Electrical power system (EPS)

Definition#

The electrical power system (EPS) generates, stores, regulates and distributes the energy of the spacecraft. In small satellites it almost always follows the same pattern: solar cells generate in sunlight, a rechargeable battery carries the loads through eclipse, and converters and load switches condition and distribute the power. According to NASA’s 2026 technology report, more than 90% of nanosatellites and small satellites use solar arrays with rechargeable batteries[1].

The energy source#

At 1 au a surface facing the Sun receives about 1361 W/m²[2]. How much of it becomes electricity depends on the cell efficiency, on the fraction of the surface the cells cover and on the angle of each face to the Sun. Multi-junction III-V cells, the standard choice in small satellites, have nominal efficiencies around 30%; single-junction silicon cells stay near 20%[1].

An uncontrolled spacecraft shows its faces to the Sun in turn and at oblique angles, so the orbit-average power of body-mounted cells is far below what the cell area suggests. For a randomly tumbling convex body the mean projected area is a quarter of the total surface (Cauchy’s theorem), which gives Eq. 1 of the PocketQube review[3]:

with solar constant $S$, cell efficiency $\eta$, packing factor $p$ and eclipse fraction $f_{\mathrm{e}}$.

Architecture#

An EPS performs four functions: generation with power-point tracking, storage, distribution and launch safety. Power from the arrays passes through a maximum-power-point tracker (MPPT) or a direct-energy-transfer stage onto the battery bus. Regulators fed from that bus create the fixed voltage rails, and current-limited load switches let the computer turn each consumer on and off and isolate it after a short circuit.

flowchart TB
  subgraph GEN["Generation and tracking"]
    direction LR
    SA["Solar arrays"] --> MPPT["MPPT converter"]
  end
  subgraph STO["Storage"]
    direction LR
    BCR["Charge control, cell balancing"] --> BAT["Li-ion battery and heater"]
  end
  subgraph DIST["Distribution"]
    direction LR
    REG["5 V and 3.3 V regulators"] --> LS["Current-limited load switches"]
    LS --> LOADS["OBC, COMMS, ADCS, payload"]
  end
  MPPT --> BCR
  BAT --> INH["Separation switches and RBF pin"]
  INH --> REG