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What can i expect from a 40 watt solar panel?


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hey all just seeing who runs solar here on their fish houses. i purchased a 40 watt panel off of an internet auction site and am curious if this is overkill or not enough. i typically have 2 batteries. run all led lighting and have two small computer fans. 10" lcd and eventually wanna run a dish network setup. Is a 40 watt sufficient or should i add a 50 or 100 watt when i can afford them?

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Consider a 40W solar panel as roughly a 2.5-3A battery charger when the sun is shining and you have good, strong light. Less if not.

Your loads seem relatively light so a 40W panel may keep the batteries at a usable level. Basically to know for sure you have to add up your electrical load and how long that load is applied vs. knowing that you have a 2.5 - 3A amp charger for roughly the daylight hours (maybe minus a couple for good measure).

For example, say your load is 5 amps and that load is on for 5 hours. That's 25 amp-hours your solar panel has to put back in the battery to maintain the charge. So, if the panel is 2.5A then you'll need 10 hours (actually a bit more) of good light to put back those 25 amp-hours. If the panel can't keep up with your load schedule the batteries will eventually go dead, just more slowly than without the panel.

Also, with that size of panel you may need a charge controller if one is not integrated into the panel.

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Make sure you have a charge controller so if you don't use your house for a while and the batteries are still in it that it doesn't overcharge them. I have gone into houses that didn't have a controller and you could smell the batteries cooking. A charge controller only costs about $40.

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You will be pulling 100-400 watts total with almost all coming from the TV and some 40 w of it from the dish set up.

Charge rate-

40W x.60 efficiency (clouds-low angle of sun etc) =24W/12V =2 amps per hour x 8hrs sun light =16 amp/hrs ... 185 amp hrs/16 amp = 11.5 days to charge from fully discharged. Probably more as there are some inefficiencies I haven't calculated.

Discharge Rate

1. @ 100 Watts and a 250 amp hr battery. The battery is actually only rated at 250 x.70 as you don't want to drain it down to 0% as it wont last=185amp hrs x.80 inverter loss(as your inverter isn't 100% efficient-please use an inverter close to your expected draw )=148 amp hrs. x .90 Puerket effect (for drawing fast which increases resistence and decreases life of charge)=133 amp hrs. x 12V=1512 watts/hr/100watts=16 hrs

Drawing 400 watts would be 4 hrs

once a week= 7/11.5=.6 x16= 9.6 hrs @ 100 watts

2.4 hrs @ 400 watts

There are prolly some more inefficiencies in the system than I listed and my math may be off, but off the top of my head that (above) would be about what I would calculate as I type.

Please correct me if I'm wrong!!

The answer is it can be used as a maintainer and buffer if it has a smart charging system but can't be used as primary charger unless you use it only weekly or for very short time periods particularly for a 400 watt TV system. You could use it for a 100 Watt TV system a couple times a week but not everyday.

How much your TV draws?, how often you use it?, how easy it is to bring the battery into a 120 V charger? and if it is a smart charging system are the questions to ask.

Make sure the charger uses "switch-mode technology" or you might damage the battery.

Hope this helps!!

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I've got a 75 watt panel with a charge controller and I have 2 group 31 battiers, It puts out 6 amp. When I'm there for the weekend it won't keep up to TV., Dish, computer fan, and a couple of lights. Probably (sp) get 8 hours into the trip and the inverter statrs chirping for more battery, then I start up the Honda. I like the solar to keep my batteries warm and topped off while I'm not there, That it does but ya still need a generator and charger if your going to use the house for more than a day

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