Here is a note I posted several days ago on the Motorola list about solar powering.
From: Chuck McCown [mailto:[EMAIL PROTECTED] Sent: Friday, November 14, 2008 9:17 AM To: Dave Crim Subject: Re: solar Continuing on a bit, lets say you have 5 lousy days and one good sunny day followed by 5 more lousy days. That one sunny day needs to store enough to charge the batts totally. 75 watts * 5 days * 24hours * 1.25 (batter efficiency loss) + (75watts * 24 hours) current day = 13050 watt hours. To make 13050 watt hours in one 10 hour day you go: 13050/.707*10=1845 watts. You need 1845 watts of panel to do this. That is 24 times the load. So, my rule of thumb of 20 times the load is still a little shy of being conservative. The thing that saves you in a situation like this is a massive battery. A one month battery with 20 X panels will never fail due to a lack of sun energy. A 2 week battery and 10X panels will fail now and then every single winter. Sometimes for several days at a time. ----- Original Message ----- From: Chuck McCown To: [EMAIL PROTECTED] Sent: Friday, November 14, 2008 9:09 AM Subject: solar Several thing you may not be including. Assuming this panel is somewhere in this neck of the woods, December 21 has 10 hours of time between runrise and sunset. But if you don't have tracking mounts (most don't) the amount of energy you get out of a panel follows the first half of a sine wave. To estimate that energy, you integrate the area under the curve. That will equal .707 of what you thought you were going to get. So, let's say you put up a 500 watt panel, your daily sunny output will be an average of 353 watts. The sun shines for 10 hours solid and you store 3530 watt hours in your battery. Now your load is on during the daytime, so if you have a 75 watt load, you are now able to make 278 watts. You are down to 2780 watt hours. You put in 2780 watt hours into a battery and you get maybe 80% back out. So you have 2224 watt hour available (if you drain the batts which is not good for them). You have 14 hours of darkness and actually more like 16 hours before the panel starts making any useful amount of energy. 16*75=1200 watt hours. But that next day is not sunny, there is frost and a light coating of snow. The whole works dies 16 hours later. About 9 pm. One other note, you only want to draw your batteries down no more than 10% each night or they won't last long. That means a minimum of 12000 watt hours. If that is a 12 volt system, 1000 amp hours. If it is a 24 volt system, 500 amp hours. And that is a minimum because here we get a week with snow and ice and no sun easy, sometimes two weeks. You really need a generator, less load, or a whole bunch more batts and panels. -------------------------------------------------------------------------------- WISPA Wants You! Join today! http://signup.wispa.org/ -------------------------------------------------------------------------------- WISPA Wireless List: wireless@wispa.org Subscribe/Unsubscribe: http://lists.wispa.org/mailman/listinfo/wireless Archives: http://lists.wispa.org/pipermail/wireless/
