ASK THE EXPERTS: Off-Grid Source Choice


I am getting ready to build on my off-grid property in New England. The property has abundant renewable resources, and I’m having a hard time deciding on how to prioritize. Here are some facts and estimates I’ve gathered:

  • 3.3 average daily peak sun-hours, with minimal shading at my site
  • 8–9 mph average wind speed estimated on a 120-foot tower, 40 feet above the tops of the mature trees
  • A stream with 30 feet of head over 400 feet, about 40 gpm available year-round, with the potential turbine location about 750 feet from the house site

I’ve heard and read in Home Power and elsewhere that hydro is “best,” wind is really hard to keep going, and solar electricity is more expensive up-front, but very reliable and perhaps the best buy in the long run. Of course, since I’m off-grid, I may need multiple sources.

My load is not firmly determined, but I’m pretty thrifty with energy, using only 8 kWh per day in my small city home. I intend to buy super-efficient appliances; I don’t need every gadget; and I expect that my off-grid home’s usage will not exceed my city usage, but may be somewhat less. I’ll heat with wood exclusively, unless there’s excess energy from the renewable energy system.

I’m getting ready to crunch some numbers, but without a great deal of experience at this, I’m wondering what I’m missing, and seeking expert advice. Perhaps you could use my example to give Home Power readers some perspective on the choices involved, so others can avoid costly mistakes and misconceptions. I’m wondering where my dollars will best be spent, what will be the best for my personal environment and the global environment, and what will give me the most personal satisfaction.

James Barber • via email

It’s a pleasure to deal with such a clearly documented case. The most important design parameter is target energy (8 kWh per day), although it’s also important to allow for losses. Energy is lost in the wires, battery, inverter, and dump loads, so you may need to generate, on average, about 12 kWh per day. If the cost of the system becomes daunting, check where exactly this energy is going—you may find it’s cheaper to save some of that energy than to generate it.

Hydro is likely to be the lowest-cost source. Plus, it also works at night and in winter, reducing battery usage. You can estimate the power available from hydro (in watts) by multiplying the head by the flow and dividing by 10. For your scenario, a well-built system could yield 120 watts. Over 24 hours, this would amount to 120 × 24 ÷ 1,000 = 2.88 (or conservatively 2.5) kWh per day. I suspect that you may have access to twice that flow for useful periods of time, and the extra cost of pipes and wires that can handle 300 watts would be small in relation to the benefits. If you go with a solar/hydro-electric hybrid system, the extra kWh in wet weather would be especially welcome as your solar production decreases. Popular brands of Pelton or turgo turbines have multiple nozzles so you can set the flow to a level that the source can sustain.

Although a 3-inch bore pipe will work well at 40 gpm, a 4-inch pipe would reduce the pipe loss from 12% to 3%, and also open the door to 100 gpm or more, when available. With only 30 feet of head, you can probably use a thin-walled pipe to reduce cost.

The transmission cable is quite long at 750 feet, so losses due to resistance have to be considered. A 48-volt battery system has one-fourth the wiring loss of a 24 V one. Consider using an even higher-voltage turbine connected via a maximum power point tracking (MPPT) controller, provided you have a voltage clamp to prevent the turbine voltage from going wild. MPPT also deals automatically with changes in flow, avoiding the need for manual adjustments to tune the alternator for changing conditions.

Solar electricity (PV) is the next option, and the good news is that the prices are falling. A PV system will complement the hydro system, as it works best in dry, sunny weather, but production could be disappointing midwinter. Assume you will get about 70% of the “peak” rating. For example, with 3.3 peak sun-hours on a 3 kW array, expect about 7 kWh per day. Again, MPPT can help improve this picture somewhat.

Wind power would be interesting and might be fun for you, but in your case it has little else to recommend it. The 120-foot tower is likely to be expensive and the energy produced at your wind speeds won’t be much. Wind is an erratic energy source that puts big demands on the batteries and the backup generator. You need more energy for your system, especially in winter, but in terms of the effort and cost, wind is the more expensive and difficult choice for your site.

Check the Home Power wind buyers’ guides for 2010 and 2011 for a comparison of turbines, but be aware that manufacturers’ energy production figures tend to be optimistic. Note that you would get twice the energy in a mean wind speed of 11 mph compared to your 8 or 9 mph. You can find the daily average kWh by dividing the annual production by 365. However, on many days you will get next to nothing; other days you may be dumping surplus energy.

Whatever sources you choose, you will need an efficient backup engine generator. Try to minimize its use, but you may need it to rescue your batteries and keep the system up and running when conditions conspire against you. If your hydro source is truly year-round, and you are willing to drastically reduce your usage at times, you might avoid using a backup generator.

The final decision about how to split your investment between these different sources will take more research. Use the wisdom of your local installers, and check out their pricing for the systems to figure out the costs per kWh. Your biggest decision, whether a wind turbine suits your marginal site, perhaps belongs more to your heart than your head (but make sure you know what to expect before taking the plunge). It’s much more satisfying to watch the battery voltage rising on a windy night and not to hear the drone of an engine.

Hugh Piggott, Scoraig Wind-Electric • Scoraig, Scotland

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