Longtime Lurker - ready to take the plunge

mart.park

I've been lurking on this site for a few months, trying to gather info for a small(er) PV system for my mountain cabin. Cabin is currently powered by a 6500w propane generator, 12 near new 305ah AGMs, and a 2500w 12v xantrex mod-sine inverter, which works ok for our needs, but is noisy.

I'm finally ready to plop down the money for a 2000w solar array, hopefully using 10-205w evergreen panels. Cabin is just outside of Las Vegas, NV, in the mountains at 6000ft, and has no access to grid power. As most people are these days, I'm on a budget, hoping to spend between 8K and 10K, doing most of the install myself as I was an apprentice electrician for 3 years.

Panels will be roof mounted on a garage, directly above the inverter/batteries, and I have 28'x17' of south facing space to work with. I plan on building a tilt mounting system to change between winter and summer orientations.

I am looking for help in sizing the various components I will need in addition to my current (minimal) setup, as well as some help with connecting it all up. I've never been "up close and personal" with PV panels/MC4 connectors, am ignorant on the benefits of more expensive Outback vs. cheaper Xantrex charge controllers, etc. Am I missing any important facts?

icarus

Re: Longtime Lurker - ready to take the plunge

In order to get a good handle on system design, you first must understand and define your loads. A couple of "rules" apply to of grid systems. The first is people underestimate their loads, and over estimate their solar gain, all leading to bad system design and premature battery failure.

A quick rule of thumb is that if you take the name plate rating of your array, divide that number by 2 (to account for all system efficiencies and loses) then multiply this by the number of hours of good sun you might expect, seldom more than four. That will give you a starting point.

So your 2000 watt might look like this (Remember, this is a quick calc, but it will get you in the ball park) 2000/2*4=4000wh/day or 4 kwh/day.

By a kill-a-watt, and measure and time all your loads. Once you get a handle on your loads, check back in and folks can give you some ideas of where to go. (Off the top of my head I would consider a real sine wave inverter to start.)

Tony

For the record, we live off grid, LP fridge, and we use ~500-1000 wh/day, plus generator for shops. (We also do some considerable load shifting so we run our big loads (like charging lap-top batteries and water pumping)

niel

Re: Longtime Lurker - ready to take the plunge

seeing you are opting for 10 pvs tells me you are going for the 24v system. most opt for the lower losses of the 48v system, but the drawback is you would need to go in increments of 4 of the evergreen 205w pvs.
the 48v battery arrangement would be 915ah capacity and would need at least 5% of that for bulk charging and preferably more, but this would require 45.75a from pv. if you would be looking at 12 of the 205w pvs you would have 3 strings of 4 pvs at 11.15a or 33.45a. 33.45a/915ah=3.6%. this would fall short on current requirements and something similar for the 24v setup of 10 pvs as it too will fall short on the battery charge requirements. this is fine with the generator keeping things up until you would add more pvs to the system.
the alternative would be to reduce the battery bank some. the big question would be if one could reduce the battery bank as we don't know what your daily kwh loads might look like. keep in mind that you don't want to deplete the batteries and 50% dod should not be exceeded to keep the battery life somewhat acceptable. this means that up to half of the battery capacity is usable and as such 21,960wh is usable. in using this and you must be able to replace it with the pvs and remember now that they don't always put out their stc ratings and even at that it would be limited to maybe 5 or 6 hours per day. winter has less sun and time available not to mention any time it rains or gets cloudy.

mart.park

Re: Longtime Lurker - ready to take the plunge

Thanks for the info. Our current load size was determined using a killawatt, but also takes into account our current budget, and we hope to expand with more panels next year when we have more $.

We are a 2-person household, and are pretty conscious of our electric usage, and turn on the generator if we need to use the microwave (*very* rarely), washer, vacuum, etc as we can use it to charge the batteries at the same time.

1) I just don't see having enough $ to buy a new pure sine inverter at this point. Our current inverter works well for us, (although it doesn't play very nicely with the aforementioned cheap micro ;-), and while I'd love a new pure sine inverter, $3000 for a new inverter seems out of my price range at the moment.

2) The fact that the inverter is a 12V dictates the input from the batteries, but not the PV array due to the charge controller abilities, correct? i am not against installing 2 additional panels if it will benefit us.

3) We are located just outside of Las Vegas, which means sunny days 80+% of the time. PV Watts says 5.0 in December, and 7.0 in mid-summer. The coldest temps up on the mountain are around 20, and the highs get to around 90.

4) We are not against running the generator occasionally to assist the batteries, but are just tired of running it for hours every day or so to charge the batteries (i.e. if we use a lot of power one day, we still fully expect to pay for it in propane fuel).

BB.

Re: Longtime Lurker - ready to take the plunge

Start with the basics... PVWatts website to estimate the amount of power you can generate with your array. Las Vegas, 2.05 kW array, derating = 0.52, fixed array (you can play with the tilt for seasonal adjustments and see how much that buys you or not):

"Station Identification"
"City:","Las_Vegas"
"State:","Nevada"
"Lat (deg N):", 36.08
"Long (deg W):", 115.17
"Elev (m): ", 664
"PV System Specifications"
"DC Rating:"," 2.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 1.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 36.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 9.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.19, 162, 15.71
2, 5.98, 165, 16.00
3, 6.57, 200, 19.40
4, 7.32, 212, 20.56
5, 7.42, 215, 20.86
6, 7.35, 198, 19.21
7, 7.37, 200, 19.40
8, 7.29, 199, 19.30
9, 7.31, 196, 19.01
10, 6.56, 192, 18.62
11, 5.75, 170, 16.49
12, 5.05, 157, 15.23
"Year", 6.60, 2267, 219.90

so, basically, you run from 157 kWH to 215 kWH per month average available power out the "AC Outlet"... (don't be shy--that 6.6 hours of sun per day average over 1 year has 7/8th's of the people on this forum jealous ).

• 157 kWH per day / 30 days = 5.22 kWh = 5,220 Watt*Hours per day
• 215 kWH per day / 30 days = 7.17 kWh = 7,170 Watt*Hours per day

Battery wise, assuming our rule of thumb of 3 days of no sun, 50% maximum discharge would lead us to an average daily load of:

• 12 * 305AH * 12 * 1/3 days * 0.50 max discharge = 7,320 Watt*Hours per day

So, your battery bank is nicely sized to your daily solar available power...

Battery Bank Charging... AGM's can accept a wider range of current and "be happy"... But using our 5%-13% rule of thumb as a cost effective starting point:

• 12 * 305AH * 0.05 = 183 amps minimum (at 12 volts)
• 183 amps * 14.4 volts = 2,635 watts minimum
• 12 * 305AH * 0.13 = 475 amps maximum (at 12 volts)
• 475 amps * 14.4 volts = 6,840 watts maximum

Because of your wonderful amount of hours of sun--the maximum current (one of our rules of thumb) is a bit "light" with only a 2,050 watt array... But since AGM's don't need "equalization"--the higher minimum current is not really needed.

Now, here is one place that you get into "trouble" with a 12 volt battery bank... Notice that Charge Controllers are rated at 60 amps (or 80 amps) maximum output current... Whether that is at 12 volts or 24 or 48 volts. Which leads to the maximum size array that a controller can handle... You can assume between 0.77 to 0.95 "efficiency" of panels + controller... I will pick 0.77 as being a bit more cost effective for you (most of the time, you will only be generating at 77% maximum efficiency--on very cool/clear days--especially with snow on the ground--You may get 95% efficiency or so--but it is not often).

• 14.4 volts * 60 amps * 1/0.77 = 1,122 watt array maximum @ 12 volt bank
• 28.8 volts * 60 amps * 1/0.77 = 2,244 watt array maximum @ 24 volt bank
• 57.6 volts * 60 amps * 1/0.77 = 4,488 watt array maximum @ 48 volt bank

So--in your case, you will need to 60 amp MPPT controller to manage that 2kW array on a 12 volt battery bank.

If you went to 24 volts--only one MPPT charge controller is needed. And at 48 volts--you have room to add another 2kW of solar panels.

Lastly, you need to look at how you parallel your battery bank... You wan the same "cable length/connections" between every battery and the common bus connections. You don't want to have one battery closest to the bus connections, and the last battery very far away--you will get uneven battery cycling (the one closest to the bus point will cycle the deepest).

Here is one thread with links:

Battery Bank Wiring

Note: Each parallel battery string should have its own fuse/breaker--One shorted cell/battery could be fed by the other 11 parallel batteries and you could be in for some exciting fireworks.

A 48 volt bank out of the same batteries would change from 12x parallel strings to 3x parallel strings... Really a much better arrangement. Fewer fuses, and less issues with trying to balance current flow between all of the batteries.

-Bill

PS: If you are "suck" with 12 volts for now--Take a look at getting a small true sine wave inverter to power the laptop, radio, tv, etc... The MorningStar 300 watt (600 watt surge) is a really nice little unit (and has some very nice power conservation/remote power switch modes too).

Morningstar SureSine, 300 Watt Sine Wave Inverter 115VAC (~$260 each)

niel

Re: Longtime Lurker - ready to take the plunge

mart.park,
ok sorry as i thought i saw you were going with another inverter, but that just isn't what you had said. the low charge rate still applies to a battery bank that large with the 10 evergreens. you will have a dilemma though as you will need more than 1 charge controller even if you opt for the cheaper pwm types. i would recommend using the mppt downconverting type of controllers if you can swing that and a good sinewave inverter to go with more pvs in the future.

mart.park

Re: Longtime Lurker - ready to take the plunge

Thanks for the help, Bill. With all the varying voltages, wiring, pv panel sizes, etc I was starting to feel more confused than educated! Seeing everything laid out and sized to "my" system really helps me see how things tie together.

Based on everyone's suggestions, I'm starting to look at upgrading our inverter, and I have a line on a couple of 3-4 yr old Trace/Xantrex SW5048s, one with a c60 controller and the other with an outback mx60. I know the outback is more expensive and has a nifty lcd screen, but is there some other great benefit that justifies its 4x cost of the xantrex? Both systems include the disconnects, and they both want around $2000 for the inverter, controller, and disconnects, which would make them about 1/2 the price of a new 5000w pure sine inverter and other included necessities. I'd obviously rather buy "new" but $2000 saved is $2000 earned in this economy.

It's hard to justify $3200(incl. shipping) on an upgraded inverter at the moment, but $1500 seems doable right now, especially as it'll be nice to go 48v instead of staying at 12v. Any comments/suggestions?

Thanks again for everyone's help.

Solar Guppy

Re: Longtime Lurker - ready to take the plunge

The SW's are at a minimum 5 years old and virtually no place to fix them when they fail ... have the seller send you a picture of the manufacturing date. A new XW-6048 is about 3K and I'll buy that over a pair of used inverters if it was my money

mart.park

Re: Longtime Lurker - ready to take the plunge

To clarify, I will only be buying ONE of the 5500w inverters (not both, as my loads aren't that crazy), if any. I'll ask both of the sellers for some detailed photos and will post here when I get that info back.

FWIW, I had an *ancient* Trace 2000w inverter repaired a few years ago in SoCal,and it was definitely worth the $200 to fix it; it is still going strong in my RV today.

BB.

Re: Longtime Lurker - ready to take the plunge

My two cents is that I would not make the heart of a new install/upgrade based on obsolete inverters that are almost impossible to get parts for...

Also, the XW does support split phase 120/240 VAC circuits right out the box--No extra hardware required. Can be nice if you need 240 VAC for a water pump--or even just to send power a bit farther to the home or an out building.

Regarding a C60 24 volt controller and a MPPT charge controller... While you can get 10-15% +/- more power out of a MPPT controller (in very cold weather)--My preference is the ability to run higher voltage solar arrays... The the case of the C60/MPPT-- ~3x higher voltage. (~35 VDC for a C60 24 volt bank vs ~100 VDC Vmp maximum for a MPPT array).

That 3x higher voltage allows you to use 1/3rd as thick copper wire (save money) or to send the power 3x farther (or mix and match).

Also--larger solar panels designed for Grid Tied systems (typically >100 watts) tend to have higher Vmp and are not designed to "match/mate" with standard battery charging requirements.

Larger panels tend to be less expensive $$$/Watt pricing and require fewer electrical connections.

One warning about high voltage GT designed solar panels and 48 volt battery banks... For panels in the Vmp of ~50-70 volt range--a single panel is not high enough voltage (Vmp hot) to properly charge a battery bank. And two panels in series is too high of Voc (Cold) to meet the 140-150 VDC max of the typical 45-80 amp MPPT charge controller.

-Bill

mikeo

Re: Longtime Lurker - ready to take the plunge

One warning about high voltage GT designed solar panels and 48 volt battery bank

I suspect the Evergreen 205.s he is talking are 12 volt nominal panels. The older inverters are a bit of a gamble but I have installed 3 of them and they are working just fine. If it will get you a few years, maybe then would be the time to upgrade to the latest technology. If these inverters come with wiring panels, breakers and a charge controller, I would be parcel to the MX60, then it would cost you close to 4 or 5 grand to buy all these things new. Ideally, and money was no object, I would always buy the latest. But for many of us these days, money in this economy is a big thing when many people are losing their houses, can't afford health insurance, much less a buy new car, sometimes buying used is the ony way to get through life. With 2 k of solar, you will need to go with 48 volts preferably with room to expand, or 24 volts knowing you have maxed out a single charge controller solution. Just my Opinion.

BB.

Re: Longtime Lurker - ready to take the plunge

Mike,

I agree with you that panels he is looking at are most likely the "12 volt" type... I just wanted to get the high voltage / 48 volt bank warning out there in general--It is one that catches people at times with MPPT controllers (and can be a costly mistake).

-Bill

chevenstein

Re: Longtime Lurker - ready to take the plunge

A note on inverters - I got a Magnum Energy MS4448 for $2200 shipped from this site's sponsor and am very happy with it. They also make a 24v version for the same money, unless you have huge loads I would say the Magnum is a good choice (the 48V unit is 4.4KW; the 24V is 4KW, both are split phase). I had a problem with the one shipped to me and Magnum replaced it under warranty with little hassle. My Magnum 4448 even starts my deep well pump with no trouble; the pump draws nearly 30 amps on 230V starting and ~7A running.

mart.park

Re: Longtime Lurker - ready to take the plunge

OK, so my options right now seem to be:

Keep the 12v 2500w mod sine inverter, buy new disconnects, 2 charge controllers, and a small pure sine inverter, ~$2000 (no warranty remains on current inverter)

Buy a "new used" 5000w pure sine inverter with charge controller, disconnects. $2000 (no warranty again) and hope it lasts a couple more years.

Buy a NEW 4000w (3-5 year warranty) inverter, disconnects, charge controller ~$4500+, and buy fewer panels now, upgrade to more panels later.

My expense list is by NO means complete, but it (hopefully) compares apples to apples as far as the used equipment is concerned.

Our HOA pumps the water for us, so I don't need 220V power. I have access to a mobile 220V welder/generator next door but have never needed it.

ONE THING OF NOTE: Our current 2500W inverter works for the two of us because we are conscious of our limits, but it can be hard to convince guests to respect that when they visit, so going larger would be refreshing.

Of COURSE I would rather buy new, but our budget for home improvements for the year is going solely to this system, and adding an additional $3000-ish to that figure is just tough to swallow. I'm pretty conservative with money, but have a tendency to "gamble" a lot on used equipment, and have been pretty lucky, but I'm not very familiar with the lifespan of these larger, older units.

What kind of experience have you all had with the lifespan of your inverters in the past 10 years?

westbranch

Re: Longtime Lurker - ready to take the plunge

Just to add a bit to your confusion, don't disregard a 'hybrid' type system where you have a MSW and a TSW inverter feeding different loads. ie MSW for lights and other stuff you already know runs on it and the TSW for that which needs good AC.

I remember that topic being discussed a fair while back .

HTH
Eric

mike95490

Re: Longtime Lurker - ready to take the plunge

This all seems to be a 12V only system, just be sure all your loads don't exceed your battery and cables limits. Ever give any thought to a 24V system, before you buy new inverters, less loss in the battery internals and cable losses, because the amps are half as much.

BB.

Re: Longtime Lurker - ready to take the plunge

My two cents--anything (power electronics, or even any electronics) running past 10 years is a gift.

A big issue (in my humble opinion) with electronics is temperature cycling and absolute average temperature...

If you had something that was exposed to cold/hot conditions every day (inverter gets near freezing at night, then in middle of day with heavy loads and hot sun/room)--Thermal cycling can be very hard on the electronics and their soldered/physical connections.

Another handy rule of thumb from engineering is that for every 10C (18F) increase in temperature, the life is cut by 1/2. If the operating temperature is 20C (36F) higher (because of room/self heating), the life is 1/2 * 1/2 = 1/4 vs the same device in a cold room/fans working well... (also works the other way, if device is in a cooler than normal area, its life will be extended).

So--if the inverters you are looking at have been in a nice cool/dry basement--they should last longer than ones kept in a small/hot closet.

-Bill

westbranch

Re: Longtime Lurker - ready to take the plunge

Mike he has to be at least into a 24 v system for 5,000 W...

found this in an early post.."3-4 yr old Trace/Xantrex SW5048s," so it might be a 48 V system if he gets one of these inverters...

Cheers
Eric