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stmbtwle

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Reply with quote  #1 

PV for Dummies... IT'S NOT THAT HARD!

I was asked to do this by another member, who while quite accomplished with Solar Thermal, had concluded that PV (Solar Electric) was too “hard” and complicated. 

That's one of the things holding PV back, perception. In reality IT'S NOT THAT HARD!!! A basic (off grid) DC system like you might install in your boat or RV has only 3 components: solar panel, battery, and controller. The battery you probably already have, so that leaves TWO components. The solar panel only has two wires and yes they're marked. Most solar panels today already have polarized wires attached, so they're “plug and play”. However most modern solar panels have enough power to cook your battery, so you'll need a controller (think "battery charger"). The controller has connections for four wires or maybe six, and yes they're marked too. The battery, well you already know about batteries. That's all there is to it. 

Here's a hypothetical project: Suppose you have a garden shed in your back yard which doesn't have electricity, and you're tired of groping around amongst the rakes, bugs, and snakes with a flashlight. Some lights would be nice so you can at least tell them you're coming. You have several options: 

Simplest and probably the best would be to call a contractor and have him run power to the shed ($$$). Another way would be to run an extension cord from the house to the shed. Simple and it works, until one day you run over the cord with the lawnmower. Oops! 

But rather than deal with the cord or the contractor. you decide to make a stand alone (off grid) electrical system. You go to Big Box and pick up a car battery (a “deep cycle” RV/Marine battery is better) , some 12v LED garden lights (it IS a garden shed after all), and some insulated wire. Up to a point, most any wire will do. You connect the lights to the wire and the wire to the battery. A switch would be good, and you can use a standard wall-mount switch from Big Box. For safety you install a fuse from the auto store. Now you simply open the door, flip the switch and watch the critters scatter. 

This is all well and good except that after a few days the battery goes dead and the lights go out, because the battery isn't getting charged (bummer). All you need to fix that is a solar panel, a controller, and some more wire. You connect the controller to the battery and the solar panel to the controller, and find a place in the sun for the panel. Now whenever the sun shines, your battery gets recharged. Congratulations, you've just built your first PV system. 

After a while you decide that while lights are fine, it would be nice to be able to run some tools. Most power tools take AC power, not the DC from the battery. While a 12v battery WILL run a power tool you will need an INVERTER to “invert” the DC from the 12v battery to AC and step up the voltage. The best inverters are “full sine wave” but in most cases the cheaper MSW or “modified square wave” will work. Your inverter (again) has two wires you connect to the battery. Now you have AC power to run small tools, simply plug them into the inverter. Big tools or appliances (table saw, beer cooler, AC) will require a bigger system. Not necessarily more complicated, just bigger (more batteries, more solar panels) That's how I started, but my “shed” was a boat.

That's OFF grid solar in a nutshell.  ON grid is a different story, with lots more power, regulations, permits, and LETHAL voltages (not for the newbie).

Anyone who has questions or additional comments, go for it. That's why we're here.


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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

BobMcK

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Reply with quote  #2 
What's the scoop on startup surge when a motor is being run off a battery-powered inverter?  I've got a 120 VAC pump motor that when running draws about 600 W so I followed an old rule of thumb that puts startup current at 1.5 to 2 x run current and used a 1500 W (psw) inverter . . . but even with a full battery the motor wouldn't start.  So I put in a 2000 W inverter and now does.  I suspect it's not the surge current that tripped the smaller inverter, but rather high voltage transients during startup . . . yes?  If that's the case it makes sense to try and filter out the transients rather than heavy handedly using a larger inverter.  Do you agree and if so any recommendations re filters?  


stmbtwle

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Reply with quote  #3 
My guess it's LOW voltage. The startup load will drop the battery voltage for a few seconds and this can trip your inverter. Possibly the 2000w is just not as picky as the 1500, as the voltage drop should be about the same with either.

If this is a new problem you may just have a bad connection somewhere.
Other possible causes could be undersized/old battery, or undersized wires from the battery to the inverter.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
jjackstone

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Posts: 94
Reply with quote  #4 
Startup current can often be as high as 6 to 7 times normal load current.
Voltage drop will also happen,
I have a 3kw inverter that I used to fire up a table saw as an experiment and it not only brought the voltage down to 80 volts, it popped a fuse in my kill-a-watt meter. However the saw did run. It just took a few seconds longer than normal to get up to speed. I will not be using the inverter with that saw. [nono][nono]
My refrigerator runs fine on the inverter though. 

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