When the junction box is screwed shut after the cables are connected, it becomes watertight. Both of our solar panels, each with 2 4 mm² cables are connected to two 16 mm² cables inside this device.

Open junction box with all cables installed

Position of the junction box between the two solar cells

Our green-and-yellow 16 mm2 cable (code H07V-K) is not designed for permanent outdoor use, and is employed only briefly during testing. Here’s why we chose this type of cable: It’s significantly cheaper than standard outdoor 16 mm2 cable normally used for ground wires. Because our construction is simply for research, this approach works for us. Those who want to make a permanent installation should use compatible outdoor cable with the required protective sheath instead. We recommend using red for the positive voltage, and blue for the negative voltage when working with direct current (DC).

Charge Controller: Battery stores excess solar energy

To make sue our PC and monitor always have sufficient power, we must use a charge controller. This device directs excess energy that the PC doesn’t use into a storage battery. That stored energy from the battery is used when bad weather hampers power collection, or during night-time hours when no power is collected at all.

Morningstar ProStar-15 charge controller

We used the ProStar-15 solar charge controller, made available to us by the German company, Wagner Solar. As the product name is meant to indicate, it can load a battery with a maximum current of 15 Amperes. This entails around 190 Watts. Because our solar generator can deliver a maximum of 260 Watts, this leaves about 70 Watts left over. At idle, our solar-powered PC consumes about 61 Watts, which makes our 15 Amp charge controller and ideal fit for our setup. The energy output from the charge controller goes primarily to service the PC.

This device can also display the actual battery voltage, power delivered from the solar panels, or energy consumed by the computer, as the following photos attest.

Readings for battery voltage (top), solar amperage (middle), and load amperage (bottom).

The solar charge controller obtains its power from the battery, because when it’s dark there would otherwise be no power available for its use (and no possibility of controller charge and discharge behavior). Because power regulation also entails energy loss, the rear side of the charge controller is bedecked with a heatsink, to dissipate related heat production.

1.5 kWh Storage Battery

On rainy days when the solar panels produce only a small amount of power, and at night, when they produce no power at all, our solar-powered PC still needs juice to operate. We use a storage battery to provide this power, and use the charge controller to charge it up during the day. In our installation, this is a lead-acid battery that weighs in at nearly 75 lbs (32 kg).

Solar battery from the Sonnenschein (sunshine) company, made by Exide Technologies Under optimal conditions, our solar array deliver up to 260 Watts of power. In idle mode, the solar-powered PC consumes 61 Watts, which leaves nearly 200 Watts available to charge the battery. Our gel-filled lead-acid battery offers a capacity of 130 Ah. At 12 volts this produces 1560 Wh. With a full charge, our battery can run the PC for about 23 hours, without any input from the solar panels. But this is possible only during optimal conditions, which occur only seldom, if at all.

Terminal connectors for the lead-acid battery