Sizing a UPS and diagnosing a shared-folder bottleneck
A school computer laboratory has 15 learner PCs rated 180 W each, one server rated 350 W, a 24-port switch rated 40 W, a router rated 20 W and a projector rated 250 W used only during lessons. All devices plug into one 24-port switch connected to the server. When two classes (30 learners) open the same video folder, transfer speed collapses. Power outages are frequent; the generator takes 45 seconds to start, and two PCs have already suffered corrupted files. The school has KSh 120,000 for power backup. Quotes: 5 kVA online UPS at KSh 150,000; 3 kVA online UPS at KSh 85,000; 1.5 kVA line-interactive UPS at KSh 35,000. The business manager wants every device protected, but the ICT teacher argues for protecting only critical loads. Assume a power factor of 0.8 and add 20% headroom.
1. Calculate the total connected load, the critical load, and the UPS capacity in VA needed for each option.
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Total connected load: 15 PCs × 180 W = 2,700 W; plus server 350 W, switch 40 W, router 20 W and projector 250 W. Total = 3,360 W. With 20% headroom: 3,360 × 1.2 = 4,032 W. At power factor 0.8, required VA = 4,032 ÷ 0.8 = 5,040 VA, so a 5 kVA UPS would be needed.
Critical load: the server, switch, router and a limited number of PCs needed to save work and shut down safely. If 6 PCs are protected: 6 × 180 = 1,080 W; plus 350 + 40 + 20 = 1,490 W. With 20% headroom: 1,490 × 1.2 = 1,788 W. Required VA = 1,788 ÷ 0.8 = 2,235 VA, so a 3 kVA UPS is adequate. That leaves the projector and the other 9 PCs on raw mains; the generator bridge time of 45 seconds is the key constraint, not long runtime.
The 1.5 kVA unit would support only about 1,200 W at 0.8 power factor, which is below even the server-plus-6-PC critical load once headroom is added. It is therefore not suitable for the stated protection goal.
2. Recommend which UPS to buy and which devices to connect, within the KSh 120,000 budget.
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Buy the 3 kVA online UPS at KSh 85,000 and connect the server, switch, router and six learner PCs. This leaves KSh 35,000 for network improvements and installation.
The 5 kVA unit exceeds the budget by KSh 30,000 and would protect loads that the generator can restore within 45 seconds. The projector is not critical: a lesson can pause while the generator starts. The other nine PCs can also ride through the outage if learners save regularly, but the server and network equipment must stay up so that files are not corrupted. An online UPS is preferable to a line-interactive unit here because it continuously conditions power and gives zero transfer time; the 1.5 kVA line-interactive unit cannot carry the critical load.
Operational rules matter as much as the purchase: label the protected sockets, disable automatic Windows updates during lesson hours, and configure the server to shut down gracefully after a set time. The UPS should be tested monthly under load. If the budget must cover installation, a 3 kVA unit plus a small maintenance bypass switch is still feasible within KSh 120,000.
3. Explain why the shared video folder slows down when 30 learners access it, and propose a low-cost improvement using the remaining budget.
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The symptom is contention at the server–switch link, not a shortage of internet bandwidth. All 30 learners share one uplink to the server. If that link is 100 Mbps, the video stream is divided among many clients; the switch backplane may be adequate, but the single server network interface and the shared folder service become the bottleneck. Opening the same file simultaneously also causes many repeated reads from the same disk rather than one efficient transfer.
A low-cost fix is to install a gigabit switch and, if the server has a spare slot, a gigabit network card, so the server–switch path is no longer the choke point. A 24-port gigabit switch and Cat 6 patch leads can fit within the KSh 35,000 remaining. If the server cannot be upgraded, copy the video file to a local PC before the lesson or use a scheduled offline copy to each lab machine. Segmenting the lab into two switches does not by itself add bandwidth unless each switch has its own uplink; simply creating VLANs would not solve the problem. The judgement is to spend on the uplink and server interface first, then on client segmentation only if contention persists.