1. The desert planet rule

In Frank Herbert’s Dune, every drop of water and every watt of effort is accounted for because the planet offers no margin. The desktop is not Arrakis, but power budgeting works the same way: add up every load, divide by the duty cycle, and remember that the sun is only available part of the time.

Entry 237 defined the power attachment’s three jobs. This entry estimates the total demand and the solar array area required to meet it.

2. The load stack

The desktop’s continuous and average loads add up roughly as follows:

Subsystem Average power
Platform bus: ADCS, GPS, command, telemetry 25–35 W
Compute attachment 40–80 W
Storage attachment 10–25 W
Communications attachment 15–40 W
Sensor attachment 5–10 W
Thermal control: heaters, pumps, valves 20–40 W
Robotics attachment 5–15 W average, higher intermittent

Total average demand: roughly 120–245 W. A practical planning number for the first desktop is about 200 W average.

This is an average over an orbit. Peak demand, when imaging, downlinking, and processing coincide, can be 400–600 W. The battery and power distribution must handle those peaks even if the solar array is not sized for continuous peak operation.

3. From average demand to array peak power

In LEO, a spacecraft spends roughly 35 minutes of each 90-minute orbit in eclipse. The arrays produce nothing during eclipse, so they must produce enough during the 55 sunlit minutes to both power the loads and recharge the batteries.

Accounting for eclipse, cosine losses from imperfect sun pointing, and end-of-life degradation, the peak array power must be roughly 2.5 to 3.5 times the average demand.

For a 200 W average demand:

  • Required peak array power: 500–700 W.

This assumes the arrays are body-mounted or have modest pointing, and that the mission life is long enough for radiation degradation to matter.

4. From peak power to area

At Earth’s distance from the sun, the solar flux is about 1,361 W/m². Multijunction GaAs cells begin life at roughly 30% efficiency and degrade to about 22% over a multi-year LEO mission. Packing factor, wiring, and diode losses reduce the usable output by another 5–10%.

A conservative usable output is therefore about 250–280 W/m² at beginning of life and 180–220 W/m² at end of life.

For 600 W peak power:

  • Beginning-of-life area: roughly 2.1–2.4 m².
  • End-of-life area: roughly 2.7–3.3 m².

The desktop should be designed with about 3 m² of solar array to have margin for degradation, unexpected loads, and seasonal variation.

5. Body-mounted versus deployable

Three square metres of body-mounted solar cells is a large surface. If the desktop body is roughly 1 m × 1 m in cross-section, only one face can point sunward at a time, giving at most 1 m² of effective body-mounted area. The rest must come from deployable wings.

A plausible configuration:

  • 0.5–1.0 m² of body-mounted cells on the sun-facing side and wrapped edges, for redundancy and for power during deployment failure.
  • 2.0–2.5 m² on one or two deployable wings that track the sun.

Deployable wings add mechanism and risk, but they are the only practical way to get 3 m² of array on a compact desktop. A single wing of 2 m × 1 m, folded against the body for launch, is a common smallsat solution.

6. Battery implication

With 200 W average demand and 35 minutes of eclipse, the batteries must supply about 120 Wh per eclipse just to keep the lights on. Adding a margin for peak loads, battery inefficiency, and depth-of-discharge limits, the battery capacity should be roughly 250–400 Wh.

A 28 V battery at 10 Ah is 280 Wh. That is a reasonable starting point. Larger batteries add mass and thermal management burden but give the desktop more operational flexibility.

7. Sensitivity

The power budget is sensitive to three things:

  • Compute power. If the desktop runs heavy AI inference on orbit, the average demand could double.
  • Communications duty cycle. High-rate Ka-band downlink is efficient in bits per joule but draws tens of watts when active.
  • Robotics activity. Robotic operations are intermittent but can spike to hundreds of watts during motion.

These are not arguments for oversizing the array to infinity. They are arguments for load scheduling and for designing the power attachment with headroom and telemetry.

What this changes

  • The desktop’s average power demand is estimated at roughly 200 W.
  • The solar array must provide 500–700 W peak power, requiring about 3 m² of area at end of life.
  • A mix of body-mounted and deployable-wing arrays is the practical way to fit that area on the desktop.
  • The battery capacity should be roughly 250–400 Wh to survive eclipse with margin.
  • The next entry can compare solar cell technologies and decide which technology the desktop uses.