We really enjoyed working with LifeEdited to provide backup solar power for their compact New York apartment. So when Graham Hill (the Founder) asked us to help with a more ambitious project, to provide off grid power for his cabin in Maui, we were keen to take it on. It was a chance to show what our small solar panels are capable of and experiment with a larger system. Besides, now we have a place to crash in Maui!

Hawaii cabin with solar panels on roof. Interior of Hawaii solar cabin
Graham wanted to power a laptop, monitor, speaker system, some lighting, and a fan. This was going to require a much bigger panel, battery, and controller than our typical system. Fortunately our friends at Earthtech Products generously provided a Solar Generator from their remote cabin kit. This is the grey box in the photo above, which provides housing for the solar batteries and regulates the output power. We paired this with 12 of our 16.8W panels and hooked the two together with a 30-amp solar charge controller also from Earthtech.

The system seems to be working well, but we are still planning a “service visit” soon just to be sure. For those of you who want more details on the system, read on.

Calculating the Power Required by Devices: We looked at various solar calculators online, but the results didn’t translate very well to a small off-grid system. They were mostly about selling large grid-tied household systems. Smaller systems ranged anywhere from 12–250 -watt solar panels and 150-3,500 watt-hour batteries, so it’s safe to say there is no standard setup. Instead, it comes down to the specifics of the devices: how much power they need, how long they will run, and what the charging conditions are like. So we started from the ground up, calculating the power requirements of each device (in Watts), multiplied by its average daily usage (in hours), to get total daily energy requirement (in Watt-hours).

The specs on the back of the AC chargers tend to show the maximum output of the charger, not the actual power requirement of the device. There are broad estimates online, but we found it was useful to actually measure how much power each device consumed using a Kill-A-Watt meter. We calculated a total draw of about 80-100 watts with everything running. After discounting Graham’s claims of 12-hour days working at the monitor (he’s bound to be surfing half the time), we arrived at a total requirement of 800 watt-hours per day.

Calculating Power Supply from the Solar Panels: Next, we looked at how many hours of sun we might expect. The measure is of “full sun hours”, not of how many hours the sun is up. In Hawaii the panels would receive, on average, the equivalent of 6 hours of sun at maximum intensity.

On this basis, we backed into a 200-watt solar system, which would be 12 of our 16 Watt 18 Volt panels.

In theory, this would produce 1,200 watt-hours in an average day. But that specification is based on the cell supplier’s testing in ideal conditions. In practice, power will be lost to multiple factors including: not having the panels perfectly oriented toward the sun (since the panels are static), transmitting power across the lines, the chemical process of storing power in the battery and extracting it, and converting from DC to AC power output. We assumed a 200-watt array would actually produce closer to 800 watt-hours over the course of a typical day, just enough to match the estimated demand.

Batteries: The batteries are necessary to store power generated by the panels so a steady supply is available even when the sun is not shining and to cover times when consumption exceeds production. We chose two 92-amp-hour 12 Volt solar batteries. Not least because this was about all we could find in Maui. These would provide 92 amps x 12 V x 2 = 2,200 watt-hours of energy storage, enough to power almost three days of typical consumption. This way, a couple of cloudy days would not mean a blackout. Similarly, there is capacity to store power from a couple of sunny weekend days with lower consumption.

It is important to use deep cycle batteries specifically designed for solar charging; normal car batteries don’t last long when regularly depleted as is the case in a photovoltaic system. Even the solar batteries last longer when full depletion is avoided, another incentive for increasing the battery capacity.

Charge Control: A charge controller is required to prevent the batteries from becoming damaged due to overcharging. Earthtech supplied us with a 30-amp solar charge controller,which was more than enough to meet our needs.
Output Power: We also needed an inverter to convert the 12-V DC power output from the batteries to the 110-V AC power required by most devices. This is the primary function of the Phono Solar Power Hub also provided by Earthtech. Earthtech typically pairs it with a 100W solar panel, but they are very helpful and likely to be able to mix and match.

The nice thing about this device is that it hides away the batteries and terminals connecting them and presents a clean face with 4 AC outlets, 2 DC outlets, and an LCD display. It can also be extended if necessary by adding a second battery box with two more batteries. It can also function as a simple battery backup which powered by AC. Panel Wiring: Admittedly, we made things more complicated for ourselves using twelve of our own panels rather than say two 100-watt glass-coated panels. Still, we were hardly going to use a competitor’s panels, and our customers find themselves in many situations where multiple lightweight, robust panels are preferable. Shipping was certainly easier in this case. The first problem we had to tackle was how to neatly connect all twelve panels in parallel while ensuring the system would be weatherproof.

We decided to use our 2-panel circuit boxes set to 6V (parallel) to combine the two rows of six panels in pairs, reducing the number of solar output leads from twelve to six.


We cut off the surplus output leads of the circuit boxes and used liquid tape to seal them. Similarly, we sealed the switch and the unused LED ports. To connect the six circuit boxes together, we used three lengths of solid 18-gauge, 4-conductor cable. This was spliced onto the output wires of each circuit box using solder, and sealed with liquid and heat-shrink insulation.

The three wires were connected to a 2-pole bus bar in parallel, mainly to make the connections clean and easy, and to leave room to add more panels if we choose to later. The bus bar was stored inside an Otterbox with holes drill into it for the wires, which was a good way to waterproof it while at the same time leaving it accessible.

To carry the output power from the bus bar, we use a section of 14-gauge extension cord ending in a male AC plug (US). At the time we didn’t know exactly how far the panels would be mounted from the inverter. This approach allowed Graham to use a standard extension cord to bridge the gap (14-gauge to minimize power loss in the line). This may not look like such a great idea if someone plugs an AC power source into the hacked extension cord, effectively feeding that power into the panels, but we made it super clear this extension is only for the DC power from the panels and asked them to tape over the plugs when connected.

At the other end of the extension cord was the female end, feeding power first to a simple but heavy-duty mechanical SPST disconnect switch.

This allowed the positive lead from the panels to be disconnected, breaking the circuit in the event of some problem. Beyond the switch, the cable connected to the solar input of the charge controller, which was powered via the solar power itself. The battery output of the charge controller was then connected to the solar input posts within the Phono Solar Power Hub. Once all of this was done, the wiring connections were tested, packed, and shipped to Hawaii along with the inverter and solar panels.

System Installation in Hawaii: Graham and his contractor Ed were responsible for picking up the two 12-V lead-acid batteries, a 14-gauge extension cord for connecting the two halves of the system, parts for grounding the system, and the materials needed to actually mount the solar panel array. They arranged the panels on the roof in two rows of six, and built a frame using long sections of aluminum C-channel. Each of our panels have screws embedded in the corners, so the C-channel was marked, punched, and drilled so as to have holes to accept the screws of the panels. Four C-channel sections were prepared this way, and the panels were attached using the plastic nuts supplied with the panels. The two rows of panels were then joined by screwing the two center C-channels together. The frame is long enough that we have room to install another 4 panels if necessary.

Once the panels were assembled onto the frame, the next step was connecting the panels to the wires we prepared earlier. The panels were all plugged into the circuit boxes, which were then affixed to the underside of the panels using silicone. Silicone was also then used to seal around the plug connecting each panel to the circuit box. With that done, the array was ready to be installed on the roof. The roof happened to be oriented fairly well for maximum sun exposure although we may tilt them up in winter to more directly face the sun. The solar output leads were routed into the house using an extension cable and connected to the DC disconnect switch.

With the two batteries installed in the Phono Solar Power Hub, the DC disconnect switch was turned on and everything lit up indicating the batteries were charging. We have only done limited testing, but so far the setup seems to be sufficient to run the laptop, speakers, external monitor, and an efficient light with a total consumption of about 80 W. Three or four weeks staying in the cabin fine tuning the system should probably be enough to totally forget whatever it was stressing us out back in New York.

About The Author

One Response

  1. Jorge Soares

    I want too bring to Brasil your sistem tu but In 5.000 popular houses In Amazônia Brasil!! This is possible?

    Other things is sell your produtos in Brasil .

    O gonna whait your mensage ?!

    See you soon

    Jorge Soares
    55 91 81524544

    Reply

Leave a Reply

Your email address will not be published.