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Author Topic: Help with Emergency Solar system sizing  (Read 12362 times)

Posts: 2

« on: January 17, 2012, 10:49:58 PM »

I want to build a small emergency solar system to power my FT-897D radio (22A at 100W transmit) and a few other small things.  I am using 25A as my max. load.  Just receiving, my load is probably 3A or less.  Max. usage is about 3 hours per day – mostly listening.  How many watts of solar panels do I need and how many Amp Hours of battery capacity do I need?  What size controller and wire do I need?

Does anyone know how to calculate the answers to these questions?  I have found quite a few solar sizing calculators on the internet, but the results they generate vary greatly.  I would think there are some good rules of thumb for this – solar power for a 100w transceiver.  Any suggestions?

73, Richard

Posts: 5482


« Reply #1 on: January 19, 2012, 06:04:55 AM »

Check this posting:,73538.0.html

At the end of the thread I go into detail on how you would set about sizing a panel.

This one goes into detail about sizing your battery:

Mark K5LXP
Albuquerque, NM
« Last Edit: January 19, 2012, 06:11:54 AM by K5LXP » Logged

Posts: 500

« Reply #2 on: January 19, 2012, 09:20:08 AM »

The simplest explanation I can give you is this:  My 60 watt solar panel is equivalent to a 3 amp battery charger when the sun shines.  So then the question becomes, how big of a storage battery do you need to operate your radio for 3 hrs a day.  Will a 3 amp battery charger bring your auto battery up enough to start your car?  Sure, after about 10-12 hrs.

In 2004 I took all the info I could and built my 60 watt setup.  As with anything, reading and practical experience are two different endeavors.

Feel free to send me a PM if you have any more questions.


Posts: 828

« Reply #3 on: January 23, 2012, 04:40:57 PM »

Richard, in round numbers, if you are pulling 3A for 3 hours a day, that's 9 amp-hours at a nominal 14 volts. Add 10% to make up for charging losses, so you'd need about 10 amp-hours at 14 volts to replace the power used in a day.

You'll find solar panels are usually rated by watts, and that's the output power of the panel in full noon daylight. those watts are usually measured at 17-19 volts in a "12" volt panel and with a conventional solar charge regulator/controller you'll lose power as the voltage is reduced. So a 150 watt solar panel is probabaly going to give you about ~85% of the rated power once it is regulated down to 14-14.4 volts for charging, and it does that for an "average" of four hours, given a full day of exposure. (Four hours is a number used by a number of labs and universities, instead of calculating the actual number of hours of daylight and the varying intensity of daylight during the year, they just figure the panel was getting full noon light for four hours.)

So you need 10 amp-hours at 14 volts, that's 140 watt-hours minimum. A 40 watt panel will produce about that much over the course of four hours in full noon sunlight, which in reality means during one average bright cloudless day. You can of course figure out hours of daylight and how bright the sun is where you are (4-land will get more) but I'd expect a 40-watt panel to stem the tide, and prefer an 80-watt panel to have some insurance on that if you can afford it. After all, sometimes it will rain.

With solar panels "as much as you can afford" usually is the stopping point. For example, you can get about 20% more power from the panels into your battery, if you use an MPPT type controller. Might cost you an extra $400-500 though at the entry level.
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