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Either it is desktop or mini PC which we have to connect to UPS or laptop with display, keyboard, mouse pointer, battery
Many inverters (the pure sine wave ones) has UPS mode, and PC won't shutdown during power cut. I am using one for about a decade now.
Do you know, you can just connect a laptop to a 24" monitor, get an external KB & mouse. Moreover the laptop's screen can be used as a secondary display. I used my laptop like that for about 2 years before moving to a desktop.
Battery capacity Ah= (Power requirement in VA * Back up duration in hour) / Battery Voltage (in volts)
Because of mass manufacturing, it is very cheap. Laptops will always have a higher demand than mini PC with battery. So there's no way a mini PC with battery will cost much cheaper than a laptop, even though it lacks a display, KB & trackpad. Just check prices of mini PCs right now, usually it is a bit more expensive than a laptop of same specs for say under 50k category. Check Intel NUC. I can see one with i5 1135G7 for 32k, this is without RAM or SSD. So 8GB RAM + 512GB SSD is another 5k, still without display, so another 6k for a very cheap 22" one, so total is 43k. You can get a R5 5500U laptop for under 40k, right one see a cheap Acer for 34k & R5 5500U is faster than that i5 as well.
For 8hr power cut, you may need about 100AH battery. Yes, without monitor. Even a 50AH battery would weigh about 20kg. The typical UPS we use for desktop has inverter+charger+battery inside. If you put all that inside the mini PC, it would become a mega PC. There are ton loads of DIY battery pack tutorial for laptops. I guess it's possible for desktop as well. But how much that would cost?
It's not his requirement. He is just questioning or wondering why they aren't making one.
We don't have the technology to build a miniaturized arc-reactor. Once we have that, say bye-bye to battery woes.
Define "regular":
I mean, you can connect a battery to a PSU, that is what a UPS is.