Creating an "Energy Budget"

Living with renewable energy means living on a budget, an energy budget. In the case of renewable energy the currency you are spending is electrical energy, rather than money. Like any budget, the expenditures in an energy budget must be carefully considered.

If you plan on having a dishwasher, a washing machine or other large appliance you will need a fairly large system. It is important to decide which items you can fit within your energy budget. For example, we have a washing machine but I do all the dishes by hand.

The first, and most important consideration, is deciding what energy needs are most efficiently met with electricity and which ones you can meet with other types of energy.

There are alternatives in home construction that can reduce or eliminate some of these needs. Passive solar heating, superinsulation, straw bale construction and earthship construction can provide heat and eliminate the need for a large heating system. There are also solar water heating alternatives that are very effective.

There are other sources of energy available that will work for some needs just as well as electricity, and that fit well in a renewable energy home. Gas is the most obvious. It can provide the energy for home heating, cooking, and water heating.

Second, as I mentioned in an earlier column, eliminate all phantom loads. What you will be left with will be appliances and lighting for which you will have to supply electrical energy out of your electrical energy budget.

At this point it is helpful to work out an energy budget worksheet. There are many different sources for example worksheets. A good one can be found in the Solar Living Sourcebook published by Real Goods Trading Company, a renewable energy supply company in Hopland, California (1-800-762-7325). A Colorado renewable energy supply company in Boulder, Jade Mountain, Inc. (1-800-442-1972) also provides worksheets.

Each of these budget worksheets will require you to understand the energy usage of your appliances. You can start this process by making a list of all of the things in your home that will require electricity. Next to each, write down its power consumption. All electrical appliances have a label or plate on them that will give their energy consumption in either watts or amps.

You will also need to note on this list the number of hours a day you will use each appliance. Decide how often you will watch TV, listen to the stereo, run a microwave, operate a vacuum cleaner, or use a hair dryer.

When you calculate your lighting usage be aware of how often you leave lights on where they aren't needed. When you provide your own electricity you quickly become good at turning lights off when you leave a room. I also suggest doing your lighting calculations two ways, once using the wattage consumption of regular incandescent bulbs and again using the wattages of good quality compact fluorescent bulbs. You will be amazed at the energy savings with compact fluorescent bulbs which have the added advantage of lasting for years. We have yet to replace a light bulb in our house after 2 years.

With the information on this list you can complete any of the worksheets to determine the size of the various components of your energy system. The worksheet in the Solar Living Sourcebook allows for the sizing of both a PV array and a battery bank.

The heart of any renewable energy system is its battery bank. Regardless of what kind of charging system you have or how you use the electricity you have to have a place to store the energy until you need it.

When I started to design our system I thought I could save money by buying a large battery bank and only a few PV panels because batteries are much cheaper. However I quickly learned that trying to charge a large battery bank with a few panels is like trying to pressurize a very large water tank with a very small pump. It would be very difficult to keep the batteries full.

A good rule of thumb for sizing battery banks is to determine the number of amphours of capacity you will need each day(using a worksheet). Then multiply it by the number of days of reserve you will need (usually 3-5 days). Then double that amount.

I sized our battery bank to provide 100 amphours of current (2.4 kilowatt-hours of energy in my 24 volt system) for 5 days. Using the formula above that means I would need at least a 1000 amphour battery bank. Our actual usage is around 60 to 80 amphours a day which gives us a little room to grow.

This extra capacity is important because batteries don't like to be in a state of discharge. Batteries that have been weakened by being discharged too deeply too often, or kept at too low a voltage, will have a shortened life and will be less likely to provide reliable service.

Batteries function best when they can be discharged less than 50% of their capacity and brought to a full charge regularly. Our PV panels provide us with as much as 190 amphours of current per day, which keeps a good charge on the batteries. Keeping your batteries fully charged as often as possible, a condition known as the float voltage, will ensure they will provide reliable service for their maximum life.

All Contents © 1997
Wagonmaker Press
Thomas W. Elliot


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