Best way to measure different inverters for battery draw/efficiency?

doctorZeus
doctorZeus Registered Users Posts: 24 ✭
in Solar Beginners Corner #1
Looking for guidance on the best way to "measure" usage (i.e. what it's drawing from batteries the batteries to power the load.) I plan to use the kill-a-watt which could give me some helpful data and one side of the story, but I was also hoping to measure between the inverters and the batteries.

My load information is as follows..
I'm looking to power my chest freezer + mostly low draw equipment like battery chargers, phone/tablet charging, small fans, etc. My calculations have me at ~2.5kwhr per day usage TOTAL (I expect it to be closer to ~1.8kwhr but 2.5 is the max and 25% of the battery bank's DOD), with highest "running at once" watts to be around 375 watts, with highest surge watts around 500. The only thing I've put the kill-a-watt on to measure for surge is the chest freezer, and the highest I've seen it surge is ~200 watts, and everything else should have minimal surge and ~5 to ~10 watts, maybe ~40 watts or so for the fan, etc.

My battery bank will consist of eight 6V golf car batteries running in series/parallel to 24V / 430aH.

So I have my heart set on the Samlex SA-1000K, which is a 1,000 watt PSW, specs here, plan to use that as my "main,"
http://lib.store.yahoo.net/lib/wind-sun/SA-1000K-specs.pdf

I also am looking at the Go Pro GP-SW600-24 as a backup, "essentials only" if necessary due to the lower wattage, etc.
http://gpelectric.com/files/gpelectric/Docs/Specs/Go_Power_SPC_GP-SW150-300-600.pdf

I also have a handful of cheaper MSW inverters I'd like to do some testing on..since I expect much of the differences to be subtle, thought I would get the forum's guidance on how the best way to measure them is.

Any opinions/experience/comments on the two inverters I'm looking to purchase is also appreciated. Thanks in advance
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Comments

  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #2
    Re: Best way to measure different inverters for battery draw/efficiency?

    The new Kill-A-Watt meters do well with non-sinewave loads. They do not capture peak currents though. A compressor in the freezer will likely draw surge of 800-1500 watts during compressor startup, with a constant run power of between 100-200 watts depending on size of compressor.

    The smallest PSW inverter for a normal refrig or freeze should be 1kW. Compressors generally do not like MSW inverters and compressor will run hot. The problem is the way the starter winding is configure with a starter cap in series with a thermistor. Normally the thermistor is low resistance when cool allowing high current through starter winding for short startup period. As thermistor heats up the starter winding current is decreased. With MSW the high frequency components of waveform means starter cap will allow more current into starter winding during normal run period, causing more heating and possibly burn out of starter winding.
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  • doctorZeus
    doctorZeus Registered Users Posts: 24 ✭
    #3
    Re: Best way to measure different inverters for battery draw/efficiency?

    thanks for the feedback.. are you or is anyone aware of some common methods that would allow me to capture that peak? Would a simple AC clamp meter be likely to capture this for me? Do they do min/max like decent multimeters? Any specific devices you can recommend (or maybe I'm completely off course here).

    What about how to capture draw from the battery to the inverter? I'm wanting to measure this to test (in conjunction with a kill-a-watt on the AC/load side) several different inverters to compare brand, size, and SW/MSW efficiency on my loads. I'm thinking I could do the same thing with a DC clamp meter like above..but I've come to learn that usually when I'm stumped/confused for longer than a few hours it's because I don't have the vocabulary to google the right things....any guidance suggestions appreciated.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #4
    Re: Best way to measure different inverters for battery draw/efficiency?
    doctorZeus wrote: »
    I'm thinking I could do the same thing with a DC clamp meter like above..but I've come to learn that usually when I'm stumped/confused for longer than a few hours it's because I don't have the vocabulary to google the right things....any guidance suggestions appreciated.

    Most DC clamps are not precise enough for inverter efficiency measurements. If you want to get the efficiency with +-1% precision, you will need current measurement with +- 0.5%. You can do that with a shunt and a precise voltmeter. However, shunts are usually only 1% precise or worse, so you will need to calibrate your shunt first, which in itself will require precise instruments too.
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  • doctorZeus
    doctorZeus Registered Users Posts: 24 ✭
    #5
    Re: Best way to measure different inverters for battery draw/efficiency?

    NorthGuy, thanks for the feedback. Five minutes of reading "shunt + solar system" searches was of tremendous help. I also now realize the subtle undertone in your response is spot on. That is, this is not a "do this," or "get that" type of question, but rather looks to be a mini project in its own right. Regardless, you've definitely helped me move in the right direction. Thank you.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #6
    Re: Best way to measure different inverters for battery draw/efficiency?
    NorthGuy wrote: »
    Most DC clamps are not precise enough for inverter efficiency measurements. If you want to get the efficiency with +-1% precision, you will need current measurement with +- 0.5%. You can do that with a shunt and a precise voltmeter. However, shunts are usually only 1% precise or worse, so you will need to calibrate your shunt first, which in itself will require precise instruments too.
    Fortunately, for A/B comparisons you only need the shunt to be precise and stable, not necessarily accurate. (using the technical sense of all three terms.)
    In NorthGuy's quote above "precise" should actually be "accurate" where the shunt is concerned, but a DC clamp is both imprecise and unstable (reading and zero issues) and inaccurate (not well calibrated).
    SMA SB 3000, old BP panels.
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  • doctorZeus
    doctorZeus Registered Users Posts: 24 ✭
    #7
    Re: Best way to measure different inverters for battery draw/efficiency?

    Thanks for clarifying inetdog..that's exactly the direction my mind wandered after reading NorthGuy's post. It would be nice to (accurately) measure the actual efficiency, but not required for what I would like to do - and that's (end result, in a nutshell) measure each inverter against a control load and device type to determine which is most efficient. Also, I've come up with next to NO empirical evidence (like what I'm trying to do) out on the Web, so that makes it that much more interesting for me to do it myself.

    I'm about to head off and do some research but figured I'd go ahead and ask here - are there any gadgets comparable to the kill-a-watt that I could put on a shunt between batteries/inverter, or inline, or whatever? Something that would accurately measure the DC current between batteries and inverter over a given period of time?
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #8
    Re: Best way to measure different inverters for battery draw/efficiency?
    inetdog wrote: »
    In NorthGuy's quote above "precise" should actually be "accurate".

    Thank you. I largely regarded these words as synonyms. But there's indeed a difference. And, in Star Wars, when Obi-Wan Kenobi said "Only Imperial stormtroopers are so precise," he should've actually said "Only Imperial stormtroopers are so accurate". I bet Yoda would've said it right :D
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #9
    Re: Best way to measure different inverters for battery draw/efficiency?
    doctorZeus wrote: »
    I'm about to head off and do some research but figured I'd go ahead and ask here - are there any gadgets comparable to the kill-a-watt that I could put on a shunt between batteries/inverter, or inline, or whatever? Something that would accurately measure the DC current between batteries and inverter over a given period of time?
    There are some good battery monitors that will tabulate and record shunt current and keep a running balance. But they will not, in general, also look at the voltage at the time so that you can determine the actual POWER flow in the way that a KAW does for AC.
    SMA SB 3000, old BP panels.
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  • Skippy
    Skippy Solar Expert Posts: 310 ✭✭
    #10
    Re: Best way to measure different inverters for battery draw/efficiency?
    NorthGuy wrote: »
    And, in Star Wars, when Obi-Wan Kenobi said "Only Imperial stormtroopers are so precise," :D

    O.k. totaly off topic . . which Star wars . . I want to look it up... :D:p
    2 - 255W + 4 - 285W PV - Tristar 60 amp MPPT CC / 3 - 110W PV -wired for 36V- 24V Sunsaver MPPT CC / midnite bat. monitor.
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    3 ton GSHP.- 100 gallon warm water storage / house heat - radiant floor / rad
    9 -220W PV - net meter - Enphase inverters and internet reporting system.
    420 Gallon rain water system for laundry.***  6" Rocket Mass Heater with 10' bed for workshop heat.
    Current project is drawing up plans for a below grade Hobbit / underground home.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #11
    Re: Best way to measure different inverters for battery draw/efficiency?
    Skippy wrote: »
    O.k. totaly off topic . . which Star wars . . I want to look it up... :D:p

    See http://en.wikipedia.org/wiki/Principle_of_Evil_Marksmanship :-)
    At the beginning of the original Star Wars movie, A New Hope, the Stormtroopers are portrayed as lethal when boarding a rebel ship, overwhelming it and killing most resistance in what seems like mere moments while taking only minimal losses themselves. At one point Obi-Wan Kenobi even comments on their effectiveness to Luke Skywalker when the pair find the destroyed Jawa sandcrawler, saying "These blast-points... Only Imperial Stormtroopers are so precise."
    SMA SB 3000, old BP panels.
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