Power Sharing

Establishing an Off-Grid Community Microgrid

Inside this Article

Brandon Greenstein and Chris Farmer, designers of the community PV system and custom weighted-metering system that helps in billing residents proportionally to their energy use.
"Tribal Condo," one of the 10 residences powered by a central 8.16 kW off-grid photovoltaic system.
"Flower Hut," one of the 10 residences powered by a central 8.16 kW off-grid photovoltaic system.
"Micro Hut," one of the 10 residences powered by a central 8.16 kW off-grid photovoltaic system.
"Wonky Hut," one of the 10 residences powered by a central 8.16 kW off-grid photovoltaic system.
"A-Frame Hut," one of the 10 residences powered by a central 8.16 kW off-grid photovoltaic system.
Twelve individual kWh meters transmit consumption data to a data logger, helping balance the energy use of residences, the community kitchen, and a diversion-load water heater.
Thirty-two 255-watt Kyocera modules in eight series strings. The two subarrays are offset in relation to a road behind them, not for solar exposure purposes.
A diversion-load relay (at left), controlled by a MidNite Solar charge controller, diverts excess solar energy to a water heater in the common area. In the middle, a Schneider Electric system control panel for remote control and monitoring of charge controllers and inverters. A Bogart TriMetric amp-hour meter sends battery state-of-charge data to the data logger.
A MidNite Solar Classic 200 MPPT charge controller for each PV subarray. One controls a relay for the water heater diversion-load circuit.
Four HuP SolarOne flooded lead-acid batteries supply 950 Ah at 48 VDC—enough storage for the whole neighborhood.

In June 2015, 22 residents of the Hut Hamlet neighborhood commissioned an 8.16 kW off-grid microgrid PV system at Earthaven Ecovillage, an intentional community outside of Black Mountain, North Carolina. The shared solar-electric system serves 10 small cabins, and the neighborhood kitchen and bathhouse. On an average sunny day, the system produces 31.5 kWh of electricity—what the average American house consumes—which is shared among the neighborhood homes.

Earthaven is a 21-year-old intentional community situated on 329 acres near the edge of the Blue Ridge Mountains. Presently, about 80 people live in the community year-round. Earthaven’s mission is to create a village that is a living laboratory and educational seed bank for bioregionally appropriate cultures.

The entire community is off-grid, producing its electricity from several PV arrays (and two small microhydro turbines). However, after solar electricians Chris Farmer and Brandon Greenstein got repeated calls from the Hut Hamlet neighborhood residents asking them to troubleshoot, fix, or upgrade their old, owner-installed off-grid PV systems, Chris and Brandon proposed an upgrade—one state-of-the-art, code-compliant system to distribute conventional 120/240 VAC power to the entire neighborhood. While this idea technically made the most sense, the notion of sharing an off-grid power system brought up many issues:

  • How to organize a group of neighbors to make decisions about creating an off-grid power system and deal with its many complexities
  • What kind of entity needed to legally own the system, how to track everyone’s different equity in the system, and how to leverage renewable energy tax credits
  • How to equitably deal with the power system being shared among different people with different electrical loads, and different levels of consciousness about usage
  • How to maintain a system with multiple owners

Sizing the Single System

Chris worked with the neighborhood’s residents on an electrical loads spreadsheet to assess all of their existing and potential future desires for electrical loads, as some knew that they would likely want to install a refrigerator in their cabin in the next couple of years.

The great majority of loads on this system are lights; refrigerators and freezers; small plug-in loads (computers, modems, printers, chargers, and occasional small appliances, such as blenders and food processors); stereos; and a few small pumps (pressure and circulating). There are occasional larger loads of juicers, electric tea kettles and cooking plates, irons, and corded 15 A, 120 V power tools. The single largest load is the electric water heater used as a diversion load. This load is in the same building and very close to the inverter and AC distribution panel, and runs at a more efficient 240 volts (since it uses diverted excess energy, it is not included in calculations used for system sizing).

There are no electrical space-heating loads in the neighborhood. Passive solar design and wood heaters provide space heating in each home. Tankless propane water heaters provide water heating in each cabin. Cooking is primarily propane, although some cabins now use electric tea kettles or electric burners for cooking and/or dishwashing on sunny days.

Before the microgrid was installed, the few folks who had refrigerators often had to run generators to keep their batteries charged. Only one household had the ability to run power tools. Now, all households have the ability to run refrigerators, power tools, and other large loads like juicers, tea kettles, cooking plates, and irons. There’s more reliable power for all of their small loads. And now there is only one generator, which rarely has to run.


Comments (4)

Robert Pollock_2's picture

So, how much? What do they end up paying per kwh? In Southern California we start at 12c and get to Tier 5 (I think the highest level) at about 50cents. TN residents pay roughly 4 or5c up to 12cents maximum, so you can see that this plan works better some places, than others.

Chris Farmer's picture

Admittedly, that's not an easy question to answer. The simplest answer to your question of how much they end up paying per kwh is "it depends".

It depends on how long the many different components last (battery, inverters, charge controllers, PV panels, etc. ).
Will the system owners have to pay for labor and materials to replace the battery (which cost $11,200) after 5 years? 10 years? 15 years? 20 years? The answer to just this one question will have a dramatic impact on the overall cost per kwh produced during the life of the system. But the same question applies to every other single component. So no firm answer can be given to your question.

That said, let's assume that on average, the system lasts for 15 years (some components will last less time, and others more).
The system cost roughly $60k in total for all labor and materials. This includes not only the costs for the main central system, but also all of the costs of distributing conduit, cable, surge protection, meters, and disconnects through the landscape to each house, and also doing some modifications of the existing wiring at each house. 
Over 15 years, the neighborhood will add roughly $30k to that total for operating costs ($1k per year for both accounting and maintenance, so $2k total each year for 15 years).
So, over 15 years, the grand total for all capital and operating costs will be roughly $90k, which works out to roughly $6k per year, including all costs for distribution and all operating costs for those 15 years.
According to PVWatts, this system will produce roughly 11,500 kwh per year, so the average total cost per kwh is 52 cents - very expensive relative to grid power just about anywhere. But I don't know of many people living off-grid for purely economic reasons.
If we had just run the same analysis with an assumption of 20 years, then the average cost per kwh would drop to ~43 cents.

To go even further to try and answer the question of "what is the range of cost per kwh with the weighted metering" gets even more difficult, as this answer would depend on not only the above mentioned complexities, but also the present weather conditions, as well as the relative consumption patterns of one's neighbors. So, I won't try to dive into that multi-variable analysis right here.

The neighborhood dreams of adding a couple more houses onto the system, as well as someday adding a micro-hydro plant to the system. The micro-hydro plant could be costly itself, but could also double the expected life-span of the battery, since it would not be deep-cycling nearly as much.

Brandon Williams's picture

This is one of the most innovative HomePower articles I have ever read. Great coverage of an incredible project. Kudos to Chris and Brandon for implementing this microgrid with community solar - not only for providing us with proof of concept, but also for pioneering a framework that encourages efficiency.

Michael Welch's picture
Thanks, and agreed. One of my favorite articles from recent years.
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