Budgeting rack power for a pantry, not a data centre
The rack is rated for far more than the room is. Every capacity conversation in a flat is really a conversation about one breaker and one extension lead.
Start from the breaker, not the kit
The pantry shares a 10 A circuit with the hallway and, it turns out, the outlet the kettle lives on. Nominally that is a bit over two kilowatts. In practice the number worth planning against is much smaller, because the circuit has to survive the worst simultaneous moment, not the average one.
| Item | Steady | Worst case |
|---|---|---|
| Storage node, four disks | 96 W | 210 W at spin-up |
| Compute node | 145 W | 240 W under build load |
| ARM sensor board | 6 W | 9 W |
| Switch and router | 34 W | 41 W |
| UPS overhead and charging | 55 W | 190 W recharging after an outage |
| Total | 336 W | 690 W |
The three numbers that actually bite
Spin-up, not run
Four disks starting together draw roughly double their running figure for a few seconds. Staggered spin-up costs about eleven seconds of extra boot time and removes the spike entirely, which is the cheapest improvement in this whole note.
Recharge after an outage
The moment power returns is the worst moment on the circuit: everything restarts at once and the UPS begins charging hard at the same time. If the budget only works while the battery is full, it does not work.
The kettle
Two kilowatts, on the same circuit, entirely outside my control. This is not a joke line. It is why the steady figure sits near a third of the circuit rather than two thirds.
What the UPS actually buys
At 336 W the line-interactive unit gives roughly nine to eleven minutes, which sounds thin until you look at what it is for. It is not there to keep the lab running through an outage. It is there to ride out the two-second dips that happen a few times a winter, and to give the storage node time to shut down cleanly on anything longer.
on battery, 30 s -> ignore, log it
on battery, 120 s -> stop non-essential services
on battery, 240 s -> ordered shutdown, storage last
battery below 40% -> ordered shutdown regardless of runtimeThe last line exists because a battery two summers past its date reports runtime with more optimism than it can deliver.
Where the headroom went
The original plan had a third node in the budget. It did not survive contact with the recharge case: adding 140 W steady pushed the worst-case simultaneous moment past what I am willing to put on a shared domestic circuit. It waits for a dedicated line, or it does not happen.
In a flat, the constraint is never the rack. It is the wall.