Gear Index

10 Things To Know About Solar Power
Tutorial Part 10 of 10: System design ABC's

Since you can't simply plug any device into any solar panel, and solar panels do not store their own power, and daylight is known to change over the course of a day, you need to design your power system according to a few basic rules so that you have the power you need. When you need it!

You've got a long list of gear that you need to keep charged while out on your expedition. Digital cameras, media storage device, GPS batteries, radio batteries, satellite phone, headlamps, the all-important iPod... have I missed anything? You are looking for a solar panel or solar charger kit that can provide power for all this gear, without weighing down your pack too much.

How do you decide what solar charger equipment you need?
The answer is... you need to do a system sizing. A simple set of calculations that will point to the size & type of solar product you will need. The steps are simple: add-up the power needed by your electronic equipment, sort out how many hours each day they need to run, choose equipment to match. So, let's get started, and we will use an imaginary system to help illustrate.

Step 1...
First, we calculate your total power demand, and choose a solar panel or solar charger to match. This does NOT mean that you need a 75 Watt solar panel to operate a 75 Watt computer. Rather, we will collect the solar power all day, and store it in a battery pack so that we will have enough juice to run the computer for up to several hours any time of day. To illustrate, we will design for a system to power a digital camera, iPod, satellite phone, and 4 AA batteries.

We will assume that the digital camera will need charging every other day, the iPod every day, the satellite phone every day, and a set of 4 AA batteries every 3 days.
Here is the list of items and the daily power demands...

Camera : 5 Watt-Hours per day
iPod : 3 Watt-Hours per day
Sat Phone : 10 Watt-Hours per day
Four AA Batteries : 5 Watt-Hours per day

Step 2...
The Camera power demand was 10 Watt-Hours to fully recharge, but since we are only planning to charge it every other day, the power demand gets divided in two. Same situation for the AA battery set... fully recharging the set would require 15 Watt-Hours, but since we only need to do this every 3 days, the daily demand is one third as much (ie 5 Watt-Hours).
Our total is now 23 Watt-Hours per day, and we must remember to store enough power to cover this demand.

We will be conservative when estimating the power production from solar panels, because it is nearly impossible to get optimum exposure during expeditions. Either it is inconvenient to deploy the panel during the peak time of day, or it is strapped to your pack, but you are walking south with the panel facing north. We will use a number for sunlight gathering over the course of the day that results in efficiencies that are half of what a roof-mounted solar power system might be.

So, our number will be '3' peak-sun-hours for our example system, because we will be using it in June & July. If we were to use it in October or March, we might find '2' is a more realistic number. Solar insolation maps will show as much as 6 peak-sun-hours for our imaginary west coast location, but again, we are going to play it conservatively and use a more realistic '3' for our hiking expedition.

Step 3...
Dividing our power demand of 23 Watt-Hours by 3 Hours we get 7.666 Watts. Looking around the industry, we can see that PowerFilm offers a 7Watt rollable panel, or we could get a 10 Watt folding panel. We could also find an 8 Watt rigid folding panel, but since we are going to be out for 2 weeks, and a rollable panel would be difficult to transport (they are really best for boats & kayaks), we will opt for the 10 Watt folding PowerFilm and be grateful for the extra power.

We want to have at least 1 day of battery pack autonomy, because our west coast weather is known to be unpredictable. So our battery pack needs to be at least 23 Watt-Hours in size. It also needs to be able to support our digital camera (7.4V Canon SLR), USB for the iPod, and offer a 12V socket for charging the satellite phone. Based on this, we will go a bit oversize and choose the Voltaic Generator battery pack. It is 50 Watt-Hours, but connects easily to just about everything at voltages from 3 to 20V.

We then will look into getting a DC battery charger for the Canon SLR, a DC charger for the AA batteries, and make sure we have the DC car charger for the satellite phone. This way we can avoid using an AC inverter, thereby saving about 15% of our generated power.

That's it, we've just outlined a simple, self-sufficient power system for all our equipment. A packable folding solar panel, a solar storage battery with USB & other connectors, and DC accessories for AA battery charging.

Thanks for reading through the entire solar power tutorial. We hope that it helps you to understand a bit more about the technology and how to use it.

Please feel free to contact us with any questions regarding your solar project... mail@modernoutpost.com

Next...Back to the tutorial index

Solar tutorial intro
Part 1: The Balance of Power
Part 2: Power & Portability
Part 3: DC not AC
Part 4: Clouding the Isuue
Part 5: Covering all the Angles
Part 6: Batteries not Included
Part 7: Amps & Volts
Part 8: Making Connections
Part 9: Weatherproof, but...
Part 10: System Design ABC's