Off Grid Electrical System

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Odalys Benitez & Leon Santen & others

When I decided to spend a semester on an off-grid farm, I really didn't know what to expect. Arriving to the Woodland Harvest Mountain Farm, I found a rudimentary electrical system — a 250W solar panel, a cheap battery charger, and a hydroelectric turbine in desperate need of some care. That was enough for the family who owned the farm, but we were about to have 15 engineering students living there! That's fifteen laptops and phones to keep charged, plus the background draw of the satellite internet.
 
First, we did some math to figure out how much power would actually be needed. A worst-case of everyone using their laptops for 12 hours/day, plus miscellaneous background draw (internet, lighting, etc), gave a requirement of about 6kWh of generation per day. The hydroelectric turbine put out a constant ~50W (1.2kWh/day), so we'd need to bridge the 4.8kWh/day gap with more solar panels. Given ten hours of usable light in the fall, we'd need panels that could generate about 480W continuous during the day. We gave ourselves some operating margin, and purchased a 1kW array of high-efficiency monocrystalline panels.

Next, we needed a way to connect the panels to the battery. This is typically done using a "charge controller," which monitors and manages battery charging. After some research, we found that largely two types of charge controllers existed — MPPT and PWM. The former was more expensive, but promised higher efficiency and more reliable power generation across temperature and light intensity. The latter was cheaper — and what was being used for the existing 250W panel. We decided to invest some money in a good MPPT controller, since it was agreed that reliable electricity access was important for everyone to complete the semester with minimal friction. Once the 1kW of panels and the new MPPT controller arrived, we got to work wiring everything up! One of our priorities was long-term maintainability and transparency for those who use the system after us, so we created a wiki page to document everything, and added stickers and QR codes to the different devices to allow for easy access to wiring diagrams, datasheets, and notes.

Overall, our electricity system worked as well as any of us could've hoped, and did a great job keeping lights, laptops, and internet on over the semester.