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Sizing a solar-electric system for worry-free off-grid living.

Some people just don’t have the right stuff to live off the

power grid. Whether the reason stems from a primal, unwavering revulsion

to storage batteries, or a pathological need to have access to unlimited

power at all times, it’s a simple fact that some folks will never

make the grade. Usually the forces of Social Darwinism sort out the

misfits quickly and efficiently, sending them packing back to the

waiting arms of civilization where hot tubs reside on every deck,

electric towel racks and hair dryers are used with giddy abandon, and

televisions and computers are left running all night.

But others persist against all odds. My wife and I know several of

them; unlikely pioneers ill-favored for the wooly wilds where lurk

temperamental wind turbines and un-tweakable photovoltaic modules,

200-pound storage batteries and persnickety power inverters. For these

fish out of water, the mysteries of off-grid living will forever persist

and multiply.

For the rest of us, however, life beyond the last power pole can be

as rewarding as it is fulfilling, so long as the limitations of off-grid

living are acknowledged and proper steps are taken to mitigate them. The

tricks to living with solar and wind energy are endless–after 10 years

spent off the grid we’re still learning new ones-but if due

consideration is given to properly sizing your renewable energy system,

the path to energy independence will be less fraught with dead ends and

bottomless pits.

Whether for monetary concerns or a misplaced belief in their own

abilities to conserve energy, lots of folks tend to undersize their

photovoltaic systems. Besides making life an onerous exercise in

frugality, it can also be hard on the batteries if they are routinely

drained beyond their safe limit every time the demands of the house

exceed the output of the renewable energy system. And, if (as is usually

the case) a backup generator is used to make up the shortfall, the

investment in fossil fuel soon exceeds the extra cost of a more robust

system, with no equity to show for it.

Admittedly, sizing a home solar-electric system is not an exact

science. It’s easy enough to derive a set of numbers and run them

through the proper manipulations, certainly, but how closely the bottom

line agrees with reality is another matter. When it’s all said and

done, the system you end up with will be a reflection of your ability to

examine your lifestyle and your needs objectively. So, as a failsafe

mechanism, you should always leave room to add more batteries and solar

panels at some point in the future.

Electrical usage

Your first step in sizing a solar electric system is to determine

how many kilowatt hours (kWh) of electricity your off-grid home will

require, on average, each day (see the reference worksheet and energy

consumption table). Most responsibly sized off-grid homes use less than

10 kWh/day. If you can squeeze that figure down to six or seven

you’ll save a bundle. As you go sleuthing for watts, you should

take into account everything that will draw power in your new off-grid

home, from the well pump to the furnace blower fan, from the clock on

the microwave to the GFCI outlets in the kitchen and bathroom. And, of

course, you should note how many minutes or hours per day each one is in

use. This includes every plug-in appliance you own. For these things you

can use a meter, such as a Watts Up? or a Kill a Watt meter, to measure

either immediate usage or prolonged usage over time (as for a fridge or

a freezer).


By the time you’re done with this exercise the $10.00-per-watt

figure you were probably quoted for your system will begin to loom large

in your thinking. Practically overnight you will become a self-made

efficiency expert, on a mission to replace every outdated appliance you

own with a sleek, high efficiency model. Do it; the planet will love you

for your efforts.

Solar array sizing

Once you have a good idea of how much electrical energy you will

need each day to run your off-grid home, the next step is to calculate

how many solar panels it will take to provide it. There are several ways

to do this, ranging from scientific to mystical (all else being equal,

science usually works best). However you go about it, the first number

you should be looking for is kilowatt hours of solar radiation per

square meter per day (kWh/[m.sup.2]/day) in the area where you live, and

the best place to find it is on the Web at: http://rredc.nrel.

gov/solar/pubs/redbook/. This is a PDF file of the National Renewable

Energy Laboratory’s venerated Solar Radiation Data Manual for

Flat-Plate and Concentrating Collectors, commonly known as the Redbook.

In it you will find, for hundreds of U.S. cities, the average solar

radiation for every month of the year at solar-array tilt angles ranging

from flat to vertical. (For a thorough explanation of the Redbook data,

read “Calculating Your Daily Solar Energy Harvest” in the

May/June 2007 issue.)

Conveniently, “kWh/[m.sup.2]/day” translates nicely into

“hours of noon-equivalent sunlight.” To determine how much

electricity your proposed solar array will produce, then, you merely

have to multiply the kWh/[m.sup.2]/ day for your location by the

array’s kilowatt rating, being sure to factor in the system

efficiency (usually around 75 to 85 percent, depending largely on the

type of charge controller you intend to use.) So, if your area boasts

4.3 kWh/[m.sup.2]/day of sunlight in December, your solar array is rated

for 1. 82 kilowatts, and you plan on using a standard charge controller,

your average daily production for December should be:

4.3 x 1.82 x 0.75 = 5.86 kWh

Or, by trading up to a Maximum Power Point Tracking (MPPT) charge

controller that constantly looks for the “sweet spot” in the

array’s power curve, you might gain as much as an extra 0.79 kWh:

4.3 x 1.82 x 0.85 = 6.65 kWh

Likewise, by perusing the Redbook data, you will quickly discover

how much you can improve your system’s efficiency by adjusting the

tilt angle of the array as the sun moves through the seasons.

Sizing the battery bank

After determining your off-grid home’s approximate electrical

usage and the size of the solar array needed to power it, the final step

is to size the battery bank. This is a comparatively easy and

straightforward undertaking, involving just a couple of variables.

Specifically, all you will need to know is the home’s baseline

energy usage and how many (very cloudy) days you want your battery bank

to be able to sustain that level of discharge before you finally relent

and fire up the smelly old generator. For most of us, that’s two or

three days.

Suppose, for instance that you have calculated your off-grid home

to use six kWh/day–at least on the days you don’t wash clothes,

sing lengthy ballads in the shower, or eat microwave popcorn–and

you’d like to be able to run the house for three severely cloudy

days before resorting to backup power. That means you’ll need 18

kWh of available power which, practically speaking, is about half of the

total battery capacity, since to drain the batteries any further may

well cause irreparable damage to them.

Batteries, however, do not come with a kWh rating; they are instead

rated in amp hours. The popular L-16 off-grid solar battery, for

example, is rated at around 400 amp hours at six volts. To translate

that into kWh you simply multiply: 6 (volts) x 400 (amp hours) = 2,400

watt-hours, or 2.4 kWh total capacity, of which you should never use

more than half. That will leave you with 1.2 kWh per battery, meaning

you will need fifteen L-16 batteries (18 kWh / 1.2 kWh per battery = 15

batteries) to power your house for three consecutive cloudy days. Call

it 16 batteries, since you will have to buy 6-volt batteries in

multiples of four or eight for 24- or 48-volt systems.

Don’t forget the meter!

Having successfully calculated the electrical usage of your

off-grid home and the size of the solar system needed to run it, do

yourself a favor: install a battery meter, such as the TriMetric Battery

Monitor by Bogart Engineering. It will tell you at a glance how much

charge remains in your batteries, plus the voltage, charging rate and a

lot more. Call it a gas gauge for the system. After what you’ve

just put yourself through, you and your family deserve it.

Solar tax credits

As unpalatable as it might have been, the ITC bailout bill that was

signed into law in early October did contain one tasty tidbit: the 30

percent tax credit for residential solar-electric and solar hot-water

systems was extended for another eight years. Best of all, beginning in

2009 it will be uncapped from the previous limit of $2,000 per system.

This means you can claim a tax credit of 30 percent of the cost of your

entire system, no matter what it costs. Go wild.

For details, see:

Rex Ewing is the author of several renewable energy books,

including Power With Nature, Got Sun? Go Solar, and the newly released

Crafting Log Homes Solar Style. He lives with his wife, LaVonne, in a

handcrafted log home powered solely by the sun and wind in the foothills

of Colorado. His books can be purchased at the Countryside Bookstore or

at www.pixyjackpress. com.



Energy Consumption of Appliances

Using a WATTS-UP? Meter, we measured the following appliances

Appliance Continuous Draw (Watts)

Computer, desktop 90

Computer, laptop 24

17″ monitor 100

17″ LCD (flat screen) monitor 50

HP LaserJet printer (in use) 600

HP Inkjet printer (in use) 15

Microwave (full power) 1,400

Coffee Maker 900

Toaster, 2-slice 750

Amana Range (propane): Burners 0

Oven (with glow bar; when heating) 380

Electric Range (small/large burner) 1,250/2,100

Blender 350

Mixer 120

Slow Cooker (high/low) 240/180

20″ Television 50

27″ Television 120

50″ LCD Television 175

Stereo System 25

Stereo, small portable 10

Vacuum, Creek 410

Vacuum, Dirt Devil Upright 980

Table-top Fountain 5

Sewing Machine (Bernina) 70

Serger (Pfaff) 140

Clothes Dryer (propane) 300

Clothes Iron 1,200

Hair Curling Iron 55

Hair Dryer (high/low) 1,500/400

Furnace Fan (1/3 hp / 1/2 hp) 700/875

Guitar Amp (ave. volume) 45

Jimi Hendrix volume 8,500

Appliance Watt Hours

Dishwasher, cool dry 736 watt-hours/load

Clothes Washer

(front-loading) 145 watt-hours/load

Air Conditioning 1,500 watts/ton

(or 10,000 Btu) of capacity

We have not listed refrigerators or freezers since

their efficiency is getting better every year. Look at

the website for the latest ratings.

There are many models at, or under, 400 kWh per

year, or about 1 kWh per day.

SOURCE: Power With Nature book by Re, Ewing (PixyJack Press)

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