4.2 Calculating Your Energy Consumption

Determine total energy consumption per day

The generating capacity you need and the battery storage you need depend on the power you consume. Regardless of how big your bank of batteries is, if you use more energy than the wind turbine and photovoltaic panels can produce, you’ll eventually run down your batteries or need to rely on a fuel-powered generator. This is why it’s important to realistically calculate the total kilowatt hours of electricity you expect to consume before you decide on the size of your wind turbine and photovoltaic array.

It’s often an iterative process. You’ll probably do this calculation based on your current consumption habits, determine the system size you would need, look at equipment, and be a bit shocked by its cost. That’s the time to look carefully for more ways to conserve by using power more efficiently, and then redo the calculation. It’s a bit like how many people fill out their income tax form: do it once, and when that shock wears off, look for a few more neglected deductions to reduce the bill.

Peak power vs. total energy consumption

An important part of designing your system is determining your peak power demand. This is the most power, in watts, that you’ll need at any given moment during the day.

It’s not the same as total daily energy consumption, which is all the power you use over the whole day, measured in kilowatt-hours.

For ballpark peak power demand numbers, you will probably need at least 3,500 watts. While some very frugal people can be happy with, say, 1,500 watts of peak output, most people need at least a 3,500 watt peak power capacity for bare-bones basic household needs, especially if you pump your own water with electricity. You’ll find 5,000 watts ample for a few more things than the basics, as long as you don’t have any heat-related loads.

Fritz and Anne’s system offers 5,000 watts in the house, and a separate 5,000 watts of AC output from batteries in the workshop. Household cooking and heating water takes a ton of electricity and should always be done using fuel-based energy sources like wood or propane, or directly using heat from the sun. Even a single stovetop element on an electric range draws 2,000 watts of power, so you don’t want to use electricity to make heat in that circumstance. Power is easy to squander when you’re on the grid but substantially more difficult and expensive when your independent electrical system has to provide the power. If you want a clothes dryer instead of a clothesline, for example, choose a model that uses propane or natural gas for the heat source, with electricity to run the rotating drum and controls.

Watch the video up next to see how a typical daily energy consumption calculation works.

VIDEO: Determining daily energy consumption

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Peak water or total water?

Water consumption makes a good analogy to understand your electricity consumption. A family’s peak water consumption is probably in the morning, when one teenager is having a shower, the toilet in another bathroom gets flushed, and someone else is running a load of laundry. That’s the same concept as peak power consumption – the one moment in the day when you’re using the most electricity.

Total water consumption is all the water you use, adding up what you use in the high-demand mornings, the low demand overnight hours when everyone’s sleeping, and all the times in between.

If the water pressure in the shower has ever dropped to a trickle, you’ve probably realized that you’re using too much water somewhere else — your peak consumption is higher than your pipes can deliver. The easiest solution is to spread out your demand by, say, not doing laundry in the mornings. But if your water bill is too high, you need to reduce your total water consumption.

Electricity is much the same. When you reduce your total daily consumption and smooth out some of those peaks, your generating and storage capacity can be smaller and cheaper.

Using your electricity bills to estimate consumption

Your power company has a keen interest in measuring how much electricity you consume, in order to bill you each month. You can use your bills to calculate how much power you now consume simply by adding up the kilowatt-hours from a years’ worth of bills and dividing by 365 to get your average daily consumption.

Don’t take one month and divide it by 30 or 31. There’s a good chance your consumption changes seasonally, so you want a full year to get an accurate picture of your usage patterns.

Because almost everyone on the grid uses power without thinking about it, this consumption figure will almost certainly be higher than it will be when you are generating your own electricity. This is especially true if you’re building a new home and have the opportunity to insulate it well, choose energy-saving appliances and generally make it very energy efficient. Think of the number calculated from your bills as an upper limit of your likely consumption.

Using electrical device wattage to estimate consumption

Even if you have bills to work from, it’s a good exercise to work the numbers out from the other end, by adding up all the demands you anticipate. The exercise is a bit tedious, but it isn’t difficult. 

First, determine the typical daily electricity consumption, in kilowatt-hours, of the devices you plan to use.

Start with the basics. If your water comes from a well via an electric pump, you’ll need to power it, of course. Refrigerating and freezing food are must-have technologies for most of us, too. Add lighting and basic charging capabilities and you’re looking at the essentials of what you need for long-term comfort and security. Total all these needs and you get the magic number for basic power from whatever source you’re looking at, whether it’s wind, solar or both. But what about computers, televisions, stereos, cooking appliances, and even your electric toothbrush? Fans or AC? You can include these too, but just remember that the more you add the bigger and more expensive your off-grid system will be. 

The easiest way to estimate what various electrical items consume is by searching online. Google “energy consumption of household appliances,” for example, for lists of typical figures. These numbers may vary a bit from your own experience, but they’ll be close enough for these calculations. Here are some numbers you’ll find for the basics:

  • Lighting: 0.5 kWh per day
  • Computer: 0.75 kWh per day
  • Water pump: 1 kWh per day
  • Chest freezer: 1 kWh per day
  • Cordless tool charging: 1.5 kWh per day
  • Fridge: 3 kWh per day

Once you’ve added up these numbers, round them up to be safe.

Total electricity usage from this basic list works out to 7.75 kWh per day, but round it up to 10 kWh per day for calculation purposes. That’s 300 kWh of electricity per month. And while this covers the bare essentials, it’s really not much compared to what most people consume while connected to the grid, and this brings us to an important point.

A word (again!) about conservation

As I've mentioned before, the first step to creating an off-grid system is economizing on items that currently uses a lot of electricity, especially when those items also use a large surge of power on startup. This is because off-grid energy systems aren't cheap, so you want to make them as small as possible while still meeting your needs.

Heat pumps and air conditioners (especially large, central air conditioners) are two items that many people have, and both use a lot of power. A learner who is interested in self-reliance wrote to ask me about this, and my response about her heat pump is instructive.

I explained that to be more self sufficient, she should consider a self-contained source of heating. Heat pumps are great in many ways (they deliver 2x to 3x as much heat energy as electricity consumed!), but the drawback is still the large electrical current draw they demand.

A heat pump draws more energy than is practical to store yourself—you pretty much have to be connected to the grid and dependent on it. In theory, you could install a very large propane generator to operate your heat pump, but that would use much more energy to heat your home than it would to burn the propane directly. 

I ask myself a question: How many days of completely independent living do I want for my household? In my home, we could probably last comfortably for a couple of months with no outside inputs of energy or food or trips to a store. By contrast, if you’re relying on your heat pump for space heating, and you have no generator backup or alternative heat source that doesn’t demand tons of electricity, in four or five hours of no electricity you’ll be feeling cold, and in a couple of days your pipes will be bursting.

Central air conditioning is another big electrical draw, and it’s much used in hot climates. But it’s highly impractical to run a large air conditioner on off-grid energy sources.

Although I don't yet have any first-hand experience with natural gas-powered air conditioners, they could provide an excellent way to keep electricity costs down while also keeping cool. People’s Gas is one company that offers a natural gas air conditioning system. The system is essentially a heat pump that creates cool air in summer (and warm air in winter). It’s probably worth investigating.

Precise power monitoring

If you’re trying to assess your demand for AC power accurately, one reliable approach is to use a power consumption meter. Tools like the Kill A Watt meter measure precisely how much you’re consuming in kilowatt-hours. Plug the Kill A Watt into an outlet, plug a 120 volt device into the Kill A Watt, then watch the screen to see how much energy you’re using.

After measuring or calculating your current demand, or looking at your electricity bills, you may feel the cost of an independent system to power essentials isn’t justified. But don’t give up just yet. The power consumption figures you’ve read about so far and most of the ones you’ll find online are for regular appliances designed to be plugged into 120 volt AC grid power. Those appliances are not particularly efficient. If you switch to appliances designed for 12 or 24 volt DC current, however, energy consumption goes way down.

Avoid wasting electricity on heat

You may have noticed that none of the items on my list of essential electric devices are engineered to produce heat. A few give off some heat as wasted energy, of course, but their intention is to produce light, motion or refrigeration. Producing usable amounts of heat takes a lot of energy. As you design your grid-free electrical system, plan to use direct combustion technologies such as a woodstove or propane appliances wherever you need heat. 

The only exception to my advice to avoid generating heat with electricity – aside from perhaps using a clothes iron occasionally or having a small hot plate or microwave – is in the emerging technology of DC heating elements, which can be used as a dump load to make use of excess electricity. This is an exciting development. A dump load is simply a way to use excess electricity when you’re generating way more than you need or can store in your battery bank. I’ll go into detail in the sections on wind turbines and photovoltaic panels.

By now you’re probably starting to realize that being connected to the grid has made most of us energy hogs. With so much power available from local utilities, it’s very easy to misunderstand what true energy efficiency means. Once you start pricing a wind turbine, solar panels and batteries, however, you have a vested interest in making the most use of the least amount of electricity.

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