Electric Load Analysis

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This is an article from Home Power #58, Posted with permission. http://www.homepower.com

(Note: This is one of the most requested articles we've ever published and doing this type of analysis is critical to designing a properly functioning independant (solar or generator) type power system. This only about half of it because much of the article makes little sense without the tables and charts the rest of the article refers to. The entire article is available in PDF format in the download section of our website including all the charts and tables as LOADCALC.PDF......DCK 8/17/99)

Doing a Load Analysis: The First Step in System Design (Partial Article)

Benjamin Root )1997 Home Power

It's not that we really care about electricity. We dont even care about the appliances that the electricity powers. Our wants and needs are even more basic than that. We want to read after dark, hear good music, and learn about what is happening in the world. We want water on demand and unspoiled food. We dont need the electricity like we dont need the drill. What we need is the hole.

Electricity is merely a tool used to meet our needs and wants. When planning a renewable energy (RE) system it is important not to lose sight of what our needs actually are. Only once our needs are defined can we then begin to design an RE system to meet them. We must analyze each need and determine how much energy it takes to meet that need. Long before we start comparing prices on photovoltaic modules we must first create a list of needs called a load profile. This article will first discuss some important considerations in choosing appliances to meet certain needs. Then we will go through a step by step discussion of the various elements in a load profile.

Why Do a Load Profile? RE systems are expensive. Costs to produce ones own electricity from renewable sources average between $0.25 and $1.15 per kilowatt hour (kWh). This is many times the price of buying power from the electric utility. Off grid, it is a waste of money to use more energy than we need to and a waste of money to produce energy that is not used.

If done correctly, your load profiles average daily kWh figure can be quite accurate. Careful load analysis can assure that we size our RE system appropriately. Which Loads are Appropriate Uses for Electricity? Most of us need to eek out as much functionality from as little energy as possible. For example, electricity is an expensive way to produce thermal energy. The electricity needed to provide space heating is generally cost prohibitive. Passive solar, wood heat, and propane furnaces are all much more practical. Domestic hot water heaters and cookstoves are also best powered by passive solar, wood, or gas.

Certain loads can be powered by electricity or by other sources. Refrigeration is a good example. Propane refrigerators are available but have their own set of pros and cons. In an energy efficient home the electric refrigerator (even the energy efficient kind) is usually the largest single load. Many RE systems use electric well pumps, but wind-powered mechanical pumps have effectively provided domestic water for generations. These choices are ours. Do we need a 1,200 watt hair dryer or will a towel do just as well? Is using candles or kerosene for light really a smart (or safe) alternative to compact fluorescents?

Some needs are surprisingly appropriate for use with renewable energy systems. Power tools, microwave ovens, toasters, and other kitchen appliances can draw a lot of power and are often mistakenly considered to be too much for an RE system. Actually, these appliances are used for short periods of time and the energy consumed is rather small.

Why Pay Extra for Efficiency? It might sound like we must do without certain luxuries in order to live with a renewable energy system. This is not the case! RE systems can provide the same amenities that our city cousins enjoy. The trick is to carefully choose how these luxuries are implemented. The most cost effective way to produce ones own energy is to first reduce ones needs for that energy. Richard Perez has a saying that sums it up quite well, Every watt not used is a watt that doesnt have to be produced, processed, or stored. When buying grid power we can dip into a limitless supply and pay as we go. But with RE systems the cost of the energy is the up front cost of expensive system components. Choosing energy efficient appliances is cheaper than renewable energy system components.

For example, compact fluorescent light bulbs have improved immensely. The light is natural colored, flicker free, and very efficient. A 15 watt compact fluorescent produces the same amount of light as a 60 watt incandescent bulbat one fourth of the power consumption. They cost about $22 but last 10,000 hours, about ten times longer than a standard incandescent bulb. More important is the money saved by power that doesnt have to be produced. Saving 450 kWh of electricity, at $0.65 per kWh (a hypothetical middle ground cost for RE based on a well designed photovoltaic system with generator back-up), over the bulbs lifetime translates to about $292 dollars. More than enough savings to cover the $7 price difference between one compact fluorescent and ten incandescents!

Refrigeration is another good example of energy efficiency paying for itself. It is often the largest load in a RE-powered home. A sixteen cubic foot Sun Frost fridge may cost $2,500 but uses only about 540 watt hours each day. A typical major brand, non-efficient fridge may cost only $600 but will use 1,500 watt hours per day. Assuming $0.65 per kWh for an RE system, the electricity to operate the non-efficient fridge for ten years costs about $3,558. The electricity to operate the Sun Frost for ten years costs about $1,281. The difference is $2,277 worth of renewable energy system components that never need to be purchased, and more than covers the $1,900 difference in price. A good rule of thumb says that for every extra dollar spent on energy efficient appliances, three dollars will be saved in energy system components. It becomes obvious that before one dollar is spent on photovoltaic panels, wind generators, or hydro turbines we must streamline our electrical demands.

Are Phantom Loads Really a Big Deal? If you read many Home Power articles then you know phantom loads are one of our biggest pet peeves. Phantom loads use electricity while providing nothing in return. A phantom load is any appliance that consumes power even when it is turned off. While they may seem small they use power twenty-four hours a day. A 4 watt phantom load can cost about $22 a year on an RE system, a lot for an appliance that is supposed to be off. Any appliance with an electronic clock or timer is a phantom load. If we want a clock we should use one that is mechanically wound, battery powered, or even electrical. But a clock in an appliance keeps the appliances entire power supply alive just to tell us the time. Very inefficient.

Appliances with remote controls remain alive while waiting for the on signal from the remote. Any appliance with a wall cube is also a phantom load. A wall cube is a small box that plugs in to an AC outlet to power appliances. Wall cubes consume 20 to 50% of the appliances rated power even when the appliance is off.

Most modern TVs, VCRs, stereos, computers, Fax machines, and other electronics are phantom loads. They may contain a transformer, much like a wall cube, that stays alive even when the appliance is off and consumes between 50 and 200 watt-hours per day. They may also contain a filter or line conditioner, to clean up incoming power for the sensitive electronics inside, consuming 8 to 40 watt-hours per day. Modern televisions have an instant on feature so we dont have to wait for the picture tube to warm up. We might as well call these TVs always on. The most direct way to overcome phantom loads is to unplug the appliance when its not in use. A more convenient technique is to use a switched plug strip. These short extension cords with multiple receptacles allow us to cut all power to multiple appliances with one flip of a switch.

Use care when shopping for appliances that will run on a renewable energy systems. Models that are not phantom loads often have the fewest bells and whistles but are the least expensive.

-- Don Kulha (dkulha@vom.com), August 18, 1999

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