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Coming from Europe, I've never seen an AC unit expressed in BTU/hour. To me it seems much more logical to express it in kW.

The only "strange" (but correct) notation was on a french train, the AC was expressed in kcal/hour

Then you've probably never had to plan cooling for a server room. BTU's are often the only metric you get from the servers datasheet.

I find that these are occasionally very inaccurate. If possible put a prototypical server on a watt meter and artificially load cpu, disk and ram to determine its real world max wattage and thermal values. Then compare to what its wattage is while idle.

At the scale of a server room, you're estimating over an aggregate and probably want some overhead/room to expand.

At the scale of a small to medium colocation facility you define W per cabinet... Such as a typical 44U height cabinet in a row designed for front to back airflow. 5kW per cabinet would be a normal low range, up to 10-12kW. Then figure out your floor spacing and sq ft usage to determine how many cabinets.

"Then you've probably never had to plan cooling for a server room."

For an American one you mean

i'm german, so no.

but i admit that its incredibly rare to find localized data sheets. they're generally en_us, not that it really matters.

1 watt = 3.412 BTU/hr.

Even British people don't use BTU

Every AC I've bought in the UK (5 so far) have been rated only in BTU.

You used the word PV a lot but never said what it means. For anyone wondering, PV means photovoltaic.

https://en.wikipedia.org/wiki/Photovoltaics

Yes, I forget that not everyone know the term. Work with solar on a regular basis and the whole industry is just as full of acronyms as x86-64 hardware.

Any developed industry has it's own terms and language - healthcare, military, government, programming, finance...

Always fun learning new languages inside languages.

> You can't run air conditioners directly off solar. PV panels natively produce DC. There are units with microinverters built into the back of the panel that produce AC.

Why can't an air conditioner run off of DC? Just lack of availability?

Can you run an electric motor on DC power?

Yes?

https://en.wikipedia.org/wiki/DC_motor

I think he meant more ‘can you run an AC motor on DC power’ (benefit of the doubt), since almost all AC units sized for residential/business operate on AC power (to my knowledge?).

Modern ACs are powered by AC, but they run IGBT inverters to power a brushless DC motor. So you could build ACs to run on straight DC, but they would be a special application (so expensive initially).

There are 48v DC air conditioners, again though you typically need a buffer with solar, either batteries or grid unless your way over paneled and even then you might get a cutout if a cloud goes over.

IANAEE but would it be possible to use a DC compressor and rectify the mains, instead of using an AC compressor and inverting the PV?

Yes, but 3-phase AC motors require less maintenance and are more efficient than brushed DC motors. Most traction applications are now using AC motors with inverters instead of DC.

Powerwalls only output single phase AC, not 3-phase.

Brushless DC motors actually have an awful lot in common with 3 phase motors, though, in terms of operating characteristics and performance.

You theoretically could, but then you would have a special weird setup that doesn't match 99.9% of the similar installation base of air conditioners on the same island. Making repairs, maintenance and such very costly. You generally want to generate usable AC power from any significant sort of solar system, so that you can use it for more than just air conditioners.

I’d also assume that you get weekends to fully charge to give you a reserve buffer.

It's implicitly in your post, but basically a battery is a lot cheaper than a ton of extra solar panels since you can use your full day of solar during the peak period.

I think we need smarter ACs, not more batteries. A smarter AC would just continue to run with reduced output when there is less available power.

Is that not exactly what a brownout would cause, as GP alluded to? (I don't know the effect of low voltage on A/C so I'm guessing here...)

I don't think a brownout would cause the desired effect (GP said "brownout and fail"). But what you could do is automatically switch off a selected set of A/C units when power output is low. That way you can prevent a brownout (drop in voltage), while still having some cooling. Of course, the A/C units should be designed such that frequent shut-offs are not a problem.

Anyway, I'm no expert either but I think the approach should consider the whole system (PV + A/C) rather than view them as different domains.

Modern mini-split heat pumps are inverter driven and can step up/down their power consumption and cooling capacity pretty much in 1% steps — at least that’s what the marketing material claims.

Normally the cooling output is a function of the set point temperature vs current ambient and the fan speed.

So I’m not sure what you would need to add to allow the units to operate/respond to variation in available power from the panels but certainly they are able to operate at variable output levels.