Easy DIY Solar Lighting
(Page 2 of 5)
April/May 2007
By Charles Higginson
On its way from the PV to the battery, the electricity is routed through a charge controller, which ensures that the battery gets all the power it needs to stay fully charged. It also prevents overcharging.
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How big a battery and panel do you need? To answer these questions, work backward from your desired end result: how much light you want for how many hours a day. We settled on three 15-watt CFLs, which could be distributed around the house for general lighting, each one operating for an hour a day (each CFL’s light output roughly equals that of a 60-watt incandescent bulb). We added an LED light bulb and assumed it would operate for five hours a day, pulling the modest total of about 3 watts. LEDs tend to provide very directional light. In the home, this means they’re not the best choice for general ambient lighting, but they’re great for focused task lighting. (If you want more light than this, the tech folks at Real Goods can help you choose the right components.)
Caution: Math Ahead
Here’s a summary of the process of sizing your system. Once you’ve chosen lights and estimated daily usage, calculate watt-hours (Wh). In our case, it goes like this:
(3 CFL bulbs x 15 watts) x 1 hour/day
+
(1 LED bulb x 3 watts) x 5 hours/day
= 60 Wh/day
Having figured daily watt-hours, you can calculate the necessary battery capacity, measured in ampere-hours (Ah). This involves several factors. First, Norman Franks, renewable energy technician at Real Goods, says battery life is determined by depth of discharge and the number of times the battery is discharged. Franks likes to specify a battery that could power the system for three days without being recharged, to cover cloudy days. Second, he says these batteries should not frequently be discharged below 50 percent of capacity. Third, in practice you’ll get about 80 percent of the rated Ah from a battery before its voltage drops below a useful level.
Taking all this into account, Franks recommended a battery for our system rated at 50 Ah. This is a little overcapacity but would allow expansion in the future.
Pick a Photovoltaic (PV) Panel
How much PV will you need to keep the power flowing? We already know we need 60 Wh per day. To account for various inefficiencies in the system, multiply that by two, yielding 120 Wh. In a typical U.S. location, you’ve got five hours a day of good sun with which to generate watts, so dividing 120 Wh by five hours yields 24 W. That’s the minimum generating capacity of a PV panel that will drive this system.
But there are several reasons to upsize from the minimum. Climate is one; in cloudier areas, you’ll need more PV to get the same power. Seasons are another; a panel that’s adequate in optimum conditions may let you down in mid-winter. Economics also enters the picture. Franks says the cost per watt rises sharply with PV panels below about 40 watts. We chose a 50-watt panel, in part because it was in stock and well priced, and in part to get excess capacity to allow for low winter sunlight and perhaps for system expansion.
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