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the UPS from Heck: a solar project that didn't quite gel

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Alan Georges:
For a while I’ve been contemplating this project, running my computer off the micro-solar system as a sort of “unstoppable UPS from Hell.”  While spec’ing out what new gear I’d have to add to make things all work, I kept notes, intending to put a success story all into one big post.  In the end the numbers just didn’t pan out, but there’s still some good information about inverters and their waveform outputs, what’s recommended for computers (and other electronics) to run on, all that stuff.  So I’m posting dumping it here to the solar forum, where it can quietly rest in peace.

Yeah, it’s an incoherent mess.  If you don’t want to bother, I won’t be offended.


My computer’s UPS gave up the ghost sometime last year.  With a decent surge suppressor and reliable grid power, I haven’t bothered replacing it, but it’s really something I need to do.  Having said all that, across the shack from my computer, there’s this beautiful battery bank that’s part of a small solar system I use to run the ham rigs...

Time to run some numbers.  Here’s the raw data on what my computer desk and its accessories draw, measurements taken using Kill-A-Watt meter:
computer only, idle      100w
computer only, busy      120
computer only, sleep      6w
computer & lamp, peak   190
computer & lamp, typical   150
computer & lamp, idle   130
just lamp            30

Let’s go ahead and jump to useful energy-per-day units now, from Andy Weir’s sci-fi book The Martian, and use kW-H/sol = 1 Pirate-Ninja (PN).  Assuming standard Terrestrial Gulf coast days (as opposed to Martian), we get something like 5 hours of average ensolificaion each day.  Also, to save decimal counting, because this system’s so small, let’s go on to mili-Pirate-Ninjas, or mPN for the standard Watt-hour/day (instead of KW-H/day) units.  Off we go.

A typical day’s computer usage: 2 hours use @150w + 22 hours sleep @6w = 432mPN.

In terms of battery usage, 432mPN / 12v = 36A-H per day.  Not enough to kill my 120A-H bank, but it’s a big daily workout, more than the 20% optimal daily dip.

In terms of 5H/day of ensolification, the panels’d better be putting out 7.2A.  In standard 12v panel sizes, a 140w panel would be needed to keep up with this.  While I do have one, this configuration doesn’t leave a lot for other uses (recharging AA batts & smartphone, talking on the radio, etc.).  So from that point of view, this project’s marginal already.  Pretty good for emergency backup but not much past that.

Oh, also, if this contraption burns my house down, the insurance company’s probably not going to pay for anything.  Something to keep in mind.

Still, let’s press on.  Look at inverters, which convert 12vdc (like a deep-cycle battery gives) into 120vac power (like a computer uses).  Computers can run fine off modified sine waves (MSWs) – for a time.  From digging around on the web, it looks like most home-grade UPSs feed grid-supplied pure sine waves (PSWs) to their computers while the power’s on, and then dump MSWs to them until you can safely power things down.  Not a problem for a few minutes, maybe MSWs are OK even a few weeks in a pinch, but you don’t want to run things this way all the time.  So if I wanted to just run with my 800w MSW Cobra inverter for a couple of weeks after a hurricane, that’s probably OK, but it’s good to understand that it’ll use up a fair chunk of the computer’s expected lifetime in that process.  Testing this today, everything ran fine, but there was a slight audible buzz coming either from the computer itself, the screen, or the backup drive – or maybe all three.

Now the questions are down to:
1- what do “good” and “bad” even mean when it comes to power?
2 - how clean of power does a computer really require?
3 - how bad are MSW inverters?
4 - how good are PSW inverters?
5 - and can we find a moderately priced PSW inverter that doesn’t put out a bunch of RFI?

Let’s get some answers:

1: Starting at has:
--- Quote ---While there is no national standard dictating THD limits on systems, there are recommended values for acceptable harmonic distortion. IEEE Std 519, “RECOMMENDED PRACTICES AND REQUIREMENTS FOR HARMONIC CONTROL IN ELECTRICAL POWER SYSTEMS”provides suggested harmonic values for power systems: “Computers and allied equipment, such as programmable controllers, frequently require ac sources that have no more than 5% harmonic voltage distortion factor [THD], with the largest single harmonic being no more than 3% of the fundamental voltage. Higher levels of harmonics
--- End quote ---
OK, so 5% THD.  Works for me.  Answers Question #2 at the same time.

3: How bad are MSW inverters?  According to this link  , THD is in the neighborhood of 24%.  That explains the buzzing.

4: How good are PSW inverters?  Same link, many are better than 3%.  That’s less than 5% recommended for computer equipment, so it sounds good to me.

5: From a try at PSW inverters a couple of years ago (link), we know that some of the cheap ones put out tons of radio frequency interference (RFI).  As in “enough to block out local FM broadcasters,” which even more so eliminates any hope of receiving AM & SW broadcasts, or working CB, ham, etc. 2-way radios.  Digging around for reviews over at (where if there’s any RFI, they’d be sure to gripe about it) there’s ONE review of ONE product, a Samlex PST-300-12  300w (perfect) in the $150 range (ouch).  But it was a good review.  Digging up the manufacturer’s manual & data sheet ( it looks alright, at < 3% THD and FCC compliant

So, the the bottom line today is that to do this I need a decent (read $150) PSW inverter for power-out situations with my computer.  Even then, it won’t be the everyday UPS, but just for power outages.  OTOH, the stereo and other electronics in the house would be much happier, and having music & movies when the power’s out for 3 weeks is a Good Thing.  And on top of all that, I still need to get a new conventional UPS.

This project’s already slid so far sideways that mission creep has taken hold.  From “gee, maybe I can use my solar bank for a cheap-o UPS, because I’ve already got the parts and everything” to “no, but a $150-ish PSW inverter will let it serve as a good get-by, and make extended power outages much more livable – and the rest of the household electronics will be much happier too.”

Of course, I could just not worry about it, and charge & run my laptop (requires maybe about 100mPN and charges fine on a MSW inverter), keep the $150 in my pocket, and move on.  But the fluorescent lights, TV/DVD, and the rest of the electronics in my house would be so much more happy.


After running the numbers, I’m not doing this.  If the power’s out and I need to do some emailing/news-gathering, the laptop and cellphone hotspot (assuming the cell system’s still available) will be just fine, and all that’s up and running now, and requires less than 1/3rd the power.  As for the rest of the lighting, there’s enough AA batt powered lights in this house for a small city, and recharging AA Eneloops off solar power is routine.  I can live without movies & stereo for a couple of weeks (definitely way down on the list), and make do with shortwave and nighttime AM DX.  Just not worth the money or hassle.  But along the way I learned a lot about running computers & electronics on MSW vs. PSW power.  Maybe it’ll answer others’ questions too.

Two years on, I still haven’t stuck ferrite beads on all the wires running in and out of my old cheap-o PSW inverter.  Have to do that in the near future, see how much it helps.  Just haven’t needed it, got sidetracked on other things.

After all that, I’ve still got to buy a new UPS.  Shoulda used the time I burned here just driving over to Best Buy!  Hope you learned something, or at least got some modest entertainment out of reading about all this flailing around.  The THD numbers at least were sort of interesting.

  The 'economy' of solar just is not up to expectations yet.

Good stuff Alan, got me thinking.

I've been charging my iPhone off my solar bank like this;

Solar bank(12 vdc) + PSW inverter(120 vac) + iPhone charger(120 vac step down to 5 vdc)
I'm wondering if at this step ................................................................^^^^^^^^^^^^^^
Dose it "clean the power" as to not harm the iPhone?

If there's even a chance it'll harm the iPhone I'll charge we grid power.

Ok ,I have to ask . Why are you going DC-AC-DC to power a laptop? Something like this

would do away with so much of your post.

In my case it's convenience.

I charge my iPhone at the power strip that is supplied by the PSW inverter, the power strip also powers 2 Li-ion battery chargers, am/fm radio, desktop lamp. I could just change the "place" where I charge my iPhone.


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