Friday, September 26, 2008

Big Performance from a Small Off-Grid System

Gasoline prices have soared over the past couple of years, and some believe that electricity rates will soon follow. When that happens, alternatives like PV (photovoltaic) systems will become an attractive option. The surge in demand for solar panels will result in higher prices, and only the well-to-do will be able to install systems large enough to meet the needs of an entire household. The average home owner will be forced to pay the high utility rates, or to learn to get by with a small PV system. The good news is that you can get by with a smaller PV system than you might have imagined, but it’s going to take some “out-of-the-box” thinking.

Much has been written about the benefits of home improvements such as adding insulation, replacing windows and doors, efficient lighting and appliances, the elimination of phantom loads, and passive solar improvements. Alternatives to electric heating and cooling are also important. These are logical, and often necessary, prerequisites to PV system implementation, and especially important to anyone wanting to get by with a small system. Since those topics have been discussed at length elsewhere, this discussion will be limited to getting the most from a small PV system.

The daily capacity of a PV system can be calculated by multiplying the capacity of the PV array, in watts, by the number of hours of peak sunlight. A PV panel array consisting of five one hundred watt panels, for example, can produce 2000 watt-hours in a four-hour period. That’s 500 watts times 4 hours. It is widely accepted as fact that an off-grid PV system with batteries will be 65% efficient, lowering the expected daily production in this case from 2000 watt-hours to just 1300 watt-hours. Now that you know the basics, let’s explore some ideas for getting most from a small PV system:

1. Load shifting

An off-grid system is least efficient when it is used to charge batteries. The inefficiencies associated with converting energy, storing it, and converting the stored energy back to electricity results in a huge energy loss. The obvious solution therefore is to use electricity directly from the solar panels as it is produced, instead of storing it in batteries for use at a later time. Doing the laundry, running a vacuum cleaner, and cooking are some of the obvious tasks you can do in the daytime, but other strategies are not so obvious.

A well-insulated chest freezer will keep things frozen for many hours in the event of a power loss. Consider putting your chest freezer on a timer, limiting its operating hours to daytime. This will require a little experimentation, as you don’t want food to partially thaw each night.

To improve its efficiency, move your chest freezer to the coolest part of the house, perhaps the basement. Allow plenty of room for air circulation near the condenser to improve operating efficiency, don't limit it to the two or three inches that the manual suggests.

2. Eliminate unnecessary appliances

Could you get by without a refrigerator? You certainly could if you had to, and you’ll reduce the load on your PV system by 1000 to 3000 watt-hours each day. You’ll be able to eliminate a dozen or more solar panels from your array, saving a small fortune. I spoke to a friend recently who, after his refrigerator failed, continued to use it to keep items cold by using ice from his chest freezer. He used his chest freezer to produce ice, and placed that ice in his refrigerator. Milk jugs provided a convenient way to do it, and he cycled three, one gallon jugs, from his freezer to his refrigerator each day.

Perhaps you’re not quite ready to shut down your refrigerator full-time. Instead, why not put it on a timer? Shutting it down for a few hours each night will reduce the system load significantly.

Converting a chest freezer to a super-efficient refrigerator is another strategy you might consider. A thermostat mounted inside the freezer switches AC power to the freezer on and off as needed. No significant modifications to the chest freezer are necessary.

3. Add diversion load control to your PV system

If you monitor your PV system during the day you probably find that once the batteries are fully charged, you have a lot of excess energy available that doesn’t get used. Putting this previously wasted energy to use can significantly increase the usefulness of a small PV system. Adding diversion load control to your system is one way to tap into that extra energy. Some charge controllers can be used as diversion load controllers, but you can also use a PLC (Programmable Logic Controller) for the task. I use a Morningstar Relay Driver, a much less expensive option.

Often, systems are designed to use the extra energy to pre-heat water, but that’s probably the least-efficient way to use it. Using the sun to heat water directly makes more sense. If your home uses a cistern for its water supply, using this excess energy to pump water is a much better idea. It is far better to use excess energy for this task than to have to pump water at night, due to demand, using energy from batteries.

You could also use the excess energy to charge a spare battery bank. The spare battery bank might be used to power some DC loads, 12-volt dc lights for example. Using this surplus power for DC loads eliminates the conversion loss that you would otherwise experience by running a DC to AC inverter. This might allow you to turn off your inverter, perhaps all night long, saving yourself the power it consumes when idling.

4. Make Peukert’s Law work to your advantage

According to Peukert, a lightly loaded battery bank operates at higher efficiency than a heavily loaded battery bank. Looking at this another way; if you increase the size of the battery bank, without increasing the load, efficiency improves. Take advantage of this phenomenon by making your battery bank larger than necessary. As a bonus, your batteries will last longer because they’ll be stressed less.

Avoid using two or more high-power appliances at the same time. Making toast, while using the microwave oven, is an example of this. Heavy current from the battery bank results in lower efficiency, according to Peukert.


Anyone who’s ever struggled to get through an extended grid power failure knows that electricity is more than simply a matter of comfort and convenience. While it’s a necessity for some, it would be hard for any of us to get by without it. Imagine doing without lights on a long winter evening, or not having the ability to keep food refrigerated. Imagine doing without air conditioning, and not even having an electric fan to circulate fresh air.

Use these strategies to the extent that you can. You might start by adding the ability to log system data. The ability to log data will help you determine if system improvements and modifications are beneficial. Adding automation to your system is the logical next step. This allows you to shift loads and divert power when it is beneficial to do so. And most importantly, be on the lookout for other ways to lighten the load and improve system performance. Please share your ideas with the rest of us, in the form of a comment, so that we can all learn from each other.


Friday, September 19, 2008

Off-grid Systems and Off-grid People

“To acquire knowledge, one must study; but to acquire wisdom, one must observe”. Marilyn vos Savant

Some off-grid systems and the people who use them:

I have an off-grid photovoltaic (PV) system, but since I’m connected to the electrical grid I can’t honestly say that I know what off-grid living is like. My system powers some loads on a daily basis, and serves as an emergency backup system, but I rely on the power grid for most of my everyday electricity needs. The closest I’ve come to real off-grid living has been during those times when my grid power failed. To understand what off-grid living is really like it’s helpful to peek into the lives of those who actually live off-grid on a full-time basis.

An off-grid lifestyle can range from primitive to luxurious, depending upon the size of the system. While some are happy with a minimalist lifestyle, it’s common to find others who after attempting to live off-grid, have become disillusioned upon realizing just how much work is involved. Some give up, and others enlarge their systems until they achieve an acceptable comfort level. On the other end of the spectrum are off-gridders who want all of the conveniences that their well-connected counterparts in the city enjoy. This group includes the well-to-do who choose to live in an area where utility services are unavailable. Most off-gridders, I suppose, fall somewhere in-between primitive and luxurious.

With these things in mind, let’s look into the lives of some off-grid people and their systems:

#1. Can you guess which country has the most residential solar PV systems installed? It’s Kenya. PV systems are replacing kerosene lamps in remote villages, greatly improving the quality of life of the residents. Typically, these small systems are only able to provide a few hours of light each night, but the importance of the elimination of fire hazards and indoor air quality improvements cannot be overstated. Because children can study longer into the evening with the extra light, these small systems also have an educational benefit.

#2. Here in the United States, Ward’s solar PV system was built for less than $700.00. It provides all of the electrical needs of this bachelor in his remote cabin, including lights, TV, VCR, and a boombox. Ward uses a wood-burning stove for heat, and a propane refrigerator. He has no indoor plumbing. His system includes a 75-watt solar panel, four batteries, a charge controller, and a 350-watt inverter.

#3. It would be a disservice to Karen to limit this discussion to her PV system. Karen transformed a 5-acre site in the Mojave desert to a comfortable homestead. Among her accomplishments Karen installed a septic system and a 4000 gallon water tank. She renovated an old cabin, including a passive solar system of her own design. Karen uses a wood stove for heat, and propane for cooking and refrigeration. And yes, she put in a solar PV system. Her system includes 400-watts of PV, 880ah of battery capacity, a charge controller, and a 3500-watt inverter. A system of this size can be built for less than $3000.00. Because Karen’s system is larger than Wards, she can do much more. Her capabilities include pumping water, running a vacuum cleaner, and using kitchen appliances.

Although Ward and Karen may be satisfied with what they have, a typical family would probably struggle to get by with such limitations. Some might opt for the prepackaged 2000 watt off-grid PV system described below. The cost of the entire system, including batteries and wiring, is in the neighborhood of $20,000.00. A system of this size will allow the use of a washer and dryer, and almost any electrical device imaginable. In spite of the size of this system, the average family of four may experience shortages of electricity from time to time. Energy efficient construction, efficient heating and cooling systems and efficient appliances will help, but some homeowners will opt for a generator to make up for periods of extended cloud cover.

System Specifications:

PV: 2000 Watts
Batteries: 6000ah
Pre-Wired Power Center with 4,400 Watts 120/240VAC

Need even more electricity? Obviously you can have as much as your budget and space will allow. Your decision to live off-grid means that you’ll have to maintain your own power systems, but it’s really not that difficult. You’ll have sophisticated equipment that automates some of the maintenance tasks and alerts you to small problems before they become big ones. You’ll know the status of your batteries, and the amount of stored power at a glance. If you choose to install a generator as a backup, it can be set to start up automatically in the event that it’s needed.

Ward and Karen may be thought of as being on the fringe of society now, but that notion will change someday. Declining fossil fuels and an increased awareness of the harm we’re doing to our environment will someday make a change to renewable technologies a necessity, not just a good idea. Folks like Ward and Karen will be typical, not the exception. We’ll all be better off when that happens.

For more off-grid systems and people, click on this link:

Here’s another site that showcases off-grid systems and people:


Friday, September 05, 2008

Are Electric Cars More Harmful to the Environment than Gasoline-Powered Cars?

Because we’re likely to see a massive shift to personal electric transportation in the next few years, this is an important question. Ideally we should reduce our carbon footprint in the transition, not increase it, but here are the facts:

• Burning a gallon of gasoline releases 19.6 pounds of CO2 into the atmosphere.

• A gallon of gasoline has as much energy as 33kwh of electricity.

• For every kwh of electricity that a coal-fired power plant produces, 2.2 pounds of CO2 is released into the atmosphere.

These statistics seem to indicate that it is better to burn a gallon of gasoline and release 19.6 pounds of CO2 than to use an equivalent amount of electricity and release 72.6 pounds (33kwh X 2.2 pounds), into the atmosphere. It seems that driving an electric car will be much more harmful to the environment than driving a gasoline-powered car. Could this be true?

These statistics suggest that an electric car, starting with a fully charged 33kwh battery pack, would travel as far as a similarly-sized gasoline powered car could go on a single gallon of gasoline. However, the Chevy Volt is expected to be able to travel 40 miles on a fully-charged 16kwh battery pack. What gives? The discrepancy is due primarily to the fact that electric motors are 90 to 95% efficient, while gasoline engines are only 20 to 30% efficient. In reality, driving an electric car will produce about the same amount of pollution as driving a gasoline-powered car, it just moves the source of the pollution from the tailpipe to the power plant.

The net result, it seems, is that we accomplish nothing by switching to electric cars, but that’s not entirely true either. It’s easier to stop pollution at a few power plants, than it is to stop it at the tailpipes of millions of cars. Electric cars also give us the opportunity to use electricity from clean sources, such as solar PV panels or hydro-electric plants. So instead of releasing millions of tons of pollution each year, we’ll soon have an opportunity to drive our personal automobiles without releasing any CO2 into the atmosphere. That’s something we’ll never be able to do with gasoline-powered cars.

The millions of electric cars we’ll see on the road within a few years will all need to be recharged each night. It’s clear that our priority as a nation should be to clean-up or eliminate coal-fired power plants. Expensive schemes, such as carbon sequestration, are not the best use of federal funds. Promoting solar-, wind-, and hydro-power would be better. Energy from a modestly-sized solar-electric (PV) array on a single residential rooftop can offset 2000 pounds of CO2 each day, so just imagine how beneficial a million solar roofs would be. That’s 2 billion pounds of CO2 each day!

Concerned about electric car performance? Don’t be. Electric cars can be built to be both highly efficient and very quick. Tesla Motors has already proven that. Increasing the size of an electric motor improves both horsepower and efficiency. On the other hand, a big motor is needed to make a gasoline-powered car quick, but gas mileage suffers as a result.

Driving a personal automobile doesn’t have to be an environmental disaster. The switch to electric transportation can have a positive effect on the environment. All that we need now is intelligent leadership and an aggressive plan. Maybe if we can get our politicians to stop taking money from coal and oil interests, we might just get legislation that is good for the environment for a change.



Carbon Dioxide Information Analysis Center:

Chevy Volt Site:

Tesla Paper: