Six Robots Everywhere modular solar cells were connected in such a way to give 27 volt nominal output. This is close to the 24V required by the printer, plus some extra voltage to actually charge the batteries. See wiring in the picture below. If modularity is not required, the panels can be distributed pre-wired, as shown.
Two 12V motorcycle batteries (sealed lead-acid type) were connected together in series for 24 volt nominal output. A small car battery will also work. Since not much current is required to run a 3D printer compared to crank-starting an engine, it is possible to reuse car batteries that are at the end of their service lives. These are easy to find everywhere, and can generally be procured locally. Remember that 12V batteries charge at 13.8V, so the best nominal charging voltage is 27.6V. Solar panel output varies greatly with weather and even temperature, so this is acceptable.
To maintain the ability to use the printer with grid power, an insulation-piercing connection ( http://archives.sensorsmag.com/articles/0501/63/main.shtml ) was made between a pair of color-coded wires and the printer's power cable, between the power adapter and the printer itself. This has the added benefit of obviating the need for more complicated electronics, as the DC side of the existing power adapter will perform voltage clamping and act as a filter capacitor.
At this point, the three subsystems (solar panel group, battery group, printer power adapter) were connected together in parallel. This allows for the solar panels to trickle-charge the batteries, and the power adapter to prevent overvoltage. The AC plug on the power adapter is left unconnected. Note that there is a very unambiguous "plus" rail and a very unambiguous "minus" rail throughout the circuit. LEGO-style sequential instructions that require no text will be easy to print and distribute.
The printer can then be powered up simply by connecting it to its own power adapter as normal. We found that with this setup, it was possible to operate the printer for one hour a day, on the winter solstice (worst case scenario). To avoid wasting energy and to keep the status of the circuit readily visible to an unskilled operator, the printer should be physically disconnected from the charger circuit when not in use. With this setup, it is possible to print from the grid or a generator simply by connecting the power supply's AC cable as it is, without modifying anything. However, with this setup, doing so will not charge the batteries. This setup can also be used for low-powered laser cutters, for example the Robots Everywhere L-Cheapo. This can in turn be used to cut sharp edges into plastic, or make a large number of identical small wood objects that are currently made by hand.
It would be possible to design a dedicated circuit for charge management rather than using the printer's own power supply. This will increase deployment costs if a small number of deployments is planned, but provide greater efficiency. The printer's stepper motors can also be used to angle the solar panels, at latitudes where this becomes necessary. However, this adds greatly to the device's mechanical complexity, for relatively modest gains. If more printing time is needed, multiple solar panel arrays can be connected in parallel. With this setup, it is safe to do so up to four times, past which a dedicated charge controller becomes necessary.
