In the first installment of this series of articles we dealt with the technologies that permit solar energy to be collected and converted into electricity, to drive a solar-powered PC.

In this, the second installment, we examine the components we chose to include in that PC, particularly regarding how they meet our stringent requirements for minimal energy consumption. To that end our Munich test labs made many measurements, which ultimately let us assemble a world-record-making desktop PC. The third and final installment in this series will provide a step-by-step description of the solar energy collection and storage system that made all this possible, so that interested readers can use it as a guide to build their own solar-powered PCs.

Our 61-Watt solar PC, including the monitor, is constructed entirely from desktop components. Of course, we could have used a notebook—which already includes a monitor and a battery, and which seldom consumes more than 16 to 20 Volts. But the Tom’s Hardware editorial staff elected to push the limits of technical feasibility, and to provide instructions to others, for a desktop PC that runs only on solar energy, without any connection to the power grid whatsoever. This makes our PC location independent, and enables it to be deployed anywhere in the world where there is enough sunshine to meet its needs. Of course, no-one is likely to do this for mobility’s sake alone, simply because the total weight of our rig, and the fact that solar panels don’t really qualify as carry-on or even checkable luggage. It’s probably best to treat this project as a kind of feasibility study, and as a stepping stone to other similar test builds. As is our practice at Tom’s Hardware, we provide a wealth of information in this series of articles, not just to expose some fascinating technologies, but also to enable our readers to follow in the footsteps we carefully document. It’s not unthinkable that a system like this one might wind up in a remote mountain cabin or a hunting lodge in many places around the world.

Power delivery: Conventional A/C (120/230V) or 16 Volts DC?

When it comes to delivering solar power to a PC, the input voltage levels and the type of power supply used are of the greatest importance. A conventional solar power module outputs the equivalent of 16 V DC.

There are two ways to route electricity from our solar modules to our solar-powered PC. Both approaches come with their own pros and cons, as we will elaborate.

Conventional Input Voltage: Too much power goes to waste

If the decision is to use conventional A/C input to the PC, lots of energy-efficient standard components may then be used. This approach requires the use of a transformer that converts the 16 V DC output from the solar modules into conventional alternating current (230 V in Europe, A/C, 110/120 V in North America). This enables the use of a standard power supply in the PC itself (which then converts back from A/C to DC, and adjusts voltage levels once again).

If you look at this sequence of power transforms, it’s easy to see that it doesn’t make much sense to switch from DC to A/C from the solar module to the PC’s power supply, which then switches back from A/C to DC a second time. This type of double power transformation could result in the loss of as much as 25% of the output energy in the form of wasted heat, and thereby increases the power requirements to service the PC.

16V DC input voltage requires a special power supply

It’s much more effective to stick to direct current, take the 16 V DC output from the solar modules straight into the PC, but a special power supply is required.

By directly connecting the power outputs from the solar modules to the PC’s power supply only a single transformer is required, which greatly lowers the amount of energy wasted in that transfer. Using 16 V DC input directly does have a disadvantage as compared to using conventional A/C power: in transporting energy from the solar modules to the PC, a lot of current is wasted in overcoming resistance in the cables that tie them together. Because the voltage levels are low, the current levels are high, but because we knew we could take certain steps to offset these energy losses, we decided to go the 16 V DC route anyway. How we managed to power the PC, choose our solar modules and storage batteries, build the necessary supporting equipment, and so forth, is covered in the third installment of this solar-powered PC saga.

Delivering 16 V DC from a Solar Panel

Since we connected the outputs from the solar panels directly to the PC, it has up to 16 V DC at its disposal. Actually, depending on the intensity of sunlight at any given moment, these modules can output anywhere between 12 and 20 V DC. That’s why we chose a very special power supply—namely, the M2-ATX from Ituner—that can handle input voltages in a range from 6 to 28 V DC.

Because this is a low-voltage power supply, the maximum rated power for this device is a scan 160 Watts. The input voltage to the PSU is also very close to the output voltage it needs to produce for a PC. This narrow range of difference between input and output voltages has the happy consequence of enabling extremely high efficiency, as compared to a PSU that handles conventional A/C power. We will use this power supply to drive all the components in our solar-powered PC, including the monitor.

Because the Ituner M2-ATX includes only a single 5.25" drive connection and a single SATA connector, we used a Y-adapter to hook up the rest of our components.

We measured the efficiency of the power supply in our solar-powered PC at idle, and under heavy load, as shown in the following chart.

When the solar-powered PC is idling, the special M2-ATX power supply achieves a stunning efficiency of 92.37%. If we load up the CPU or the graphics subsystem, efficiency sinks to 88.4%. Internal power consumption in the PSU itself varies from 5 W at idle, and up to 14.2 W under load.

These measurements bolstered our decision to go with a 16-Volt DC solution. Using conventional A/C power, the best PSUs achieve efficiency values of 87% under optimal conditions. This approach not only does away with energy losses inherent in two-stage transformation of output, it also does better at dealing with DC input that the best conventional PSUs do with A/C input. This PSU is built in the USA, and costs $80-90 (well-known mini-ITX vendor Logic Supply and automobile PC vendor SolarPC are good sources for this part in North America).

Power Supply Connectors and Components

• Power LED
• To motherboard on/off switch
• To external power on/off switch
• ATX connector (20-pin)
• P4-12V power connector
• User jumper settings
• Positive voltage input (battery)
• Ignition (switched battery, positive)
• Negative voltage input (battery)
• Amplifier control via remote on/off
• Configuration for on/off timer settings

In cases where the power supply has to work at peak output for any length of time, the manufacturer recommends installing a cooling fan. We chose the Papst 512 F/2 using a 5-volt adapter, which caused the fan to draw only 160 mW.