Power Budget
Overview
A power budget balances generation (solar arrays, RTGs), storage (batteries), and loads (platform + payload) over all mission modes and eclipses. Best practice keeps at least 20 % contingency on both array and battery capacity through end-of-life (EOL).
Budget Table Template
| Mode | Array In (W) | Battery Δ (Wh) | Loads (W) | Margin (%) |
|---|---|---|---|---|
| Cruise-Sunlit | 300 | +120 (Charge) | 240 | 20 |
| Cruise-Eclipse | 0 | −110 | 110 | 10 |
| Science (peak) | 320 | +15 | 305 | 5 |
| Safe-Mode | 150 | ±0 | 120 | 20 |
Generation & Degradation
- Solar array degradation: 2–3 % per year (LEO) due to radiation and thermal cycling.
- RTG decay: ≈ 4 W yr-1 for Voyager’s Multi-Hundred-Watt RTGs; load shedding maintains essential science.
Storage Sizing
Battery capacity is sized for the worst-case eclipse (LEO) or safe-mode coast.
with Depth-of-Discharge (DoD) ≤ 30 % for long-life Li-ion.
Case Studies
Voyager RTG Management (good)
Gradual power decline is mitigated by selectively powering off heaters and instruments, extending mission life beyond 47 years.
Philae Lander (power-limited)
Philae came to rest in deep shadow on Comet 67P, receiving ≈ 1.5 h of sunlight per 12 h rotation – insufficient to recharge, leading to hibernation after ~60 h of operations.[1^]
DANDE Nano-sat (margin example)
Mission study showed that 40 % array margin was needed to stay power-positive with spin-stabilised body-mounted panels.
Design Standards & Guidelines
- ECSS-E-ST-20-20 — Power subsystems requirements.
- NASA “Nano-Satellite EPS” guide – margin and array sizing examples.
References
[1^]: NASA “Radioisotope Power FAQ,” 2024.
[2^]: BBC News, Rosetta: Battery will limit life of Philae, 2014.
[3^]: NASA Nano-Satellite EPS Lecture Notes, 2024.
[4^]: CubeSat Project Handbook (power contingency examples).
[5^]: ESA CGS “Spacecraft Power Budget” slide, 2014.