After 2 days light building my power died for 2 minutes… is the light progress saved somewhere or I should start from the beginning?
You need to restart.
I seriously question why it’s taking two days though?
-make sure that you use a lightmass importance volume -> will decrease light building time
-when it’s something like an open world map, use dynamic lights 
It’s archviz without direct light =( There’s lightmass importance volume, I’ve been building the whole level on production lightning.
Is it building all levels (maps)? I need only one umap in the project not all, but I create new one from time to time as backups if I don’t like some changes on the level.
Is it actually still progressing to build, or is it just stuck at 99%? 2 days seems too much, even for a heavy archviz scene.
Hmmm… At at preview it’s done in 3-4 hours at productions continues for 2-3 days. It’s not stucking keeps working. The only thing is the models are with high polycount between 5k-30k…
Aw yeah and the light bounces are at 100 in the world settings. Maybe that’s the reason 
Light bounces dont affect build time that much. You should be careful with Indirect Lighting Quality and Static Lighting Level Scale though, and dont go beyond 2k lightmap resolution(even 2k is overkill most of the time.)
@
I’m using between 256 and 1024. Good news is after certain percentage the light building increases its pace
Ouch. 
If you can’t fix your build time issues I might suggest buying an UPS. 
Hah yeah… will get 900w one eventually, currently it’s 10%, once the processing mapping starts it goes faster, but if the power dies one more time you can watch me on the news for a psycho attacking the local electro company. 