Wednesday, January 10, 2007

What to Expect from a Small PV System

If you’re thinking about starting really small, perhaps just one solar panel, you might be wondering what to expect from such a system. This post will attempt to answer that question.

To simplify, I’ll assume that you’ve chosen the following equipment:

- A 85-watt solar panel ($400)
- A 110ah, 12-volt marine battery ($60)
- A 10-amp charge controller ($50)
- A 125-watt sine wave inverter ($205)

You’ll have to add wire and mounting hardware to the list, but the entire system can be built for about $800.00. To illustrate how much power you’ll be able to generate, let’s start with the solar panel.

The 85-watt rating is the result of laboratory tests. Under ideal conditions the panel can generate 85-watts, but it’s unlikely that you’ll get that much power from the panel in your installation. Let’s be optimistic and assume you’ll get 80-watts from the panel. Let’s further assume that you’ll have 4-hours of sunlight each day. The daily production of your panel is 80-watts times 4 hours, or 320-watts per day.

Next, let’s consider the battery.

It’s important to note that you should never fully discharge the battery, as that will shorten its life. We’ll assume that you use only half of the energy stored in it, or no more than 55ah. An alarm will sound when your inverter senses low battery voltage, so you’ll know when to stop using stored energy.

I’ve found that a 100-Watt inverter load results in about 10-amps of current at the input to the inverter. It follows then that the battery should be able to provide power to the load for 5.5 hours. (55ah divided by 10-amps equals 5.5 hours). Considering conversion losses, we’ll drop that by about 20%, and assume that the battery can provide 100-Watts for 4.4 hours. Since 100-Watts for 4.4 hours equals 440-Watt/hours, the solar panel doesn’t quite measure up to the capacity of the battery on a day-to-day basis. There’s nothing wrong with that, it’s good in fact. I prefer to somewhat oversize the battery for a given solar array output. A little extra stored energy helps to make up for deficits caused by cloud cover.

A typical use for a system of this size is to provide emergency power, or as a primary source of power in an off-grid cabin or camper. Here is an example of how you might use the available power each day:

Radio - 5 Watts - 4 hours per day - 20 Watts per day
CF Light Bulb - 13 Watts - 5 hours per day - 65 Watts per day
Fan - 30 Watts - 2 hours per day - 60 Watts per day
Portable TV - 60 Watts - 2 hours per day - 120 Watts per day
Cell-phone charger - 25 Watts - 2 hours per day - 50 Watts per day

The total energy used is the sum of the energy used by each device, or 315-Watts per day. As long as you have plenty of sunshine, you’ll have power to spare. Unless you upgrade to a more powerful inverter, you won’t be able to operate a microwave oven, toaster, or other high-power devices.

While the system described here won’t significantly reduce your electric bill, it can really come in handy during an extended power outage. If you need more power than the solar panel can provide, you can use your automobile’s electrical system to recharge the battery. This increases the usefulness of your system, but at the expense of burning fuel.

If you live in an area that experiences frequent power outages, you’ll probably want to enlarge the system to a point where it’s able to provide refrigeration in order to keep food from spoiling. I’m in the process of enlarging mine so that I can do all of these things, and also keep the motors running on my corn-burning stove. Meanwhile, I’m producing clean, quiet, non-polluting power. This is so much better than getting up in the middle of a cold night to refuel the generator.

Please let me know what you thought about this, and other articles on this blog, by posting a comment.


1 comment:

Jay Draiman said...


In order to insure energy and economic independence as well as better economic growth without being blackmailed by foreign countries, our country, the United States of America’s Utilization of Energy sources must change.
"Energy drives our entire economy." We must protect it. "Let's face it, without energy the whole economy and economic society we have set up would come to a halt. So you want to have control over such an important resource that you need for your society and your economy." The American way of life is not negotiable.
Our continued dependence on fossil fuels could and will lead to catastrophic consequences.

The federal, state and local government should implement a mandatory renewable energy installation program for residential and commercial property on new construction and remodeling projects with the use of energy efficient material, mechanical systems, appliances, lighting, etc. The source of energy must by renewable energy such as Solar-Photovoltaic, Geothermal, Wind, Biofuels, etc. including utilizing water from lakes, rivers and oceans to circulate in cooling towers to produce air conditioning and the utilization of proper landscaping to reduce energy consumption.

The implementation of mandatory renewable energy could be done on a gradual scale over the next 10 years. At the end of the 10 year period all construction and energy use in the structures throughout the United States must be 100% powered by renewable energy.

In addition, the governments must impose laws, rules and regulations whereby the utility companies must comply with a fair “NET METERING” (the buying of excess generation from the consumer), including the promotion of research and production of “renewable energy technology” with various long term incentives and grants. The various foundations in existence should be used to contribute to this cause.

A mandatory time table should also be established for the automobile industry to gradually produce an automobile powered by renewable energy. The American automobile industry is surely capable of accomplishing this task.

This is a way to expedite our energy independence and economic growth. (This will also create a substantial amount of new jobs). It will take maximum effort and a relentless pursuit of the private, commercial and industrial government sectors commitment to renewable energy – energy generation (wind, solar, hydro, biofuels, geothermal, energy storage (fuel cells, advance batteries), energy infrastructure (management, transmission) and energy efficiency (lighting, sensors, automation, conservation) in order to achieve our energy independence.

Jay Draiman
Northridge, CA. 91325

P.S. I have a very deep belief in America's capabilities. Within the next 10 years we can accomplish our energy independence, if we as a nation truly set our goals to accomplish this.
I happen to believe that we can do it. In another crisis--the one in 1942--President Franklin D. Roosevelt said this country would build 60,000 [50,000] military aircraft. By 1943, production in that program had reached 125,000 aircraft annually. They did it then. We can do it now.
The American people resilience and determination to retain the way of life is unconquerable and we as a nation will succeed in this endeavor of Energy Independence.

Solar energy is the source of all energy on the earth (excepting volcanic geothermal). Wind, wave and fossil fuels all get their energy from the sun. Fossil fuels are only a battery which will eventually run out. The sooner we can exploit all forms of Solar energy (cost effectively or not against dubiously cheap FFs) the better off we will all be. If the battery runs out first, the survivors will all be living like in the 18th century again.

Every new home built should come with a solar package. A 1.5 kW per bedroom is a good rule of thumb. The formula 1.5 X's 5 hrs per day X's 30 days will produce about 225 kWh per bedroom monthly. This peak production period will offset 17 to 24 cents per kWh with a potential of $160 per month or about $60,000 over the 30-year mortgage period for a three-bedroom home. It is economically feasible at the current energy price and the interest portion of the loan is deductible. Why not?

Title 24 has been mandated forcing developers to build energy efficient homes. Their bull-headedness put them in that position and now they see that Title 24 works with little added cost. Solar should also be mandated and if the developer designs a home that solar is impossible to do then they should pay an equivalent mitigation fee allowing others to put solar on in place of their negligence.

Installing renewable energy system on your home or business increases the value of the property and provides a marketing advantage.