Sizing a System

Sizing a solar electric system can be tricky, especially if you are installing a system on a house you’re just moving into or a house you are building. In such cases, how do you know how much electricity you’ll need?

What most people do is make a list of all the appliances and electronic devices that will be used in the house. Once this is complete, they estimate how long each appliance or device is used each day. They then multiply the power consumption (in watts) of each device, (Power consump­tion is listed on all electronic devices on a small plate or sticker on the back of the unit. Go check your microwave or TV right now,)

Another way to calculate usage is to consult a table that lists typical wattages of household appliances and electronic devices. Figure 8-12a shows a typical appliance wattage chart. It gives the elec­trical consumption for each device.

Multiplying wattage of each appliance by the estimated time it is used each day gives you the daily electrical consumption in watt-hours.

From this analysis, you should be able to calculate your total daily energy requirement in watt-hours. Note, how­ever, that some appliances or electric devices may be used more during some parts of the year than others. Lights, for example, are on more often during the winter than the summer because the days are shorter in the winter. The television may run less frequently during the sum­mer because the kids are outside playing. Try to take such differences into account when computing energy consumption (Figure 8-12b).

Once you know how much electricity you consume, you need to perform some additional calculations. These are typically in a worksheet form. You can also complete this process using computer software.

The worksheets available through Solar Energy Internationals book Photovoltaics: Design and Installation Manual help you determine electric load (demand for elec­tricity) as just described. You are then led through a few computations to determine the size of the inverter you’ll need. Once you know this, you can determine the size of the battery bank you’ll need if you are designing an off-grid home. The work­sheets then lead you through calculations to size the array — that is, to determine the number of PV modules you’ll need. This calculation takes into account your demand, the efficiency of the entire sys­tem, and solar availability.

If your eyes are starting to glaze over, don’t despair, A local installer can run the numbers for you. Even if you are planning on installing the system yourself, consider hiring a local installer to consult with you at this stage. (Be careful, if you install a system yourself. You’ll be dealing with high-voltage DC and AC electricity that can be dangerous!)

Off-grid PV systems should be sized for the month with the highest energy demand. You can learn how to do this by taking a solar electricity course or by reading more advanced books on solar electricity. An installer will do the math for you, at no charge.

Grid-connected systems are sized for annual energy production. That is, you determine how much electricity you need annually. Once this is determined, you can size a system based on average daily use and the amount of sunshine in your region. Again, if your system is going to be installed by a professional, they’ll run the math.

In an existing home, it is easy to determine the amount of electricity you consume each year. But in a new home, you’ll need to make an educated guess.

Table 8-12a

Typical Wattage Requirements for Common Appliances

General household

Kitchen appliances

DC pump for house….

…….. 60

Air conditioner (1 ton)

…. 1500

Blender……………………

….350

pressure system (1-2 hrs/day)

Alarm/Security system.

……… 3

Can opener (electric)…

…100

DC submersible pump.

…….. 50

Blow dryer……………..

… 1000

Coffee grinder…………..

….100

(6 hours/day)

Ceiling fan……………..

….10-50

Coffee pot (electric)…..

..1200

Central vacuum……..

….. 750

Dishwasher………………

..1500

Entertainment

Clock radio…………….

……… 5

Exhaust fans (3)………..

….144

CB radio………………..

…….. 10

Clothes washer……….

… 1450

Food dehydrator……….

….600

CD player………………

…….. 35

Dryer (gas)…………….

…… 300

Food processor…………

….400

Cellular telephone…..

…….. 24

Electric blanket………

….. 200

Microwave (.5 ft3)……..

….750

Computer printer…..

…… 100

Electric clock………….

……… 4

Microwave………………

..1400

Computer (desktop)….

…80-150

Furnace fan…………..

….. 500

(.8 to 1.5 ft3)

Computer (laptop)….

… 20-50

Garage door opener. ..

….. 350

Mixer………………………

….120

Electric player piano….

…….. 30

Heater (portable)……

… 1500

Popcorn popper………..

…250

Radio telephone…….

…….. 10

Iron (electric)…………

… 1500

Range (large burner)……

..2100

Satellite system……..

……… 45

Radio/phone transmit.

..40-150

Range (small burner)…

.1250

(12 ft dish)

Sewing machine……..

…… 100

Trash compactor……….

..1500

Stereo (avg. volume)…

…….. 15

Table fan……………….

….10-25

Waffle iron………………

..1200

TV (12-inch black & white)…. 15

Waterpik………………

…… 100

TV (19-inch color)…..

…….. 60

Lighting

TV (25-inch color)…..

…… 130

Refrigeration

Incandescent (100 watt)

…100

VCR……………………..

…….. 40

Refrigerator/freezer..

….. 540

Incandescent light……

…. 60

22 ft3 (14hrs/day)

(60 watt)

Tools

Refrigerator/freezer..

475

Compact fluorescent….

…. 16

Band saw (14")………

…. 1100

16 ft3 (13 hrs/day)

(60 watt equivalent)

Chain saw (12")……..

…. 1100

Sun Frost refrigerator…

…….112

Incandescent (40 watt)..

….. 40

Circular saw (7%")…..

…… 900

16 ft3 (7 hrs/day)

Compact fluorescent…

…. 11

Disc sander (9")……..

…. 1200

Vestfrost refrigerator/.

……. 60

(40 watt equivalent)

Drill (УО………………..

…… 250

freezer 10.5 ft3

Drill (У2")………………

……. 750

Standard freezer……..

….. 440

Water Pumping

Drill (1")………………..

….. 1000

14 ft3 (15 hrs/day)

AC Jet pump (% hp)……

….500

Electric mower………

…. 1500

Sun Frost freezer…….

….. 112

165 gal per day,

Hedge trimmer………

…… 450

19 ft3 (10 hrs/day)

20 ft. well

Weed eater…………..

…… 500

Fig. 8-12: Sizing о Solar Electric System, (a) This table (b) This worksheet can be used to list all of your

provides typical wattage (power consumption) readings appliances and to determine how much power they use

for major appliances and household electronics. each day.

Sizing a System

But remember: reduce your demand first! Apply energy conservation measures vig­orously before you estimate how much electricity you will be using,

BUYING A SOLAR ELECTRIC SYSTEM

When it comes to buying a solar system, many people wisely turn to a local PV

dealer/installer who can select the com­ponents and ensure that all of them work well together. Although this option may cost a bit more than those I’ll explain shortly, it’s a good approach. A compe­tent local installer can answer all of your questions and take care of problems that may arise. (Be sure they really know what they’re doing.) Unless you’re mechani­cally inclined and pretty knowledgeable, embarking on this process yourself can put you on a steep and treacherous uphill climb. "Even licensed electricians often need help from experienced PV install­ers," notes Weiss. You can find a list of local installers at homepower. com or at findsolar. com. Also, be sure to check out the local business directory your phone company provides.

Another approach is to buy a system from an Internet supplier. This approach can save you a substantial amount of money, as deep discounts are available through the Internet. Once the system arrives, though, you will have to install it yourself, or try to hire a local installer to do it for you. However, local installers may not be happy that you cut them out of the first part of the deal. (They make a little profit on the sale of equipment.) If you do buy through an Internet supplier, you’ll need to know quite a bit more about PV modules, racks, inverters, charge control­lers, disconnects, and batteries than when purchasing a system from a local supplier/ installer. If you decide to take this route, you should read one of the books on solar electricity in the Resource Guide to deepen your knowledge. I’ve provided a lot of information in this chapter, but there’s much more to know. As Weiss points out, "PV systems are not plug and play."

For years, my favorite book on solar electric systems has been The New Solar Electric Home by Joel Davidson. It pres­ents a lot of technical information in a way that is amazingly comprehensible, and it was recently updated. Unfortunately, most of the books on solar electricity are penned by engineers or tech-heads for whom writing is not their strength. There is at least one exception, though, that might be useful to you: Johnny Weiss and his colleagues at Solar Energy International in Carbondale, Colorado have written Photovoltaics: Design and Installation Manual. This book is a man­ual for individuals who want to size, design, and install solar electric systems. It is generally well-written and full of good information. It should bring you up to speed on the subject so that you can size and design your own system and purchase components with confidence. I’d also strongly suggest taking a workshop or two to hone your skills. Hands-on experience is vital. You can take classes at The Evergreen Institute’s Center for

Renewable Energy and Green Building, and a number of other places, including Solar Energy International in Colorado, the Solar Living Institute in California, or the Midwest Renewable Energy Association in Wisconsin* Many other educational programs have come on line in the past few years. Be sure instructors have experience and can explain this dif­ficult subject clearly*

Be very careful when shopping for and purchasing the components of a solar electric system* This requires a lot of knowledge and attention to detail; be certain that you are working with a very knowledgeable dealer who really knows what he or she is talking about and offers solid technical support* Look for a supplier whos been around for a long time and who will sell you what you need, not what they have in surplus* In recent years, there’s been an onslaught of Internet renewable energy suppliers* Some of them operate from remote sites; they have no inventory, so everything must be drop-shipped from the manufacturers. They typically offer little, if any, technical support* They lack exper­tise and may have lousy return policies* They may even charge you to replace items that they shouldn’t have sold you in the first place* As a starter, I recommend that you visit Solatron Technologies’ website (solaronsale. com) and read “Six Important Questions to Ask before Choosing an

Alternative Energy Dealer*’’ (As a side note, I’ve purchased PV modules, Water Miser battery caps, batteries, and an inverter through this company — and even sold a used inverter through their website — and have found their service and products to be exceptional* Moreover, their website is one of the most secure sites from which you can order* I also order a lot of compo­nents from Northern Arizona Wind and Solar* There* End of free advertisements.) Another highly reputable supplier is Real Goods* They’ve got excellent, highly knowl­edgeable people who can help you size and select components* Other online suppliers provide top-notch service and products, too, but you need to be careful when shop­ping for an online supplier* Look through Home Power and Solar Today magazines* Check out each company’s website, its return policies, its expertise and level of technical support, and other aspects of their business* Does the company actu­ally have a location, or are they operating from someone’s home? Ask friends who may have dealt with online dealers for recommendations*

Updated: September 20, 2015 — 12:23 am