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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.
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.
compact fluorescent light (120 vac) compact fluorescent light (120 vac)
radio/cassette player (ac or DC)
Maximum watts Maximum amps
vacuum (12 VDC) computer (120 vac) hard drive (120 vac)
Maximum watts Maximum amps
How much Storage?
Watts Hours Watt-hrs drawn /day /day
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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