1. The elevator and the watt

Arthur C. Clarke’s The Fountains of Paradise is about building a space elevator, and much of the drama is not the carbon fiber but the energy budget: how much power the climbers draw, how fast they can ascend, how the structure pays for every joule. The desktop is not an elevator, but its compute attachment is a climber of sorts. It pulls power up from the solar arrays and dumps heat down to the radiators.

Entry 219 decided the first compute attachment is for Earth observation processing. This entry estimates how much power that costs.

2. Building blocks

A compute attachment built from commercial system-on-modules might contain:

  • One or more processing modules. A modern ARM or x86 SOM draws roughly 5 W at idle and 15 W under sustained image-processing load.
  • An accelerator for dense operations. A small GPU or neural-network accelerator can draw 10–30 W when active.
  • Redundant solid-state storage. Each module draws 3–7 W depending on capacity and activity.
  • Network interfaces. Ethernet or SpaceWire adapters add 2–5 W per link.
  • Power conversion and management overhead. Budget 10–20 % on top of the payload for DC-DC losses.

A single board with processor, accelerator, storage, and networking is therefore in the 25–50 W range when working hard.

3. The Earth observation workload

A reference Earth observation processing job takes raw imagery from one or two sensors and produces cloud-free composites, change maps, or feature masks. The steady-state load is moderate: the computer is busy when imagery is being acquired and mostly idle afterward.

For one sensor stream, two processing boards plus storage and networking give redundancy and enough throughput. For two independent sensor streams, three or four boards are safer.

4. First attachment power estimate

A conservative estimate for the first compute attachment is 100–150 W continuous, with peaks near 200 W when multiple pipelines run simultaneously. This assumes two to four processing boards, redundant storage, and active networking.

This is a back-of-the-envelope number. The real value depends on the exact boards, the duty cycle, and whether the accelerator is shared across pipelines.

5. Against the desktop budget

Entry 187 sized the desktop power budget in the kilowatt range. A single compute attachment at 100–150 W is a small fraction of the total. That is good. It means the first compute attachment is not the dominant power consumer; the solar arrays, batteries, and thermal system are.

It also means there is headroom for growth. A desktop with several compute attachments, storage arrays, and customer payloads could still fit within the overall budget.

6. Thermal consequence

Every watt drawn becomes a watt of heat. A 150 W attachment must conduct that heat through the cell wall into the desktop’s radiator loop. This is manageable if the thermal interface is designed into the cell from the start, but it is not negligible.

The lesson is that compute power and thermal design are the same conversation.

What this changes

  • The first compute attachment is estimated at 100–150 W continuous, peaking near 200 W.
  • This is a small fraction of the desktop’s total power budget, leaving headroom for other attachments.
  • Thermal design must account for the attachment’s heat from the beginning.
  • With workload and power known, the next entry can look at the robotic replacement procedure or the software architecture.