There might be a problem with the UPS and PSU.
There might be a problem with the UPS and PSU.
The situation you described is unusual but not necessarily alarming. It seems the PC won't start when connected to the UPS in "AC Mode," despite the display showing "Input." This could indicate a mismatch between the power supply's capabilities and the UPS's expected behavior in this specific configuration. I'm curious about how this is supposed to work, as it differs from your usual experience.
Have you explored alternative functional power cables between UPS and PSU? Or tested that particular "specific PC" in another recognized working Battery and Surge port? The goal is just to narrow down possible causes: a good cable, a suitable port, etc.
The correct UPS should function out of the box without any setup. Simply connect your power cord from the PSU to the UPS, press its power button, and that’s all. The UPS will handle everything automatically. No extra steps are needed from the user.
This applies to my CyberPower CP1300EPFCLCD (1300VA/780W, line-interactive, true/pure sine wave) UPS. I’ve been using two of these for about six years now.
You might find that what you have is a simulated sine wave UPS. It’s worth noting the specifications:
https://www.forzaups.com/products/internal/ST-1501-eng/
Specifically, a simulated sine wave UPS creates a zero output during phase changes, which can lead to power interruptions for active PFC PSUs when switching from AC to simulated sine wave in battery mode.
When using a simulated sine wave UPS with an Active PFC PSU and it switches to battery power:
1. The UPS shows an error and the PC shuts down right away.
2. The PC shuts down immediately, causing the PC to turn off.
3. The UPS then switches to battery power, but the PC continues running (stays on).
It’s important to recognize which PSUs have Active PFC—just look for certification labels like 80+ Bronze or Gold. These indicate Active PFC capability.
Keep in mind that while your PC can operate from a simulated sine wave UPS, be ready for potential problems. If issues occur, consider replacing it with a true/pure sine wave UPS.
I'm not familiar with this particular UPS model, but I understand that certain ATX power supplies generate a very strong, brief current spike from the mains when they're first connected. In some cases, the initial surge can reach up to 100A in just a few milliseconds. This rapid increase quickly charges the main bulk electrolytic capacitor inside the PSU, often pushing it to around 380V DC from a standard 230V AC supply. Unless the surge arrestors include additional components like series inductors, they may not effectively reduce this sudden current flow.
The way your UPS handles such intense initial surges depends on its design and whether you're drawing power directly from the AC mains (through the UPS) or via a battery-powered DC-to-AC inverter inside the UPS. Some users report that certain ATX PSUs can trip circuit breakers or fuses if they encounter these high initial currents.
If replacing a sensitive 16A circuit breaker is difficult, opting for a new ATX PSU with a lower maximum surge rating—such as 40A instead of 90A—might prevent the breaker from tripping. Likewise, another ATX PSU could work well with your Forza UPS. The RM850x model, for example, might not be suitable if its surge protection is too sensitive for the Forza.
I own a Corsair RM850x and an RM750x in two of my systems, both ATX 2.0, not 3.1. I can run both PCs at the same time from an APC 1500VA UPS without issues, along with a third PC using an RM650x and various network switches.
ATX 3.1 PCIe 5.1 compliant power supplies are better equipped to manage higher short-term voltage spikes on the +12V PCIe rail when high-power GPUs are in use. These may have larger bulk capacitors for extra transient support, possibly leading to more significant mains switch-on surges. This remains just an assumption based on my understanding.
Even with a 1,000W PSU, I'm unsure it fully utilizes the available power unless it contains an RTX4090/5090. In that case, it might draw less than 600W and even less than 400W.
In short, you may be encountering the large initial current spikes from many ATX PSUs when they first connect to the bulk capacitor. Standard surge arrestors likely won't fully mitigate this issue. The PSU would benefit from a higher resistance NTC thermistor.
I agree with your assessment about the PSU. I was only slightly aware of the initial surge draw, but this experience has clarified things.
The UPS connected to this PC is a solid Cyperpower model. As noted in the post, it seems to manage surges more effectively, which is why I just needed to plug and play.
I actually tried this myself using a 1000VA/700w UPS—it worked fine, but since it's from the same manufacturer, I had to switch it to "AC mode" to power on the PC, assuming it couldn't handle the surge. I ended up refunding it and opted for a larger one.
I understood my position. I use a Cyperpower UPS on another PC with the same 1000W PSU from the same model, and it works smoothly without any issues. This was what I expected, which is why I didn’t mention it earlier.
Through my own research after comparing interactive/offline/online UPS models and their behaviors, I realized but hadn’t considered the simulated wave section. Luckily, I have a one-year warranty, so I’ll have to wait and observe the outcomes.
I plan to proceed with this approach.
I would return it right away if it weren’t for the fact that this brand is common in my area and shipping a higher-quality model is quite costly.