Help me design an off-grid DC solar system

I want to start with something small just as a proof of concept. The basics of this are simple, but there are lots of little details I don't know the answer to.

What panel do I use? Most ~200w panels seem to be hovering around $250. Do any have a serious advantage over any others?

What about mounting system? My house faces east/west, so I'd need something to angle it up a bit to the south. Something adjustable for summer/winter? Something motorized that can adjust on its own with some sort of controller?

I want to keep this system DC, so no inverter nonsense. I'd like it to charge a bank of 1-?? 12V AGM batteries. Do I need a charger of some sort for this? How do I wire up 24v to charge 12v batteries? Is there some sort of A/C powered backup charger I can use (would charge batteries only when they drop below a certain level & solar charging is not taking place)?

What wire & connectors is generally used to get from the panel through the roof down to the battery area? Is there a waterproof interface box I can install in the roof?

Running off these batteries I plan to have a 12v->5v converter to a USB hub of some sort to charge phones, PS3 controllers, power my 2 RPis, etc. I will also have a 12v output to power router, modem, & other 12v electronics. What type of connectors should I use here?

It would be great if I could power my TV & PS3 off DC directly since they are both internally DC, but it appears the power supplies are internal to both so they still need to receive A/C power.

Solar panels come in various voltages. For a 12V system, you want something around 18V at the panel. Yes, you need a charge controller. HINT: Harbor Freight sells 50 amp 12V charge controllers. Use a 20% off coupon. HF also sells pretty nice looking inverters, which would solve all your "what connector/how do I do this" questions for using the power.

200 watts at 18V is 11 amps. Wire for 20 amps for the length you have to run. I'm planning on getting some big Romex at Lowe's for mine, but I have about 500W worth of panels to put up. I think the 4ga stuff they sell will work for me. At 11 amps-ish, about any Romex will work for you.

Motorized tracking mounting systems start to get complicated. I suggest starting out with just pointing the panel south. The right angle depends on when you want your maximum power, summer or winter, or if you want a compromise for all year. Teh Maths are out on teh Intr4w3bz. Every web site seems to have a different "perfect angle."

Would a 50A charger be inefficient with an 11A max load? Would I be better off with a 20A like this? http://www.amazon.com/Docooler-Controller-Battery-Regulator-Protection/dp/B00L37KZI6/

Dr. Hess wrote: HF also sells pretty nice looking inverters, which would solve all your "what connector/how do I do this" questions for using the power.

I hate the concept of going DC -> A/C -> DC again. I'd rather keep everything DC. I'll start with all the DC stuff, then possibly get an inverter just for the stuff I can't separate the power supply from (TV, PS3).

Yeah going DC-AC-DC is really wasteful and not necessary. I'd recommend using 4-pin Molex connectors. You might also want one big 12V-5V converter for charging your USB devices. The Molex connector just happens to have 4 pins for carrying 12V and 5V at the same time...

I don't think the 50 amp charge controller would be less efficient (or more efficient, for that matter) than the 20 amp one. It gives you "room to grow." Gonna cost more than $13, though.

Think of the voltage feed thing this way: You have to run wire from the battery to whatever you're powering. A 12V circuit will have 10x the line losses to heat as a 120V circuit would. Depending on how far your battery is from what you're powering, you would have to run a much larger size wire to get the same efficiency as you would have with 120V. Converting the 12V to 120, then running that over a smaller wire with less losses then converting it back to 12V (or 5V, whatever) could have greater overall efficiency. Like how the power companies boost the line voltage to Kilo-Volts or Mega-Volts to run longer distances at less loss, then drop it back down for your game box.

Maybe converting to high-voltage DC and back would be more efficient for reducing line losses? I was assuming the wiring would be short. 48v is used in data centers for this purpose.

I understand the line losses thing. I'd be dropping this from a panel, straight through into a cubby area behind my TV which houses my Modem, Router, 2 RPis, a PS3, some other USB powered/Charging stuff, A VCR, the TV, etc. I'd like to put a small box back there containing the charger, battery, and 2 DC outputs (5v/12v). From there no power is going to run more than 5 feet, tops.

MY goal is: 500 Watts of panels on the roof, wire down to the charge controller under the porch, where I have a concrete pad poured to hold a battery bank. Batteries in a plastic box. 1.5KW Inverter to 110V to a sub-panel backup generator switch underneath my main panel in the kitchen. From here, I can switch individual circuits to run off the batteries/solar or off the main line. So, I can have my computer room switched to the batteries, or the bathroom, bedroom, etc, as long as I'm careful about the total load. I'll probably invest in some LED lighting at that point.

I have most of the parts I need now, except for the switching panel, batteries/box and the Romex. I might go with a 3KW inverter the next time HF has it on sale instead of the 1.5KW I have now.

The line losses you see with 12V DC is going to be the same at higher voltages. The difference between 120V AC household current and 12V DC from a battery is AC versus DC! AC power can travel long distances with very little loss while DC will drop off quickly.

Line losses may be the same at the same current, but not at the same load. Do the maths, Jim. 120 watts at 12V = 10 amps. 120 watts at 120V = 1 amp. Assuming the same line resistance, say 1 ohm for discussion, then the power loss of the line is P=I^2*R = 100 watts loss versus 1 watt loss for the 120V system.

Does anyone have a link to details of how the panels are mounted to the roof? Also, how the wires pass through the roof without any leaking?

The wiring for a lot of roof-mounted stuff simply wraps around the edge of the roof and then comes in through a wall, rather than punching through the roof.

In reply to Dr. Hess:

For 500w, how much panel area is that? And you are in So Carolina, right?

I've been pondering a solar system for my in-laws in Puerto Rico- so the passive angle is really easy- sun doesn't move much for them throughout the year. And a solar system would do some good with the sun load on the house- lowering the a/c requirements.

edit- considering the location, I should be asking Gameboy...

alfadriver wrote: And a solar system would do some good with the sun load on the house- lowering the a/c requirements.

I've always wondered how much that helped with cooling in the summer. It seems like it would keep the attic quite a bit cooler because the roof itself isn't getting heated up by direct sunlight.

GameboyRMH wrote: The wiring for a lot of roof-mounted stuff simply wraps around the edge of the roof and then comes in through a wall, rather than punching through the roof.

Gotcha. Any details on this?

Just wondering what kind of conduit is used, and what the area looks like where it actually passes through the wall.

DC doesn't easily transform to higher voltages like AC does. This is actually one of the reasons why we ended up with AC way back when anyway. The copper requirements for running DC were enormous and required more generators.

Con-Ed still provides DC electric service in older parts of NYC still running old elevators with DC motors (or at least they did 15 years ago when I was doing supermarket engineering in NYC).

There are new sections in the NEC regarding PV systems and DC wiring, although it changes with every issue as new technologies and techniques become available. Everything with this is changing so fast it's hard to keep up.

What are you, some sort of revolutionary? As a customer/hostage of a big, bad, power company, I must insist that you continue to shoulder your fair share of the burden.

I dunno how many sq ft. I've got 3 145 watt panels that are like 3 x 5' or so and 2 HF 45 watt panels. I got the big ones from these guys: http://www.amazon.com/gp/aag/main?ie=UTF8&asin=&isAmazonFulfilled=&isCBA=&marketplaceID=ATVPDKIKX0DER&orderID=&protocol=current&seller=A21XZF4K65PP5Y&sshmPath=

Their current panels are 18% efficient, so do teh maths. 1KW per square meter is what falls on us from the big bright thing out there. 180 watt panel would be 1 square meter at 18% efficiency.

I'm in Arkansas.

They have whole turn key sets at 10KW with inverter for like twenty large. Depending on how bad The O butt bangs our energy industry, that could be a decent deal with about a 5 year payback.

Side note: Do any charge controllers have a switch that will close when the battery is full? Something I could use to trigger a solid-state relay to kick on a heater or heaters? In winter if the battery is full during the day, but there is extra capacity, might as well use any excess energy to heat the house.

Not that I've seen. You could make one easy enough. Op amp, transistor, relay, few resistors and maybe a Zener diode. That ought to do it. Google up something like "adjustable voltage switch circuit schematic." Then set the voltage to wherever you think the battery is fully charged at. Like 13.4 or something.

ProDarwin wrote:

GameboyRMH wrote: The wiring for a lot of roof-mounted stuff simply wraps around the edge of the roof and then comes in through a wall, rather than punching through the roof.

Gotcha. Any details on this? Just wondering what kind of conduit is used, and what the area looks like where it actually passes through the wall.

I'll use an antenna on my house as an example - the thick antenna wire curves around the edge of the roof and is then stuck to the concrete with those nail & hook things (can't remember what they're called). It goes down into a plastic conduit shared with some network cables, across the side of the house, and then just outside the living room wall where the TV is, there's a hole drilled in the wall about 1cm wide, it's not too noticeable on the outside.

These walls are all-concrete so if you have brick & drywall it might be more complicated. Also the house is painted white, and the cable, conduit and hooks are all white so this helps make them less noticeable.

ProDarwin wrote:

GameboyRMH wrote: The wiring for a lot of roof-mounted stuff simply wraps around the edge of the roof and then comes in through a wall, rather than punching through the roof.

Gotcha. Any details on this? Just wondering what kind of conduit is used, and what the area looks like where it actually passes through the wall.

Conduit would be nice, but typically not used in residential installations. Usually just multi-conductor wiring rated for outdoor installation (UV resistant). Drill a hole through the wall and caulk the crap out of it after running the wire.

I ran wire in PVC conduit for the A/C condenser at my ex's house as well as conduit for a future generator, but we're engineers... anything worth doing, is worth over-doing... and by that point all thoughts of budget had long been forgotten.