Voltage rating of solar panels query

Bug's on the money.

I think I see what you're doing, take solar panel from roof and use it for backup power with batteries in an austere situation?

Solar panels of roofs are usually in series [strings] with fairly high voltage [might be 400V if 10 panels] though that's not so important as you can rewire them into parallel and keep the 0-40V if you needed to.  The watts 200-300W [as per bugs] give you an indication of their max output, typically summer at 12pm.  A better way to think of them is they will generate 'about' 2.5x panel watts of power per day.  So 200W panel will generate 500W/hr per day.  This varies a lot depending on the time of year and the weather.     

What you'll need to use the panels on the cheap is a regulator/solar charger for 12V batteries, with MPPT [more efficient] or PVM [cheaper]. You can pick up crappy chinese ones for very little, I have 3 to be sure.  Their ratings give you an idea of their capacity, 10Amp x voltage [12] give you a max output of 1200W.  So if you were using 200W panels you could use up to 6.  

From there an inverter, preferably pure sine wave, though not essential

You can get better equipment [much more expensive] that does both regulate and then invert [to 240V AC] with more features and a pretty panel that tells you things like how much charge you got from the sun etc.   Taking a multimeter voltage check [see pic below] also works.

Batteries

If you have quality lead acid deep cycle/AGM batteries you can run them down to 50% of their rated power and get plenty of charge/discharge cycles.  If you're hooking up car batteries, then keep it about 80% to keep the battery life.   To work out the capacity of the battery you multiply voltage x amp/hr to give you the watt/hr. 

This is easier to figure out, your laptop might use 50W/hrs [electrical items have wattage ratings], so if you have a 12V AGM battery with 100amp/hrs.  12V x 100amp/hrs = 1200W/hrs.  Since it's an AGM battery you have 600W of usable power, so 12hrs of using the laptop. 


Batteries are the weak link and most expensive part of solar power.  You can get around this by using appliances when the sun is shining and the panels are charging the most and then have a small amount of power for say lights at night.  Most houses use 18kwh+ of power each day, quite an investment in batteries.  Personally my preps for power focused on charge torches/lights/radios/NV gear/water pumps.  I consider radios critical so each has it's own small usb panel/spare batteries. 

A kit might look like this
Two solar panels 250W, 500W total, 1250W/hr day.
Battery bank - 4 x 12V 100Amp AGM batteries = 4800W/hr, 2400W/hr usable
Regulator - 10Amp 12v - 2400W max [easy]
Inverter - 10Amp 12v - 2400W max
Multimeter
Battery connection wire to join batteries, can use an aluminum bar from Bunnings [$5] with heatshrink tube [insulate it]
Enough wire for the solar panels.  Note that wire diameter might matter as we're working with lower voltages. 

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If instead you're looking to have an inverter that you can use off-grid and/or charge batteries then you would want to get an electrician sort it all out. 

Already got solar for camping. Waiting for the ability to attach pictures to the forum again to do a thread on really basic solar.

Plan is: (haven't done the sums yet)
Some of these grid power houses have dozens of panels.
Take a dozen or so panels.
Give an appropriate number to a house.
Connect to a vehicle battery or three.
Regulate volts and amps by shading part of the panel if required. Not sure how to work it yet.
End up with charged batteries without needing fuel to run engines to run alternators. Also don't need electronics.
Use batteries to run lights and comms.

Need to mess around with shading uncontrolled panels to see what happens. In theory this is possible if very unwieldy. Would probably require constant or regular monitoring by someone during charging. Shrugs.

Yup. Hence the constant monitoring required. Far from an ideal method but if the correct equipment isn't available, 'needs must'.

Yes, anyone can have lights then. Handy for comms too.


Yes. Anyone can buy stuff and make this work. But the shops may be empty, destroyed or radioactive. What then? Use whatever is laying around somehow.
In the same way it may be possible to set up comms using smartphones and routers, people can have lights and charging for small electronics.
Not looking to buy enough controllers to make this happen for an entire town or neighbourhood!
Also could be very profitable during bad times.

Gee those regulators have gotten cheap. As a general rule, if buying cheap, buy a spare.
If someone wanted to set up a 12V supply, it's pretty cheap now. You can get second hand/obsolete panels vastly cheaper than new ones. Get onto gumtree. There are plenty selling them. They'll be from houses getting demolished, or from warehouses with panel stocks that are no longer up to scratch. But don't let that scare you. The only real thing to worry about is that with older panels you'll need double the square meterage to get the same output. Unless there's a real problem, solar panels don't die. They gradually degrade over time, but will last 40yrs or so. Generally 75% generation is guarranteed at 25yrs. The failure point is generally the inverter. 

Batteries are best charged with a 'pulsing' wave form rather than a flatline DC. It may still work, but will reduce battery life.

If I ever get a home battery (the economics are getting close, but not there yet),  I'll be buying a bunch of old panels and a cheap inverter. As I have a 2 phase supply, I can put them on the other phase and not clash with my existing inverter. Downside is no green credits or govt subsidy. They'll be there to charge the battery, not for export. I don't have much of a 12V setup, just 2 x car batteries and 2 x 240/24V trickle chargers. Not much on 12V to run here other than air compressor, car, and a 100W 12/240V inverter to run laptops and charge phones from.

Good read. Interesting discussion on the pulsing there. Seems it's not a settled discussion. For the batteries I've worked with, we allowed a little gassing as there was a scheduled maintenance visit every 6 months to check cell voltage and top them up if needed.

Definitely agree with the 'ripple voltage' commentary.

We use a 120v system here in the States.  Seems it would have been smart to standardize the worlds systems by now.  I’ve been setting up an off grid system here.  I’m actually making three separate systems, mostly for grid down scenarios.  Here, and likely everywhere, it’s still much cheaper to just buy electricity.  I’ve lived in hurricane prone areas and have had to use generators for extended times so the solar is a much better option for having power due to no need to keep getting fuel.  My first system is 8 250w panels with dual 60amp charge regulators and a 24v 3500w inverter.  I wired ten 100AH AGM batteries together in series fro a 24v system.  The inverter was a cheap Chinese one and even though it works fine I just bought a much better one for my second system that was triple the price.  I am making all three systems 24v with similar setups so I can swap parts if one goes down.  I will likely get an extra of each part eventually for security.  The first system is up and running now, powering just my chest freezer and refrigerator right now.  So far my battery bank hasn’t dropped hardly at all even after cloudy days so I will add some light circuits to it before long too.  Since food refrigeration was my top priority I won’t max out this system to ensure I don’t loose my frozen food preps.  The second setup will run all the regular circuits in the house, just not the central heat and air or laundry.  I will also be able to run a small window AC unit for the master bedroom for hot summer nights.  The third system will be smaller but the same voltage and will just run the well.  I bought all the solar panels allready and two of the inverters but still need a third inverter, and the batteries for two systems.  It seemed like a big investment but electricity really does make our lives so much easier that I felt it was worth it.