My issue with these Mercer units is that I don't know whether they are capable of effectively recharging LiFePO4 batteries.
The voltage delivery curve when charging these two types of batteries are different, and I believe the Mercer units only have lead acid based battery charging curves "hard coded"/Wired into the unit.
This means that the inverter unit would not be delivering the right voltage to the LiFePO4 battery in order to effectively charge them, and may damage the batteries in the long run.
If you are looking for longevity, the LiFePO4 based batteries will last quite a lot longer than Lead Acid based batteries, there is a trade-off in price, though.
If it's for the duration of 2-4 hours, and using a generous consumption of 600 Watts for a gaming PC, you'd need an equivalent of ±2500Wh worth of power. (Portable solutions 1000Wh @ ±R11k, 2000Wh @±R20k)
While typing this, I noticed that you mentioned a 700Watt power supply - most modern power supplies supply power "as required" by the components demanding.
Ryzen 5 3500X - ±70Watts (Given it is not overclocked to any degree)
Monitors - No more than ±50 Watts each
Radeon RX 6700 XT - ±250Watts.
Total: ±420 ~ 450Watts.
GeeWiz has
this solution with 2560Wh LiFePO4 battery, based on a 400-600 wat consumption, this solution can last 4.2 - 6.4hrs. for R16,000.00 with a replacement battery (after 2000 cycles) costing anywhere between R7.5k-R10k - Depending on brand and quality. Given that Geewiz couples this with an inverter (RCT Brand) which I have read is quite reliable.
Comparing this to Lead acid type batteries, 12v 100Ah AGM batteries are priced at ±R2800 each, in order to provide the same endurance as the above mentioned solution, the expense on a pair of 2x (12v 100Ah) batteries is R5,600.00.
This costs ±R3000 less than LiFePO4
HOWEVER - These AGM batteries hardly ever last more than 8 months during load shedding Stages 4-6) with 2x 4-hour sessions a day. - The cycle life on Lead Acid is anywhere from 600-800 charge/discharge cycles, 8 months of loadshedding @ 4-hours each time, twice a day, equates to 480 cycles.
LiFePO4
starts at 2000 cycles guaranteed life (most manufacturers also include a 2-3year warranty), with the capacity of the battery hardly dropping below 80%. (which is still effectively 2048Wh.)
In comparison, Lead Acid based batteries' capacity drops to 50% after 600 cycles, and becomes unusable after ±75% capacity, due to the voltage supply becoming inadequate over time, whereas Lithium/LiFePO4 batteries have a much more stable voltage supply.
In addition to this, you
can jerry-rig this inverter to a circuit in your house, or insert the power solution at the front of the chain of things you want to power.
The optimal way to do this, is to incorporate a reserve power circuit breaker and plug into your distribution board, and leave the unit standing nearby your DB Board. - this way, the house will have uninterrupted power, as long as you don't use the Oven, Stove, Geyser, etc.
TL;DR: Buy LiFePO4 batteries, the Lead Acid stuff sucks and needs to be replaced every 10 months (that's being generous).
