Junction boxes and inverters

ryan1
ryan1 Registered Users Posts: 6 ✭
in Solar Beginners Corner #1
Hi everyone. I decided to take up making solar panels as a hobby and plan on starting with the 3x6 ebay stuff for off-grid set up.

Regarding junction boxes and cables-
Are they specific to PV panels, or would home depot carry equally good generic for less money?
,
Regarding inverters-
The ones that have a built in charger, does this mean built in charge contoller?
And how is that different than "automatic line to battery" charger? (tripp lite models on amazon; aware of pwm etc.)

Thanks, Ryan


If anyone is interested, my short term goal is 600w 24v system with a 60a controller, 2000w inverter and two 6v 225h batteries (http://www.solar-electric.com/crrepoba6vo2.html). Nothing set in stone, but I want to plan for future growth. 2k plus inverter is a must because I'll want to use my washing machine in the day, possibly a portable ac to help out.

I don't have a lot of cash to burn, and I use very little energy. My bill last month was $50. As of right now, the joy and benefit I'll get out of it is worth it. Plus living in the valley of the sun, it seems foolish not to take advantage of the immense resource we have.

BTW, does the store side havea phoenix pick up location? I didn't think you guys were in Flag for some reason. Thanks again.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: Junction boxes and inverters

    Regarding questions for NAWS (our host)... None of us here work for/with NAWS (with the exceptions of Rick and Windsun--our admins) and they do not monitor the forum for business related questions.

    Please call them directly about your delivery/pickup questions.
    Northern Arizona Wind & Sun
    4091 E Huntington Drive
    Flagstaff, AZ 86004

    Sales & Service: 800-383-0195 | 928-526-8017
    Fax: 928-527-0729

    Our office hours are from 8AM to 4PM, MST Monday through Friday. We are closed for major holidays. If you are in the Flagstaff area, please stop by our store and visit.

    Take care,
    -Bill "one of 4 Volunteer Moderators" B. ;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Junction boxes and inverters
    ryan1 wrote: »
    Hi everyone. I decided to take up making solar panels as a hobby and plan on starting with the 3x6 ebay stuff for off-grid set up.

    Regarding junction boxes and cables-
    Are they specific to PV panels, or would home depot carry equally good generic for less money?

    They are specific to panels, but that doesn't mean you couldn't adapt a standard outdoor box from the hardware store. It all depends on how clever you are. :D
    ,
    Regarding inverters-
    The ones that have a built in charger, does this mean built in charge contoller?
    And how is that different than "automatic line to battery" charger? (tripp lite models on amazon; aware of pwm etc.)

    The built-in charger is an AC powered battery charger, not a PV powered charge controller. It's easy to get confused about this as GT inverter take input from the panels directly, but have no batteries. Until you look at the hybrid types like the Xantrex XW - which need a separate charge controller! Kinda makes your head swim, doesn't it? :p
    If anyone is interested, my short term goal is 600w 24v system with a 60a controller, 2000w inverter and two 6v 225h batteries (http://www.solar-electric.com/crrepoba6vo2.html). Nothing set in stone, but I want to plan for future growth. 2k plus inverter is a must because I'll want to use my washing machine in the day, possibly a portable ac to help out.

    I don't have a lot of cash to burn, and I use very little energy. My bill last month was $50. As of right now, the joy and benefit I'll get out of it is worth it. Plus living in the valley of the sun, it seems foolish not to take advantage of the immense resource we have.

    Since your just experimenting I'll skip the standard-issue "loads lecture". But ... two six Volt batteries isn't going to get you a 24 Volt system; you need four. And to give you an idea of how that would work out for panels:

    225 Amp hours of 24 Volt battery. Recharge at 22.5 Amps and 28.8 Volts = 648 Watts, not including typical derating and system losses. So the 600 Watts of panels would be a little bit shy.

    Another observation: "I'll want to use my washing machine in the day, possibly a portable ac to help out." Either one of those can overwhelm a 2kW inverter, depending on the model of everything involved. Some washers use less power, some inverters can deliver more "surge" capacity. The AC is probably a no-go at any rate, as they can be enormous power users with huge start-up surges.

    I'll also politely skip the standard admonishment on homemade panels. :roll:
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  • ryan1
    ryan1 Registered Users Posts: 6 ✭
    #4
    Re: Junction boxes and inverters

    Thanks, that gives me more to consider.

    I was thinking the 24v panels, wired to 12v with mppt controller. You're right about the head wrapping, just when I think I have it figured out red flags start popping up.

    Sounds like what I want is a 24v system on a 12v budget lol. It will be an accumulation thing.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Junction boxes and inverters

    Some of the confusion is terminology used. When we refer to "system Voltage" around here we're talking about the nominal Voltage of the battery bank. Panels, alas, have three different Voltages! They have a nominal Voltage (usually 12 or 24) and a Voltage open circuit and a Voltage maximum power.

    You can charge a 12 Volt system from a 24 Volt panel, but to get the full power you need to use an MPPT type charge controller. And at 2 kW the DC Amperage on a 12 Volt system gets very high:

    2000 Watts maximum output / 10.5 Volts minimum input = 190 Amps

    Wired and fused properly, occasional draws to that level aren't a problem. Sustained use becomes another matter.

    You might want to do a bit of reading up on things like the Glossary: http://forum.solar-electric.com/showthread.php?t=6136

    And the all-important Battery FAQ's http://www.windsun.com/Batteries/Battery_FAQ.htm as batteries are the heart of an off-grid system.

    So far no one has found a way of doing this off-grid stuff cheaply. :roll:
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  • ryan1
    ryan1 Registered Users Posts: 6 ✭
    #6
    Re: Junction boxes and inverters

    That makes more sense, yes a 12v system. I bought mostly energy efficient. New Kenmore washer is 120/60/12. I have no idea what it might surge to though, I thought 2k would cover it. I haven't looked at portable ac's yet.

    MPPT is the way to go imo. Heat and loss on the 12v is what steered me towards 24v.

    Correct me if I'm wrong here but the main heat and loss occur at the panel level to the battery level, right? The AC from the inverter isn't affected?

    And as for cheap, I just took myself out of the lottery to be the first one .
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #7
    Re: Junction boxes and inverters

    Vmp falls with increasing temperature.

    Imp increase (very slightly) with increasing temperature.

    For MPPT type charge controllers: Pmp=Imp*Vmp -- so, with falling Vmp (increasing temperature), then Pmp falls with temperature.

    For PWM type charge controllers--They are on/off switches... All that is required is that Vmp-at-temperture>Vbatt-charging ... So having Vmp~17.5 volts means that for most conditions, the Vmp-hot>Vbattery-typical charging voltage.

    That is why it is recommended to run solar panels at least ~5" or higher off the roof surface--Better air circulation, cooler panels.

    For wiring, hot copper wire has higher resistance than cool copper wire.

    And for resistance heating, the power equation is Pheat=I^2*R -- Notice that heating (losses) in copper wire increase with the square of the current... Double the current, get 4x the losses.

    Batteries are more "active" at higher temperatures (more apparent AH capacity) than at cooler temperatures. Standard Conditions for a battery are around 77F (25C). So--too cold of battery (roughly below water freezing, you have to account for lost AH capacity).

    On the other hand, there is a handy engineering rule of thumb regarding "life" of components... Basically, for every 10C (18F) increase in operating temperature, there is a factor of 2 decrease in life. Running a battery bank at 95F (35C), its lifetime is about 1/2... Run it at 20C over "standard"--then you are looking at 1/2*1/2=1/4 life and so on...

    Conversely, running things 10C cooler will give you 2x life time.

    The issue between 12 volt and 24 volt (and 48 volt systems)... Basically:
    • Power = Voltage * Current
    Double the voltage, the current falls by 1/2. Also, the voltage drop that can be tolerated goes up too (~1 volt wiring drop maximum for 12 volt, ~2 volt maximum for 24 volt, and ~4 volt maximum for 48 volt systems).

    Higher voltage, lower current allows smaller copper wire and/or the ability to send power farther away.

    Very roughly, it gets expensive to handle >100 amps of current...
    • 100 amps * 12 volts = 1,2000 watts
    So, roughly, I like to recommend that you look at your power requirements... >1,200 watts, look at 24 volt system. Over 2,400 watts, look at a 48 volt system...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Junction boxes and inverters

    If the last number on that Kenmore is the Amps it draw running it will in all likelihood fault out a 2 kW 12 Volt inverter. 12 Amps is a lot of power. A standard outlet is rated for 15, and usually washers plug in to a 20 Amp outlet (GRFI protected and 12 gauge wire is standard practice). That's 2400 Watts capacity. Some inverters might handle it, but any time you run something close to its maximum limit, even for a short time, you affect its longevity.

    Which brings us to Electrical Enemy #1: heat.
    There are heat losses all over in a solar power system. Start with derating he panels, as they heat up their power output goes down. This is why you can don't expect to see the rated output very often. 80% or less on average. Since the heat is internal to the panel and glass is a poor conductor of energy they tend to stay hot all day. You really have to lower the ambient temp to get any good cooling of the P-N junctions and keep the power up. As has been explored many times on this forum, there's no practical way to do this - outside of living in the frozen North - so you put up with the losses.

    The batteries suffer from heat too. That's why putting a remote temp sensor on them is a good idea; it will vary the output of the charge controller (if it can be so equipped) and keep it from over-charging when the battery gets hot - coincidentally from being charged.

    The reason we like to keep the DC Amps down is ... you guessed it: heat. More Amps means more heat, hotter wires make poor conductors - causing higher current draw and more heat.

    AC wiring can also suffer this fate. Lots of houses have burned down as a result. Again the culprit is too many Amps for the wire size. Like plugging three 500 Watt halogen lamps into one 18 AWG extension cord. The outlet can handle the 1500 Watts, but the tiny wire can't. It becomes a "fuse" of sorts - albeit one coated with flammable vinyl insulation.
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  • ryan1
    ryan1 Registered Users Posts: 6 ✭
    #9
    Re: Junction boxes and inverters

    Thanks, you're right on the "what I need" recommendation. I keep telling myself to plan for expansion but the cost will outweigh the benefit at this time.

    I live in Phoenix so over 110 in the shade isn't uncommon. The washer 12 is amps...I'm going to have to go bigger if I want to run something more than my TV and dish receiver.

    Thanks again for the equation info, it's a lot easier to understand broken out like that. Most of my supplies aren't being delivered until next week, so I have some time to decide if I'm going to pull out the debit card or the credit card! LOL

    Have a great day, I'll be back around with updates.
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  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    #10
    Re: Junction boxes and inverters

    I often use my Xantrex 1800W Prosine inverter for washing clothes. Details on the actual numbers is in this article: http://www.alpharubicon.com/altenergy/altwashclothes2manytoyz.htm

    The wash cycle was ~450W, the spin cycle peaked ~1000W, as noted on the Kill-A-Watt meter. I completely agree that the Kill-A-Watt meter is too slow to capture the actual startup current spike, but my 900AH battery bank also helps the inverter deal with the brief spike. This inverter is rated at a 2900 watt surge capability.

    I've also run a 5000 BTU A/C unit from this inverter. My A/C draws 4.9A @ 120VAC with the fan on high, and the compressor engaged. With a special Fluke meter (model 334), capable of capturing In Rush current, the A/C spiked at 22.6A.

    I know my inverter will run either, or both, but might get into an overload situation if the A/C compressor engages and washing machine starts a spin cycle, at the same time.

    Key thing to keep in mind is these high current events are brief, and the running current for both is relatively low.

    Your inverter, batteries, wiring, and appliances, will all play a role in how effective your setup is. I also have multiple inverters, allowing me the option to move a heavy load to a separate inverter. Not found it necessary yet...
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #11
    Re: Junction boxes and inverters

    I have one of the smaller Sears HE front loaders, Very pleased with it! 100 to 200 watts washing (depending on load) and about 400 watts on spin. NO big surges! Variable speed inverter run, 3 phase motor is computer controlled to very gradually picks up speed for spin. VERY easy to run, just a light load on my Xantrex 1800/12 volt puresine. Lots of power left over to run the water pump (1/4 HP) and other things at the same time. I couldn't ask for anything better.
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