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Back to the Basics

Me and My

Panel Therese Peffer

©1992 Therese Peffer

Idid it. I decided to make my own power. I live in a trailer about a 100 foot extension cord length from

Home Power Office & Power. The batteries are filled with electricity from photovoltaic panels and a wind generator — I can’t complain about the source. But, well, sometimes the extension cord gets “borrowed”, and there are “black outs” when Richard changes inverters (he counters that I haven’t been paying my bill). I decided to create my own system to learn and to do a system for my folks.

The Plan

I decided to use a photovoltaic (PV) module to make electricity from the sun, and a battery to store it in, but what kind of PV module and what size battery?

First I made a list of the appliances I use and whether they use Direct Current (DC) or alternating current (ac). I also have a clock, but it is a wind-up model. Since my only ac loads right now are lights, I may buy a DC light instead of buying an inverter to change DC to ac current. I’ve seen DC compact fluorescents and halogen lights ranging from 11 to 50 Watts.

Then I looked at how much power or watts each appliance draws. The figure is usually stamped on the back or bottom of the appliance and often is not exact, but gives a fair estimate. Next, I listed how long I use each during the week. I also thought about expanding in the future. Our area is really dusty, so a small car vacuum would be nice. I’ve been thinking about getting a computer someday, too. I multiplied the wattage drawn by each appliance by the hours

used per day to get an idea of how much power I need. Now I have an idea of how much electricity I need — about 100 Watt-hours per day. In the future, I may need over 212 Watt-hours.

Energy Consumption

Appliances

compact fluorescent light (120 vac) compact fluorescent light (120 vac)

radio/cassette player (ac or DC)

Maximum watts Maximum amps

Future Appliances

vacuum (12 VDC) computer (120 vac) hard drive (120 vac)

Maximum watts Maximum amps

How much Storage?

Watts Hours Watt-hrs drawn /day /day

Total

Future

Total 212

The capacity of a battery — how much it can store — is rated in Ampere-hours. To figure out how big of a battery bank I need, I converted Watt-hours to Ampere-hours. Since power (Watts) equals voltage (Volts) times current (Amperes), I divided the number of Watt-hours by the Volts. I will use a 12 Volt battery, so I divide 100 Watt-hours by 12 Volts to get 8.3 Amp-hours.

Another concern is the usable capacity of the battery. Lead-acid batteries should not be fully discharged — you can’t regularly use the full capacity. A 40 Amp-hour lead-acid battery cannot deliver 40 Amp-hours. If the battery is a deep-cycle battery (designed for deeper or fuller discharges), one should never use more than 80% of the capacity. For car batteries, only 20% of the capacity should be used — any more will decrease the life of the battery. I’ll use a car battery a friend gave me for now, and maybe get a deep cycle or alkaline battery in the future. So I divided 8.3 Amp-hours by 30% to get 27.7 Amp-hours. I need a battery rated at least 27.7 Amp-hours; the car battery is rated 40 Amp-hours.

But what if the sun doesn’t shine? We have stretches of cloudy days, about three in a row on average. Since I want to be able to turn on my lights during this period, I want to have a battery capacity of at least three days: 27.7 Amp-hours per day x 3 days = 83.3 Amp-hours. I don’t have this capacity right now, but will make do with what I have. I will just watch my use on cloudy days until I get a different battery.

78 Home Power #32 • December 1992 / January 1993

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