Estimating Power Usage

Before building your off-grid power system, you will need to know how big a power system to build. We get that information by looking at what electricity we use now and how much we expect to use while off-grid. This will only be an estimate, but any estimate is better than none.

Conserving Power

The first step in estimating power usage is to minimize usage wherever possible. Conservation makes sense (and cents) on-grid as well as off, but when we go to generate all our own power it is doubly important to use electricity wisely.

Does this mean we can't have a night light? Or leave an outside light turned on when we are away? Not at all. But it does mean you should use a compact flourscent for that night light and shop for the highest efficiency on your outdoor lights as well.

Estimating

There are two approaches to estimating power needs: tables and nameplates, and two types of usage estimates that must be done: peak and average.

Tables can provide a rough guide for initial system sizing and will generally be sufficient so long as this estimate is modified to account for your particular lifestyle. A table of typical appliance power needs and the procedure for using it is included below.

The nameplates on your own appliances and can be used rather than the typical numbers provided by a table. This will sometimes provide a different total where your appliances are non-typical.

Estimated peak power is your best guess at how much stuff you&quote;ll have turned on all at once. At that once a year dinner party or when you have all thirteen grandkids visiting, you will be using way more power than normal. Is it reasonable to size for this? That is a question of economics. You decide.

Making a good guess at average power is critical to sizing the components in your system, especially batteries. Average power estimation is individual. Given the same house, two different occupants may have widely different monthly power usages.

Once a list of power usage numbers is obtained, either from a table or from the acutal appliances, the proceedure for adding up your total power demand is the same. Our first thought might be to simply add up the power usage of every single light and appliance to arrive at our total need. This would be far more power than you would normally use. When was the last time you had every light on at once?

So the first thing we'll do is make an assumption about how much of your lighting is on at the same time. There are no hard and fast rules here, just good guesses, so I'll give you mine with that understanding.

Rule 1: Everyone is different in their light usage. For example, I seem to be the only one in my house that knows that lights turn off as well as on. I also have decent night vision so I often don't use lights in places like hallways just to pass between rooms. My family always uses hallway lights.

Rule 2: People use rooms, not lights, and there is typically more than a single light in each room. Unless you have a very specific room and lighting layout in mind, figure at least one area light and one close/work light in each room. For standard lights this would be a 60 watt and a 100 or 150 watt bulb. Don&quote;t use dinosaur incandescent lights, use equivalent compact flourescent lights of 15 to 25 watts each to get the same light output of power hungry incandescents.

Rule 3: People use lights only where/when it is dark. Duh! OK, so it is dark while the sun is down, which happens a bunch more in the winter than in the summer, and in places where the sun don't shine--like basements and windowless hallways.

Rule 4: One person uses less light than multiple people. If everybody stayed together and moved from room to room as a group (weird!) then this would not be true, but what actually happens is one person is reading in the living room while someone else is in the bathroom and a third person is in the kitchen. One person, assuming they knew about the the &quote;off&quote; position, would not be using the lights in three rooms all at once. On the other hand, families do spend sometime together, so I figure the total lighting demand for each additional person is about 75 percent of the lighting demand of one person. For example, I would estimate that three people use about two and half times as much lighting as one person does.