Quote:
Originally Posted by BillG
Pool Man, there are a couple of things to consider regarding sizing of the battery and the panel.
1) How many days do you want the light to operate in the absence of sunlight?
2) How fast do you want the battery to recharge to full capacity?
I think that the first one is the most important, and that determines the battery size. Lets say that you want to turn on the lights at sunset, and leave them on for 12 hours. And you want them to be able to run for 10 days with no good sunlight.
7watts x 12 hours/day x 10 days = 840 Watt hours. Because you don't want to ruin your battery, size it for double that, and don't let it discharge below 50% capacity. So for this example, you might want two 24 series deep cycle batteries, which have about 1000 Watt hours of capacity each.
You will want a charge controller that prevents overcharging and over discharging, the two enemies that destroy batteries. The charge controllers that give the best utilization of your solar panel are called MPPT, or "Maximum Power Point Tracking."
In some cases, you can find a charge controller which also controls the lights. Morningstar makes one, although I don't think it uses MPPT for charge control, rather, the less efficient PWM.
The size of the panel depends on how fast you want to fully charge the battery after being fully discharged. Some also depends on how efficiently the charge controller uses the panel capacity to deliver the charge to the battery.
Figure that the panel can only deliver its full output for 2-3 hours per day on a good day - when the sun is shining directly at it. Before and after that, it is shining at an angle and the capacity is reduced. You will probably want it to deliver in a day, about double what the light bulb draws (84 watts hours in the above example.) If you had an efficient MPPT charge controller, you might want at least 100 Watts of panel. With a less efficient charge controller, you will need a bigger panel, maybe double that.
For what it is worth...
|
Bill, a 100 watt module for 2.5 hours (this is low, I believe the average for the US is 3 to 4 hours of maximum module output per day), gives 100 x 2.5 = 250 watts. A 5 watt source quadrupling the required power needed for a 10 hour night would be 200 watts, that's at ~25% battery discharge, so while, if you can afford it a bigger module is always better, you can go with a smaller module especially if space is limited.
I don't believe there's any place in the lower 48 that doesn't have sufficient sun for 10 days straight. Also 50% is too low a discharge point it reduces the max battery cycles appreciably. My home system is designed for 75 to 80%.
Two 75 aH group 24 batteries will run about $140 each, he's just lighting a 5 watt LED. His moniker is Poor Man!