From my repairs, if budget allows, I would go with an online low frequency pure sine online UPS.
Generally, cheap and Chinese branded PS online inverter/UPS are high frequency (multi stage SMPS style), they have exponentially poor <0.9 PF capacity, rely on MOSFET tech due to the high frequency, they cannot handle load surge well such as plugging in a 800W+ PSU that's been sitting for hours off due to the internal post rectifier capacitor, shortening it's service life, but that usually does not happen in UPS scenario as it's already on, they're specifically designed for electronics. Low frequency rely on a (single stage) transformer, they're generally bigger and heavier due to the larger transformer (lower the frequency in TFs, the larger). They're more suitable for home circuit installations, inductive loads such as a garage door motor, and no noise for sensitive audio equipment. They last much longer, and due to low switching speeds on the primary, they use pricey IGBT modules that may last a lifetime, they can handle shorts, even huge back-EMF from near welding tools. However they do suffer from high idle consumption, worse than offline as it's on all the time. Expect around 40-70 watt idle no load for a 3000VA unit on the input, same when using the batteries.
Protection wise, a good UPS is a power conditioner, it does not protect your devices from current surge or voltage (from lightning). The MOVs on them can be considered a type 3 (last stage) surge arrestor.
High frequency UPS/inverter are incredibly useful for dynamic loads, such as having TV and a router during blackout as load or both off. While the TV is off and the router is on, the UPS/inverter will consume the same amount of power needed from the batts to load in this
rough example: Load = 15 watt, High freq battery side = 16 watt. Low freq pure sine battery = ~30 watt. Low freq square wave battery = ~70 watt.
While a 100 watt TV is on: ~120, ~125, ~140 respectively. With <0.85 PF, the low freq pure sine wins them all but not by a huge margin.
Due to lower idle, fan will rarely run.
Battery date is sometimes more important than quality or condition. A recent used battery with 100 cycles less than 6 months manufacturing date may perform better than a new 1 year old battery. Cheap small batteries have no catalyst for hydrogen/oxygen, unlike cars (non calcium) VRLA.
For a "sane" or limited budget, cheap high frequency PS online UPS and high quality square/MSW offline/line-interactive w/AVR UPS are the way to go, the latter lasting much longer and is able to handle overload/short but nosier due to relays, both offer the same protection. Cheap square wave offline should be avoided, especially the ones with AVR.
Generally the high end specifies VA at 0.85 PF, while the low end specs at 1 PF. E.g if both are 100VA, the latter maxes out at 100W with a heating element (resistive = PF 1), while the former leaves 25W. Bear in mind that their 0.85 spec also follows the efficiency curve that is usually specified, going lower not only reduces the total reactive power, load from battery/mains will exponentially increase below that point. Low spec UPS do not mention that and their PF mentioning is only for liability purposes with unknown performance or max load under PF 1.
Square wave low freq inverters handle bit less capacity at poor PF compared to pure sine, but a bit better than high freq PS.
Almost all SMPS and general electronics have a PF of >0.85, including dirt cheap constant voltage LED drivers. (commonly used in retrofitting halogen fixtures)
Efficiency is generally the same with low frequency square/MSW wave inverters from high-end to dirt cheap, they're mostly efficient at a certain power gap, higher end having a wide gap such as 40% to 80% of total load due to having an MCU (decreases RMS frequency in MSW mode on certain load, cheap UPS don't even have a MCU or MSW mode, they use a pulse generator circuit/IC). This gap eliminates core loss load, you should never under/oversize a square/MSW UPS. E.g 20% or 40% load will have the same backup time, the latter only producing less heat from the UPS.
Most of these cheap offline interactive UPS out there offers fake capacities and miss the following:
-No frequency clock skewing, this is used to synchronize to the line peak timing after the line is back. Not having this is basically the same as flipping the switch on then off quickly at around 80-200ms, I've heard that TV backlight drivers getting hit by this as some TVs have a dedicated limited PS for backlight for increased efficiency. (A++). Some cheap non-MCU UPS wait for the right moment to sync, they may stay in backup/inverter mode with a flaky generator (e.g swaying 50 to 50.5 hertz every second)
-No modified sine wave, this is useful for active PFC PSUs over certain voltage, usually around 210v for 230v base spec. High end UPS usually switch to 190v using the internal AVR before switching back to square wave at ~50% battery due to batt voltage drop, square wave is actually very efficient for SMPS psu compared to MSW.
-High "battery/backup-mode no load" power consumption due to imperfect core ratio in the transformer, this shortens the "efficiency gap".
-Mismatched weekly manufacturing date of the FETs, causing untimed back EMF and poor cold starts due to manufacturing tolerances, Eaton and APC are known to install weekly equals. Very important for handling overloads and main line load surges.
-Hysteresis only AVR switching and inverter mode, no dormancy (2 factor hysteresis). Expect tons of relay clicking when the "Ishtirak" voltage sways all the time at around 10V +/-/
-Linear low ceiling voltage trickle charging, not being able to charge >80%, single stage and very slow, unlike cheap online PS, they already offer a decent way to charge batteries using one of the conversion stages, most have 3 stages, preserves battery service life when charging from low voltage state (e.g around 10v per 12v battery not lower, lower = already heavily worn)
-Time based backup function, no thermal sensing/load. Using the UPS at 80% load or 20% will always shutdown after 7 minutes for example, commonly found in fanless models.
Whenever they mention "Generator friendly" = microcontroller insideā¢, and freq skewing/sync is there, usually a good UPS.
It's a safer bet to go with a cheap online UPS when unsure, although they offer very low quality components and may die early, they do less mistakes on the output. Bear in mind that they don't really offer *pure* sine wave unless PF is 1.0 and less than half the max load rating, else its a smooth jigsaw..
Wave pics, some of these models are sold in Lebanese market:
https://www.hardwareinsights.com/database-of-ups-output-waveforms/5/
TL;DR: Go with brand names and whatever fits your bill for set and forget experience, oversizing =/= longer backup time, aim for chinese no brand online if ambient noise is nil, no relays, and fans rarely run. Brand name online are incredibly overpriced here unfortunately. New mid range APC models are terrible after Schneider bought the company, models affected 2012 and up. MGE is APC.
*PS: pure sine wave