It is available for download for free at the end of this article.

Cover.png

WHAT THIS POST DOES NOT COVER

There are many, many topics involved in the design, installation and use of an off-grid electrical system. Some things we will cover in later posts include:

  • Science and theory behind off-grid / vehicle-based electrical system design

  • Specific system designs

  • Specific product comparisons

  • Charging batteries via an alternator, shore power or generator

  • Installation of off-grid / vehicle-based electrical systems

  • Maintenance of off-grid / vehicle-based electrical systems

AND THE VERY IMPORTANT AND OFTEN OVERLOOKED TOPIC:

  • The size and placement of the components that comprise an off-grid / vehicle-based electrical system. You will have to determine if there is enough room to safely and sensibly install each of the items in your system. This will be based on the size and shape of your rig, as well as the size and features of your electrical system. You cannot fit the same system from a Class A RV onto a teardrop trailer. At least not yet…

WHERE TO START? WHAT TO LEARN?

Electrical systems can be intimidating to work with due to numerous safety concerns and the technical nature of the components involved. While you should only do work that you’re comfortable with, and always understand the risks you’re taking, you certainly do not have to be an expert in order to design and install your own electrical system. While you can get through this exercise without an in-depth knowledge of electricity, it will be helpful to be familiar with the following concepts:

  • How chemical batteries work

  • How Photovoltaic (PV) solar panels work

  • Difference between AC and DC power

  • How direct current circuits work

  • How alternating current circuits work

  • How an inverter works

BASICS FOR THIS ARTICLE

  • Loads - electrical appliances, devices and components that use electricity. Lights, your cell phone, fans - all loads.

  • Power - measured in watts (W) - the rate at which energy is being used by a load, or supplied by a source 

  • Current - measured in amps (A) - the rate of flow of electric charge

  • Voltage - measured in volts (V) - makes electric charges move 

  • Battery Capacity - measured in amp hours (Ah)

  • Battery Capacity - sometimes measured in watt hours (Wh)

And for our purposes:

  • Power (W) = Current (A) x Voltage (V)

At its most basic level, an off-grid electrical system will have:

  • Solar panels that COLLECT electrical energy and recharge the batteries

  • Batteries that STORE electrical energy

  • Loads (electrical appliances) that USE the stored electrical energy

  • Several “in-between” components to help with the collecting, storing and using of electrical energy

Solar Power Diagram-01.png

START WITH YOUR LOADS

The first step in designing your electrical system is determining what things you want to power while you’re off the grid. We are going to make a list all of the loads, calculate (or estimate) their power usage, and add them all up. For each device, we will take the power it uses and multiply that by how long you anticipate using it each day to calculate the total power you will want to have access to while off-grid.

110V AC APPLIANCES:

These are your normal, everyday appliances that you plug into the wall at home (or wherever you have standard 110V outlets). Toaster, hair dryer, Vitamix, etc. These appliances rely on Alternating Current (AC) power in order to run. Our system will need an inverter in order to power these loads.

What we need to know:

  • Power required (W) for each of these loads. It will be listed on the device itself, or on its packaging.

The handle of a hair dryer (not mine). Power (W) = 1875W

The handle of a hair dryer (not mine). Power (W) = 1875W

110V APPLIANCES WITH AC/DC ADAPTER:

These appliances plug into a standard 110V outlet but actually run using DC power. They include an adapter (brick) that converts the AC power into an equivalent DC power at a different voltage. In many cases, it may be possible to run these appliances with DC power (but may require a different adapter). These include printers, monitors, etc.

What we need to know:

  • Output Voltage (V)

  • Output Current (A)

You will find this information on the adapter. We will use this info to calculate the amount of power required for these devices.

The AC/DC adapter (brick) from a laptop charger. Output Voltage (V) = 19.5V Output Current (A) = 10.3A

The AC/DC adapter (brick) from a laptop charger. Output Voltage (V) = 19.5V Output Current (A) = 10.3A

DC APPLIANCES:

These appliances are designed to run off of Direct Current (DC) power, usually at 12V or 24V (or both). There are many appliances such as refrigerators, fans, lights, etc. that are available in DC compatible versions. These can be run directly from the off-grid batteries (with proper wiring and fuses in between).

What we need to know:

  • Power required (W) for each of these loads.

It will be listed on the device itself, or on its packaging. You will find it listed in one of the following formats:

  • Voltage (V) (if this is missing we will assume 12V)

  • Current (A)

OR

  • Power Required (W)

The label on the back of a portable document scanner. Voltage (V) = 6V Current (A) = 1.5A

The label on the back of a portable document scanner. Voltage (V) = 6V Current (A) = 1.5A

APPLIANCES WITH BATTERIES:

These are pretty easy to identify. If you’re going to use your off grid system to recharge the batteries of any devices we need to add those in too. This includes cell phones, camera batteries, bluetooth devices etc.

We will add up how many times they need to be charged per day (it might be twice a day or once every 4 days = 0.25 times per day).

What we need to know:

  • Battery Capacity

You will find it listed in one of the following formats:

  • Watt hours (Wh)

  • Amp hours (Ah)

  • Milliamp hours (mAh)

This is usually available somewhere on the device or the actual battery. If not, you can find it in the manual or on the manufacturers website.

The back of a generic Canon battery. Battery Capacity (Ah) = 2000mAh

The back of a generic Canon battery. Battery Capacity (Ah) = 2000mAh

AVERAGE USAGE

To finish determining how much electrical storage and power you will need while you’re off-grid, you will need to estimate the average daily use of your appliances. This can be a rough estimate, because you will rarely use all of your appliances and devices over the course of a single day, but you want to be prepared for some extreme cases (needing lots of power in one day, or not having any sun for several days to recharge your batteries).

TASKS:

  1. Make a list of each electrical device you want to bring with you and power while off the grid.

  2. Find the power required for each device and fill in the information required for each type of load.

  3. Enter the estimated time (in minutes) you will use each device per day.

CALCULATE TOTAL AMP HOURS USED PER DAY

For each AC and DC device:

(DC Amps) x (Hours Used Per Day) = Amp Hours Used Per Day

For each battery powered device:

(Battery Size mAh)x (Charges Per Day) = Amp Hours Used Per Day

Add all of those up we will have “Total Amp Hours Used Per Day” AKA “the total.

CALCULATE RECOMMENDED SYSTEM SIZES

LiFePO4 Batteries

If you are considering using a LiFePO4 (Lithium Ion / Lithium Iron Phosphate) battery bank:

  • Round up your total to the nearest 100Ah.

  • For example, if your total is 312Ah, round up to 400Ah (amp hours) of LiFePO4 batteries.

AGM Batteries

If you are considering using an AGM (Absorbent Glass Mat / Lead-Acid) battery bank:

  • Round up your total to the nearest 50Ah and multiply by 2.

  • For example, if your total is 312Ah, round up to 350Ah x 2 = 700Ah (amp hours) of AGM batteries.

Solar Panels

To determine the total wattage of solar panels you need:

  • Round up to the nearest 100Ah and multiply by 2.

  • For example, if your total is 312Ah, round up to 350 x 2 = 700W (watts) of solar panels.

Inverter

To determine the size of inverter you need:

  • Multiply the maximum wattage of 110V device by 1.2 (for efficiency loss) and round up to the nearest 1000W.

  • For example, if your max wattage device uses 1200W, multiply by 1.2 = 1440, round up to a 2000W inverter.

Download The Featherbuilt Worksheet

Annotation 2020-01-24 180705.jpg

FEATHERBUILT Off Grid Power Estimate Worksheet

1. Click this link to open the “view-only” Google Sheets document (no login required)

2. “File - Make a Copy” to start your own power worksheet

Featherbuilt Worksheet Formulae…

Let me know how it goes…

This template has been developed to be flexible enough to meet the needs of any type of rig. If you find any issues or have trouble using the spreadsheet, let me know and I’ll do my best to help out.

Hint: If you send me your completed worksheet, I’ll post some examples of van, truck and trailer estimates for different off-grid scenarios. Feel free to comment below with any questions, issues or ideas. Looking forward to seeing what you come up with!

Thanks for reading!

Footnotes:

The Featherbuilt Off-Grid Power Estimate Worksheet is an expanded version of the work done by Nate and Stephanie Yarbrough at Explorist.life. They have many helpful articles for electrical systems for campers.