The reality and truth for realistic expectations please...

Dizastrous

I have lurked here for months and have learned so much that I now want to put it to practical use and I could use some real truth behind the paper math please:

Potential system would be for home backup use for power outages, and if it works well for that, I would throw it in the camper for weekend use too. Made in USA is also very important to me, although it costs a bit more.

Here is my paperwork - I would ask the resident experts to please set me straight on what I can REALLY expect please...and thank you!

Materials:
1 Panel - Suniva MVP Series - Max Power 290w, Max Voltage 35.74, Max Amps 8.11 (Voc 44.91 and Isc 8.55)
(Yes, I know it is big, but my paperwork says this is the right way to go) 100% Made In USA too

1 Battery - Sun Xtender 1530T AGM - 12V-153Ah/24hr - Heavy, Expensive, but looks to be the best out there that is 100% Made In USA

1 Charge Controller - Rogue MPT-3024 - Based on the reviews and tests from the 23 page thread here and is 100% Made In USA (I have also spoken to lorelec/Marc via email)

1 1500w Pure Sine Inverter - Mandalay Wind - No bad reviews and 100% Made In USA

My paperwork:
My example appliance is the family 25cu ft Samsung Refrigerator built in 2004. Semi-efficient. Kill-A-Watt says it sucks down about 170watts an hour - every other hour. So, in a day - it hogs up 2040 watts @ 120V. Hourly math says 170w / 120V = 1.42A. This, of course, does not include in-rush at start time. Simple AC/DC wisdom says that for every 100watts of AC use, subtract 10amps from your battery +10% for inverter waste. So, to run my fridge on my battery of 153Ah - I subtract 17amps+10% from 153amps per hour of running time, or 18.7amps/hour to run the fridge, right?

My battery and inverter combo start to confuse me. General consensus says do not run a battery past 50% DOD (Depth of discharge) for Lead-Acid...BUT, AGM's can go to 80% discharged. The SunXtender documents do not really say this though - they reference 50%. So, is there truth to 80% on AGM's, or do I stick with the 50% rule?

Battery Usage - when a 153Ah battery reaches 50% DOD - what does that even mean? I see that OCV per cell is 2.03 and that OCV per battery is 12.2 at 50%...but what is really left? DO I still have a 153Ah battery but for half the time, or do I now have a 76.5Ah battery for twice the time? I see I am still ~12V. Does it even matter?

Recharging the battery at 50% DOD - What do I really need to recharge this battery to 100% capacity. Tiered charging and the various cycles aside - 153Ah at 50% seems to tell me that I have removed 76.5 amps of DC from it and now it wants it back +10%. So, to restore this battery to 100% capacity, I need to give it back ~84amps DC from the charging source. Is that correct? So, if my charging source (Suniva Panel and Rogue MPT can give me 10amp DC per hour - my battery would be back to 100% in about 9hrs in full sun?

Rogue MPT 3024 - When I asked Marc a few quick questions, he replied that PERFECT conditions would have 290w at the panel and then figure ~95% efficiency with the controller and wire losses and some tracking loss may put 260w-270w available to charge the battery. The current from the controller is dependent on the voltage of the battery at charge time (which is a moving target, I know). So, we assume 50% which is 12.2V at the battery puts my charge current ~21amps DC during perfect sun. As the battery voltage goes up from the charge, the current from the controller has to go down - I think - if I understand charging principles.

Putting it all together - (and here is where I think the paper lies)....

I have a 153Ah battery and the power goes out at midnight. I plug my fridge into my inverter and 100% capacity charged battery at midnight. My fridge runs for 1 hour and took 18.7amps from my battery. My battery is now at 153-18.7 = 134.3Ah? Is it now discharged by 12.2%?

1am - 2am - only loss from inverter sitting idle
2am - 3am - Fridge on again for 1hr for a loss of another 18.7Ah. Battery now at 134.3Ah - 18.7 = 115.6Ah and discharged by 24.4%
3am - 4am - only loss from inverter sitting idle
4am - 5am - Fridge on again for 1hr for a loss of another 18.7Ah. Battery now at 115.6Ah - 18.7 = 96.9Ah and discharged by 36.7%
5am - 6am - only loss from inverter sitting idle
6am - 7am - Fridge on again for 1hr for a loss of another 18.7Ah. Battery now at 96.9Ah - 18.7 = 78.2Ah and discharged by 48.9%
7am - 8am - only loss from inverter sitting idle - (Very weak sunshine becoming available on my panel face)
8am - 9am - Fridge on again for 1hr for a loss of another 18.7Ah. Battery now at 78.2Ah - 18.7 = 59.5Ah and discharged by 61.1%
But - do I now get to ADD for my panel getting sun? Say it is weak sunshine at 8am and my battery is over 50% DOD. Can I maybe expect 12amps back this hour
78.2Ah - 18.7 + 12 = 71.5Ah and the battery now discharged at 53.2% instead of 61.1% Does it work that way?
9am - 10am - only loss from inverter sitting idle. Panel providing me 12amps this hour. Battery now at 71.5Ah + 12 = 83.5Ah and discharged by 45.4%
10am - 11am - Fridge on again for 1hr for a loss of another 18.7Ah. Battery now at 83.5Ah - 18.7 = 64.8Ah and discharged by 57.6%
ADD for my panel getting sun? Say it is better sunshine at 10am and my battery is over 50% DOD. Can I maybe expect 15amps back this hour
83.5Ah - 18.7 + 15 = 79.8Ah and the battery now discharged at 47.8% instead of 57.6%
etc, etc, etc

As you can see, the paper math says it will be give and take for the day and keep me around the 50% DOD mark until the sun goes down and then I am dead in the water, but to run the fridge since midnight and be able to cover the power needed for about 20 hours, would this work? I realize I have options to only let the fridge run every 3 hours instead of every other - this is more for the math and theory rather than the real-world appliance being used. Only once in my life has the power been out for more than 24 hours, so 24 hours is my likely goal. Camping would be much less draw.

Basically, what I am looking for is confirmation or correction that my typed-out theory above will be similar to real-world experiences, or if I should just scrap this entire idea. It is an expensive adventure, but could have years worth or reliability.

Fire away and thank you for your time!

icarus

Re: The reality and truth for realistic expectations please...

The quick rule of thumb for any off grid system is this: Take the name plate rating of the PV, divide that number in half to account for all cumulative system loses, then multiply that number by four to account for the average number of hours of good sun you. An reasonable expect, per day over the course of the year.

So,, 290/2=145*4=580 watt hours of power per day,, on average.

All design criteria start with the loads. Define y our loads, and work backward to figure out batty bank sizing and PV sizing.

A 50% DoD on a regular basis is a recipe for short battery life. Ideally one might design for a 25% discharge rate, and a
~15% of ah rating charge current.

I suggest you read the following links: http://www.batteryfaq.org/
http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries


Welcome to the forum, keep in touch,

Tony

Cariboocoot

Re: The reality and truth for realistic expectations please...

Oh boy, it's fun with numbers time!

See, all those numbers are really guidelines. For example, a "200 Watt" panel doesn't produce 200 Watts except under lab conditions and rare occasions in the real world. Likewise any battery can be discharged 80%, but the deeper the discharge of any battery the shorter its lifespan. Recharging is based on the individual battery's Peukart factor, and luck. For one thing, a "200 Amp hour" battery only has 200 Amp hours at a fixed rate of discharge. In the real world this does not happen. Lighter rate of discharge gives you more Amp hours, heavier discharge rate gives you less. Thus at a steady 5 Amps it may be 200 Amp hours, but at 10 Amps it can be less than 50% of that.

So you go back to the loads. You have a K-A-W number for about 2 kW hours per day (that's just the 'frige; you need to include the inverter draw and system losses). On a 12 Volt system that's approximately 166 Amp hours average. Rounding UP here is a good idea. To keep the discharge to a maximum of 50% you need at least 334 Amp hours of battery.

Nothing is ever precise, and if you try to calculate it all out to the tenths you will be disappointed in the performance. You need margins for error, which is why there is the Icarus formula. That estimates an over-all system efficiency of 50%, which is abysmal but more often true than not. Using that formula, your one 290 Watt panel would provide about 580 Watt hours of AC out the door which is no place near the 2kW hour load demand.

Frankly, for a couple thousand Watt hours on the occasions the power goes out a small inverter-generator is a much wiser investment. Run it at night before bed to cool down the 'frige and it will still be cold in the morning.

Dizastrous

Re: The reality and truth for realistic expectations please...

And there is the truth...50% of label and only 4 good hours of sun in a day for an average. It is kind of amazing when you look at the cost of this "system" and then see what you really do not get. Granted, trying to run the family fridge like it is on-grid is a bit presumptious, but it was the example I started with. Really makes me glad I didn't buy the "solar generator" with even smaller specs.

Thank you for the links also Icarus.

That is actually very helpful and I thank you for reading all of my post - I didn't realize it got so long.

Dizastrous

Re: The reality and truth for realistic expectations please...

Cariboocoot wrote: »

...Recharging is based on the individual battery's Peukart factor, and luck. For one thing, a "200 Amp hour" battery only has 200 Amp hours at a fixed rate of discharge. In the real world this does not happen. Lighter rate of discharge gives you more Amp hours, heavier discharge rate gives you less. Thus at a steady 5 Amps it may be 200 Amp hours, but at 10 Amps it can be less than 50% of that.

Very valid and not incorporated into my "paperwork". Thanks for that reminder.

BB.

Re: The reality and truth for realistic expectations please...

I would start with a Kill-a-Watt meter or similar. It allows you to run the appliance for 24 hours (or longer) and read the kWH used (very handy for cycling appliances and computers/entertainment systems that have variable on times).

Also, regarding discharging a battery by 25/50/80/100%...

For a battery bank that is used daily (cabin/off grid home/etc.), discharging 25% per day is a good compromise between system capability and long battery life. And avoiding going below 50% discharge too often helps the battery bank have a long cycle life.

For RV's where space is tight and you only use it on weekends or a few weeks a year--I have less problem discharging to 50% or even 80% of battery capacity--As long as the battery bank is quickly recharged (solar, AC generator, home/shore power). Even if you get 500 cycles vs ~2,000 cycles with deeper discharge--50 weekend uses a year would still give a 10 year cycle life--The battery bank will probably die of old age/mistakes by the operator long before that.

If you discharge below 80% depth of discharge--You do run the risk of taking one or more cells past dead and even begin to "reverse charge"--That is pretty much instant death for those cells.

For large/expensive battery banks and/or installations where you risk very deep cycling--I would suggest a Battery Monitor (Victron is another good brand). Also, very useful for AGM/Sealed batteries where you cannot measure specific gravity.

Monitoring a battery bank by DC voltage only is tough to do accurately.

-Bill

RandomJoe

Re: The reality and truth for realistic expectations please...

Dizastrous wrote: »

And there is the truth...50% of label and only 4 good hours of sun in a day for an average. It is kind of amazing when you look at the cost of this "system" and then see what you really do not get.

So true! Your specific use-case - the fridge - was the initial design criteria for my system when I started. My specifications were for 3 days without sun (makes the battery larger) to 50% DOD, 4 hrs sun (my "full sun" hours in winter, worst case) and I wound up with over 500W in solar panels and 8 Trojan T-105 6V batteries! Not to mention all the associated hardware. Just for the fridge!

Another issue with your design above, you'd be really hammering that battery to handle the starting inrush of the fridge. Mine pulls about 110W while running but for several seconds at startup it hits over 1200W. At 12V you're drawing over 100 amps from the battery! A 153AH AGM is *NOT* going to like that! Odds are the terminal voltage would sag so low the inverter would trip off, but it could cause damage to the battery as well.

Dizastrous

Re: The reality and truth for realistic expectations please...

Thank you BB for the voltage monitor links. I have actually looked at those from other posts you put them in and that looks like another tool I would be wise to add. As for the Kill-A-Watt; I did buy one specifically for this research and that is where I learned the family fridge runs about 170watts/hr for about 12 hours a day. For anyone else learning - I also highly recommend buying the Kill-A-Watt device...you will learn alot about what your house is really eating for power. Great $26 spend for this knowledge.

Based on all the "real-world" experience on this forum, I see I need to either reel in my expectations, or cast out my budget for more support...I thank everyone for the comments and experience.

Back to lurking and learning...

niel

Re: The reality and truth for realistic expectations please...

imho you will need to up your battery capacity and possibly your pv capacity too. that particular battery is rated at 153ah, but at a 24hr rate. the 8hr rate is closer to your discharge rate, but favoring a lesser actual capacity than even this. 136ah at 8hrs. note that the rates are to 100% dod and it is never a good idea to do that or even 80% dod as damage can occur to the batteries as well as a very shortened lifespan. anyway, 136ah/8hrs=17a. you will certainly be drawing more than that so the time drawing may need to be shorter in reality. for the record mine draws 18adc and it's a smaller older refrig than yours.

moving on to bad news #2 we find that the sunxtender according to their manual will need adjusted by 1.39 for battery temps that typically are around 68-76 degrees f. this is not ambient air temps, but the battery temps. in a backups there usually isn't much rise above the ambient air temps unless deep charge/discharges are taking place. this means your 136ah battery is now 136ah/1.39=97.8ah. 50% dod of this would be about 48.9ah usable capacity. if you draw about 20a from the battery you have now crossed a bit over into a 5hr rate realm with about 2.25hrs as a good approximation on the 50% dod time, but those 5hr figures aren't available from concorde and you need to take a good guess based on facts and figures supplied. most likely it would be good for about 2hrs if the refrig doesn't bog down the voltage too much. this dipping of the voltage could stop the inverter from operating depending on its specs.

i use 4 pvx 1040t batteries for mine and can take it out to about 4hrs give or take. higher capacity may necessitate more pv if it isn't at least about 5% for the charge rate with your proposed pv on a larger battery.


http://www.sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf
http://www.solar-electric.com/cosuagmba.html

Dizastrous

Re: The reality and truth for realistic expectations please...

Coot - I just read your thread on "small fridges". not sure how I missed that one before - http://forum.solar-electric.com/showthread.php?12272-Just-how-bad-a-small-frige-is&highlight=dometic

Again, bottom line and truth is that the fridge is about the biggest 'necessary' enemy to these power systems...A/C and others are not as 'necessary" as a fridge is.

If you want efficient DC systems, you're gonna pay and if you want efficient AC systems...keep looking as they don't seem to exist yet. Maybe the old "ice-box" isn't such a bad idea again...

Thanks everyone!

Sparkletron

Re: The reality and truth for realistic expectations please...

Something to consider is that you don't need PV if your goal is "home backup use for power outages". I have a 330Ah bank @ 12V with the same idea in mind: to power a fridge and small electronics in the event of a power outage. And while it would be fun to charge the bank using PV, the math tells me that I would need around 600-700 watts, a 150Vin MPPT controller, and lots of expensive wire. A smart AC charger can do the same thing at a fraction of the cost. But let's say we did get PV systems for our banks. What do we do with all the energy we're generating when there is no emergency?

My advice is to go with a smart charger and a small efficient tri-fuel generator. I would only go with PV-as-backup if cost was not an issue, or if I thought the grid was going to go down for more than a month.

The Only Sarge

Re: The reality and truth for realistic expectations please...

Yup. Well sorta
I have 1000 watts of PV and 300 watts of wind power. 600 amps of battery. I run most of my lights, TV's and ceiling fans in my 3500 sq ft house on solar alt power 7X24X365. No issues at all.

I came to the same conclusion in a power outage lasting more than my gas/gen supply. So I bought a 12v freezer/fridge and it is in the shop running all the time. Paid around $1K for it. Much easier on the alt energy than the "standard" fridge.

So you can have your cake and eat it too. Dont have to build a PV array to peak amp draws etc. Look @ 12V solutions.

Sparkletron

Re: The reality and truth for realistic expectations please...

It sounds like you have a very cool setup. I also use a 12VDC fridge/freezer (Sundanzer). However, 12V solutions do not exist for many home appliances. For example, there is no 12V microwave or air conditioner (and you won't be able to get enough in-rush and sustained amps even if you use an inverter). You could run a washer (via inverter) but not a dryer. Running a TV off 12V is easy if it's a small LCD unit. But you can forget about a 50" plasma entertainment system. Bottom line: A 12V system has a lot of advantages, but major amp-hogging appliances is not one of them. You also have to think about how you're going to get this energy to the devices you want to use. Let's say you have a battery bank in the basement or garage. Fine. Great. But the ceiling fan you want to run is far away. Are you prepared to rip down drywall, rewire, and repaint? Factor in those costs as well...

The Only Sarge

Re: The reality and truth for realistic expectations please...

As for 110v wiring. I have a transfer box next to the house breaker box. Took the commons and ran the circuits I wanted behind the inverter to it. As for the 12v fridge yes I have it next to the battery bank. I am lucky and live close to a RV custom builder. He has 12V microwaves. I was in George Straits custom RV and he had 2 of them. Small yes. But fully functional to heat a Lean Cuisine etc. Not for me.....but the fridge works great...and I did not fill my roof with panels.

Here...for your files
http://www.skingco.com/convenience/food_related/2cook_appli.htm

Sparkletron

Re: The reality and truth for realistic expectations please...

Thanks for the link. I was curious how a microwave would be able to draw the standard 1000W via 12VDC, and the answer is it doesn't:

Wattage
660w AC/Direct Connect
240w Vehicle Power Outlet

Nothing wrong with 240W, but it does mean that all the times listed on all the frozen foods will be off, presumably by at least a factor of 4 (not sure if power levels are linear or exponential).

So the trick is to use existing wiring, and just choose the circuits that you want to power and add a transfer switch. My problem is that my batteries and inverter are nowhere near my breakers. In my area, code requires the breaker box to be mounted outside for easy access by the authorities (never in the garage or basement).