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Comment by madaxe_again | Hacker News Reader

I mean, it is ok, as long as everything is appropriately sized - I live entirely off grid, with several battery banks and inverters, which are essentially giant UPSs - and have regular UPSs for various equipment so that when I shut the power down for whatever reason the network and servers stay up.

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There are a vast number of problems that all can be summarized as "this is a really stupid thing to do, unless you really know what you are doing". This is one of them.

This article is about daisy chaining UPSs into each other's AC outputs. I hope you're not doing that off grid. Daisy chained UPS means the energy takes the path of DC battery > AC inverter > DC rectifier > different DC battery > AC inverter > DC rectifier to whatever device you're powering. You could be losing up to 20% of the energy (server uptime) at each step of the daisy chain. And since there's only one path for power to take, if anything fails, it all goes down. The article's solution is super simple, just use them in parallel instead.

UPSs typically operate in passthrough mode unless a deviation in supply occurs - so no efficiency loss day to day - and no, I don’t chain UPSs, but as I said, the house batteries are essentially big damn UPSs.

Also, not particularly caring about efficiency currently as we always have more energy than we can use, and I’m currently looking at filling a shipping container with sand as a dump.

>UPSs typically operate in passthrough mode

Oh, that makes a lot of sense. In that case, daisy chaining two sounds like no big deal.

>filling a shipping container with sand as a dump

That's pretty cool. I didn't know private folks were doing that.

Same here, I run a lot of different equipment on grden variety UPS's on our solar grid - but that is fed by redundant 18KW three-phase inverters and 72KWH of batteries... so that probably looks a lot like utility power from the perspective of the UPS's.

Overall, the discussion tangentially reminds me of a common theme in aviation - twin vs single engine aircraft. With two engines, the chance of having an engine failure at a critical time is doubled.

But you also have a good chance to get through the failure with one working engine. A two engine plane with one working engine climbs slowly, and a one engine plane with zero working engines doesn't climb at all.

The problem is that fully loaded, many piston twin planes do not climb at all or can only descend slowly on one engine.

There is a saying about light piston twins…. The other engine will take you all the way to the scene of the crash. If you are willing to fly your light piston twin half full, you have great safety margins, but then you are paying twice as much to move less payload than a comparable single engine aircraft.

I should say that some piston twins are much better than others in this regard, but for many it seems the extra engine mainly hauls the weight of the extra fuel and airframe you had to add to carry it. Asymmetrical thrust adds a lot of drag.

Moving to turbocharged engines or turbines tends to improve things a lot, but even some big twins can’t climb with one engine. (C130 in a many configurations, for example)

I’m pretty sure modern designs are much better as a rule. Composite construction and power to weight ratios of modern power plants have pushed the numbers safely to the left of the tipping point.

This is a lot harder to achieve using 1930s technology engines and construction as found in many light aircraft built up into the 1980s. There’s only so much you can get out of an air cooled, low RPM carburated gasoline engine,and complex shapes in metal construction cost big in labor and weight.

Not really. In your case, you probably have a very high quality pure sine wave inverter that's much larger than the individual UPSes, which will make the downstream UPSes mostly happy as long as you don't approach max load.

Many cheap inverters are not pure sine, and a UPS seeing this waveform may decide it needs to go to battery also.

Practically, a UPS also adds to the current draw, and many people may accidentally exceed the circuit limit because they only look at the useful load but not also the UPS charging load after a power failure ends.

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