Quick question for the group - working with an Entergy customer here in New Orleans who has 8 rooftop units on a retail building. Currently they all start up together after overnight setback and it's creating a 180 kW demand spike. Normal operation is around 95 kW. What's the optimal staging interval for rooftop units to minimize inrush current overlap? I'm thinking 2-3 minutes between starts but wanted to get some input from folks who've done this before.
Entergy demand charges - rooftop unit staging question
Juan, I typically recommend 3-5 minute intervals for rooftop staging, depending on unit size. The compressor inrush current usually lasts about 30-60 seconds, so you want to make sure one unit is fully started before the next one kicks on. With 8 units, you're looking at a 25-30 minute startup sequence which might be too long for occupant comfort. Consider staging them in pairs - start 2 units, wait 5 minutes, start the next 2, etc.
We do a lot of this with OG&E customers in Oklahoma City. The key is balancing demand reduction with comfort. Greg's suggestion about staging in pairs is good, but I'd also look at the building's thermal mass and occupancy schedule. If people don't arrive until 8 AM, you can start the staging process at 6:30 AM and have all units running by 7:30 AM. What's the building's occupancy pattern like?
Thanks Greg and Susan. The building opens at 9 AM so we have some flexibility with startup timing. I like the paired staging approach - that should cut the demand spike in half while still getting decent comfort levels. The units are 7.5 ton package units, nothing huge, but 8 of them starting together is definitely hitting Entergy's demand meter hard. I'll propose a 6:30 AM start with 2 units every 4 minutes.