Build a 40-Watt Solar Power Kit: A Step-by-Step Guide for About $200

The finished system: charge controller, fuse, battery, DC outlets and inverter. (Image: Lonny on Appropedia, CC BY-SA 4.0)

This guide builds a small 40-watt solar power system with 120 watt-hours of battery storage that can run both DC devices (through USB) and ordinary AC appliances (through an inverter). It uses parts that are easy to find and simple connectors, no soldering. The whole thing, tools included, costs about $200, depending on where you buy.

What you need

The design is meant to be flexible: read the notes on your parts and check they work together.

Wiring it up

1. Prepare the panels. Cut the alligator clips off both panels' red and black wires and strip about 3/8 inch of insulation from the ends.
2. Cut the panel leads. Cut 6 to 12 inches each of red and black 10-gauge wire, depending on how far the panels will sit from the controller, and strip all the ends.
3. Connect the panels in parallel. Join the three red wires (one from each panel, plus your new lead) in one lever nut, and the three black wires in another.
4. Fit the fuse. Cut the fuse holder's wire so one side is short, to sit near the battery, and push the 25 A blade fuse into the holder. A fuse has no direction.
5. Make the battery leads. Cut a black wire about as long as the fuse holder's, crimp a spade terminal onto the short end of the fuse wire and one onto the black wire, and put lever nuts on their other ends.
6. Connect the battery. Push the fuse wire's terminal onto the battery's positive (red) blade and the black wire's onto the negative. The battery is the one dangerous part: never let anything metal touch both terminals.
7. Connect the charge controller to the battery. Screw a new pair of red and black wires into the controller's battery terminals, then plug their other ends into the battery's lever nuts, red to red, black to black. The controller's screen should light up; set it for your battery type (for a lithium battery: type "Li", 12 V, charge voltage 14.4 V).
8. Connect the panels to the controller, screwing the panels' leads into its solar terminals.
9. Add the accessory plug, pushing its red and black wires into the free openings of the battery's lever nuts.
10. Plug in the inverter to the accessory plug's socket, and switch both on.

Step 3: the panels joined in parallel with lever nuts. (Image: Lonny on Appropedia, CC BY-SA 4.0)

Step 7: the charge controller connected to the battery, through the fuse on the positive wire. (Image: Lonny on Appropedia, CC BY-SA 4.0)

The complete system, running the inverter (panels not shown). (Image: Lonny on Appropedia, CC BY-SA 4.0)

Using it

Set the panels in full sun and tilt them toward it; in Arcata, California, in summer, the builders tilted theirs at 26 degrees. Charge a phone from the accessory plug's USB port, or plug an AC appliance such as a fan or lamp into the inverter. Don't draw more than the inverter can supply continuously, here 200 W, and check that the accessory plug is rated for it too.

The system in the field, its panels tilted toward the sun. (Image: Lonny on Appropedia, CC BY-SA 4.0)

How long will it last? Two examples, ignoring small losses:

• A 10 W LED light. In good sun the panels' 40 W runs it easily without touching the battery. At night the battery keeps it lit for almost 12 hours. Recharging an empty battery takes about 3 hours with the light off, or 4 with it on. In summer, a light like this could run with hardly any downtime.
• An 80 W box fan. It uses more than the panels make. In full sun it runs for about 3 hours before the battery's low-voltage cut-off shuts the inverter off; with no sun, about 1.5 hours. With the fan off, the panels recharge the battery in about 3 hours.

Loads that run on and off can often be powered indefinitely.

Why build one

For charging laptops and phones, emergency preparedness, remote air-quality sensors, gate openers, electric fences, van life and camping, or lighting in an off-grid home. It is also a good first project before building a custom system, a class project for students, or a start for an energy business. It is easy to make larger or smaller.

Safety: electricity is dangerous, and a battery's stored energy can do real harm if its terminals are shorted. Build at your own risk.

Sources

• "Step by step 40W solar manual" from Appropedia, by its contributors, under CC BY-SA 4.0. Changed: rewritten in hubnx's own words and condensed; product links and the step-by-step photographs of every part left out (the original has them, and a printable PDF); each image under its own licence, credited in its caption. This page is shared under CC BY-SA 4.0.