Before installing solar panels I used any random UPS that had been thrown away. I'd modify it to remove the noise it makes when it's on power supply, connect the internal circuit to a 150amp truck battery instead of the miniature one that is inside. If power is out for long the internal inverter of the UPS would not be able to recharge the battery fast enough, so I'd get a car charger instead in that case. Also included a fan to avoid heat. Such setup would last something like 12h or more depending on how hungry your desktop is. As far as surge protection goes, we have it installed at the house level, not per-appliance.

Pic of what that actually looks like, connecting these two to the battery:


Not sure if that helps, but it keeps the conversation flowing.