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Electrical Power Systems (EPS)

This section covers power generation (solar panels), storage (batteries and BMS), conditioning and distribution (EPS and power buses).

Power Requirements and Budgets

A power budget is the bookkeeping exercise that tells you whether your spacecraft will stay alive in orbit. On one side: how much energy each subsystem draws and for how long, across every mission mode. On the other: how much your solar panels and batteries can supply across every orbit phase. If the second number isn't comfortably bigger than the first, you don't have a flyable design.

Power budgets are rarely a one-pass calculation. They evolve from coarse estimates at concept stage to detailed worst-case-orbit analyses by CDR, and they're updated continuously as components are selected, tested, and replaced.

Estimating consumption

  • Component-level draws: collect current and voltage figures for every active component from datasheets – receivers, transmitters, OBC, ADCS sensors and actuators, payload, heaters. Use worst-case (max) values, not typical.
  • Operational modes: most CubeSats have 4-6 modes (e.g. safe, nominal, comms, payload, detumble). Each mode activates a different set of components and so has a different power profile.
  • Duty cycles: many components don't run continuously. A transmitter might be active 5 minutes per orbit; a payload might pulse for 10 seconds per minute. Average power = peak power × duty cycle.
  • Orbit-average power (OAP): the time-weighted sum of consumption across all modes over a representative orbit. This is the headline number generation must beat.1

Estimating generation

  • Solar input: depends on cell efficiency, panel area, sun-incidence angle, and beta angle (which drives eclipse fraction).
  • Eclipse fraction: a typical LEO orbit spends 30-35% of its period in eclipse, but this varies seasonally and with orbit geometry. Equatorial orbits eclipse longer than high-beta-angle orbits.
  • Pointing: a tumbling spacecraft generates far less than a stable, sun-pointing one. Account for this honestly during early phases – assuming nominal pointing before you've proven your ADCS works is a classic source of negative-budget surprises.2
  • Degradation: solar cell efficiency drops over time in the LEO radiation environment. The rate varies significantly by cell technology and orbital altitude – triple-junction (TJ) cells degrade the least, silicon cells the most, with small satellites (under 10 kg) experiencing higher rates than larger ones due to lower thermal inertia and shielding.3 A common planning figure is 1-3% per year for quality cells with coverglass; size your panels for end-of-life, not beginning-of-life.4

A more detailed photovoltaic generation analysis (often called a PV budget) breaks out these factors panel-by-panel and over time, and is worth doing separately once orbit and attitude assumptions firm up.

Margins and derating

  • 30% margin at PDR is conservative; 20% at CDR is typical; 10% at flight is acceptable if the budget has been validated against test data.2
  • Battery depth-of-discharge is usually kept below 20-30% to preserve cycle life over the mission.
  • Worst-case scenarios worth checking explicitly: maximum eclipse, off-nominal attitude, transmitter active during eclipse, end-of-life cell efficiency, all heaters on at coldest case.

Coupling to other budgets

Power is just one of many connected things to consider:

  • Thermal: every watt consumed becomes heat. Heater duty cycles change with thermal environment, which changes with orbit and attitude. See Thermal.
  • ADCS: detumble and slew operations are power-hungry, and pointing accuracy directly affects solar generation. See GNC.
  • Comms: transmit power often dominates the budget during pass windows. See Comms – Link Budget.
  • Mission ops: payload duty cycles and downlink schedules need to fit within what the power system can sustain across an orbit, a day, and a season.

Templates and worked examples

For ready-to-use templates and reference analyses, see Calculators and Reference Tools.

Solar Power Generation

To be added here:

  • Body-mounted vs. deployable solar panels
  • Cell technologies and efficiencies
  • Illumination, eclipses, and incidence angles
  • Wiring, blocking diodes, and degradation over time

Solar cell datasheets

MPPT and power management

Energy Storage

To be added here:

  • Battery chemistries used in CubeSats
  • Capacity sizing and depth-of-discharge
  • Charge and discharge limitations
  • Lifetime, degradation, and safety considerations

Battery Management Systems (BMS)

To be added here:

  • Cell balancing approaches
  • Over-voltage, under-voltage, and over-current protection
  • Temperature monitoring and cutoffs
  • Interaction between BMS and EPS logic

Power Conditioning and Regulation

To be added here:

  • Maximum power point tracking (MPPT)
  • Buck, boost, and buck-boost converters
  • Efficiency vs. noise tradeoffs
  • Startup and brownout behaviour

Power Distribution and Buses

To be added here:

  • Common bus voltages (e.g. unregulated battery, 3.3 V, 5 V, 12 V)
  • Switched vs. always-on loads
  • High-side vs. low-side switching
  • Connector and harness considerations

Power Switching and Protection

To be added here:

  • Load switches and current limiters
  • Fuses, polyfuses, and electronic protection
  • Inrush current management
  • Fault containment and isolation

Inhibits and Deployment Safety

See also: Inhibits and HDRM.

EPS Monitoring and Telemetry

To be added here:

  • Voltage, current, and temperature sensing
  • Telemetry resolution and sampling rates
  • Using power telemetry for fault detection
  • Ground-based analysis and trending

EPS Integration Considerations

To be added here:

  • Coupling with thermal design
  • Interaction with flight software and modes
  • Ground testing and power simulation
  • Common integration pitfalls

👉 Please consider contributing!

  1. University of Hawaiʻi, A Guide to CubeSat Mission and Bus Design, §5.9 "Power Budget and Profiling" – walks through the OAP methodology step by step using the Artemis CubeSat Kit as a worked example. Open access. ↩

  2. Craig Clark and Ritchie Logan (Clyde Space), "Power Budgets for Mission Success", Cal Poly CubeSat Workshop, April 2011. Practical slide deck on estimating OAP, managing loads, and avoiding negative budgets. One of the most-cited introductory treatments. ↩↩

  3. Yermek Amangeldi et al., "Degradation Modeling and Telemetry-Based Analysis of Solar Cells in LEO for Nano- and Pico-Satellites", Applied Sciences, 15(16), 2025. Open access. Reports that GaAs cells degrade 4.5-7.0% over typical mission lifetimes at 300-700 km altitude, with TJ cells showing the highest radiation resistance and Si cells the most pronounced loss below 500 km. Smaller satellites (<10 kg) show higher rates than larger ones. ↩

  4. University of Hawaiʻi, A Guide to CubeSat Mission and Bus Design, §5.5 "Power Generation". Notes that ionizing radiation effects on solar cells can be mitigated by coverglass, with typical loss figures of 1-10% per year depending on cell technology and shielding. Open access. ↩