It depends on battery capacity and what you're running. Essential-loads backup (fridge, lights, internet) lasts much longer than running AC. We size it to your goals.
Usually yes. Most existing systems can be retrofitted with a battery; we'll confirm compatibility during the assessment.
Yes — storing daytime energy to use during expensive peak hours can lower your bill on NV Energy time-of-use plans.
Essential-loads backup often needs one; whole-home or long-duration backup may need more. We size it to your loads.