Power budget by mode
Power Budget by Operating Mode
A spacecraft never draws one fixed power. Safe mode sips; a payload pass or a downlink spikes. Budget the orbit mode by mode and see whether the energy balance actually closes.
// SMAD Ch.11 energy-balance method. per-mode average draw weighted by orbit fraction, sunlight generation vs. orbit load, eclipse battery depth of discharge. orbit-mean approximation. flight design needs a duty-cycle timeline + array degradation + thermal model.
This spacecraft power budget calculator works mode by mode: each operating mode's average draw is weighted by its share of the orbit period, and the orbit-averaged load is checked two ways (the energy-balance method of Space Mission Analysis and Design (SMAD), Ch. 11) — does the solar array's sunlight-only generation cover the load over a full orbit, and does the battery's eclipse depth of discharge (DoD) stay inside the chemistry's cycling ceiling? A design passes only when both close. It is a first-order trade-study model for low-Earth-orbit (LEO) missions — flight design adds a duty-cycle timeline, array degradation, and thermal effects.
How this model works & what it omits
A spacecraft moves through a small set of discrete operating modes over an orbit — safe, nominal, payload-active, comms-downlink — and each mode draws a different average electrical power. The single-number "spacecraft power" figure hides the design risk: a payload pass or a high-rate downlink can draw three to five times the nominal load, and that peak is what the battery and array must actually survive. This tool budgets the orbit one mode at a time.
The method is the standard orbit energy-balance accounting (SMAD — Space Mission Analysis and Design, Ch. 11, Power Subsystem). Each mode contributes its average draw weighted by the share of the orbit period it occupies; summed across modes that gives the orbit-averaged load. Over one orbit the solar array generates energy only while in sunlight E_gen = P_array · (1 − f_eclipse) · T_orbit, while the load runs the entire orbit E_load = P_avg · T_orbit. The orbit energy balance is the difference: positive means the battery net-charges each orbit; negative means the state of charge ratchets down every orbit and the mission eventually browns out.
The second check is the battery. During eclipse the array produces nothing, so the battery alone carries the load. The eclipse depth of discharge (DoD) is the eclipse-period load energy as a fraction of usable battery capacity. In LEO a satellite eclipses thousands of times a year — cycling deeper than the chemistry's DoD ceiling (≈ 40% for lithium-ion, per SMAD Ch. 11 guidance) sharply shortens cell life. A design only passes when the orbit is energy-positive and the eclipse DoD stays within the allowed limit.
Either supply a circular-orbit altitude (the orbit period follows from Kepler's third law, T = 2π√(a³/μ)) or enter the orbit period directly. What this tool does not model: a time-resolved duty-cycle timeline within an orbit (mode order and clustering relative to eclipse matter for the true peak DoD), solar-array degradation over mission life, array pointing and temperature effects, battery charge-rate limits, MPPT and harness losses. It is an engineering trade-study model for first-order power sizing — not a flight-design power simulator.
// pick a mission, then tune the operating modes and power system.
Operating modes
// average power per mode + share of the orbit period. orbit fractions must sum to ~1.
Orbit
// circular orbit; period from altitude via Kepler's third law.
Solar array
// electrical generation flows only in sunlight.
Battery
// carries the load through eclipse; depth of discharge bounds cell life.
Orbit energy balance
// orbit period 94.6 min · 4 modes
// eclipse DoD exceeds the allowed limit.
34 W
Orbit-averaged load
71.8 Wh
Energy generated / orbit
53.3 Wh
Energy consumed / orbit
+18.5 Wh
Orbit energy balance
46.6%
Eclipse depth of discharge
40.0%
Allowed DoD limit
payload
Worst-case mode
FAIL
Verdict
// per-mode average power draw
// eclipse depth of discharge exceeds the allowed limit
The battery is cycled to 46.6% each eclipse against a 40.0% ceiling. In LEO that is thousands of cycles per year — deep cycling sharply shortens cell life. Add battery capacity, cut the eclipse-period load, or relax the DoD ceiling to the chemistry's real limit.
// shareable URL encodes every input. no backend.
// average power draw by mode
// ai-generated breakdown of what these numbers mean — with diagrams.
Common questions
Why does a spacecraft power budget need operating modes?
Because the single-number "spacecraft power" figure hides the peaks the hardware must survive. The default template's modes average from 15 W in safe mode to 54 W with the payload active, yet the orbit-averaged load works out to only ~34 W — and that peak-mode draw is what the battery and array must actually survive. The standard mode set — safe, nominal, payload-active, comms-downlink — weighted by orbit fraction is how SMAD's energy-balance accounting captures both the average and the peaks.
Why can a design fail with a positive energy balance?
Because passing takes both checks, and the battery check bites first. The default template is comfortably energy-positive — +18.5 Wh per orbit from its 70 W array — yet fails: through eclipse the battery alone carries the load, and its depth of discharge reaches ~47% against the ~40% lithium-ion ceiling. The fix is a bigger battery or shedding eclipse-period load, not more solar array. Two of the tool's three presets fail exactly this way.
What battery depth of discharge is allowed in LEO?
A LEO satellite eclipses on nearly every orbit — roughly 5,500 charge/discharge cycles a year at ~15 orbits a day — and cycle life falls steeply with depth. SMAD Ch. 11 guidance ceilings: about 40% for lithium-ion (the smallsat norm), ~60% for nickel-hydrogen, ~20% for nickel-cadmium. The margins are thin in practice: the imaging-smallsat preset lands at 40.8% DoD against its 40% limit — a hair over the line is still a fail.
What eclipse fraction should I use?
Around 0.35 for a typical LEO altitude (the presets use 0.30–0.36); a dawn-dusk Sun-synchronous orbit can be effectively eclipse-free. The fraction moves both checks at once — it shortens the generating part of the orbit and lengthens the battery-only part — so it is worth computing rather than guessing: get your orbit's actual eclipse duration and its variation through the year from the eclipse & beta-angle timeline tool.
How much solar array does a CubeSat power budget need?
The energy balance sets the floor: with ~35% eclipse the array must beat the orbit-averaged load by at least 1/(1 − 0.35) ≈ 1.5× just to break even, and the presets run nearer 2× for margin — the science-CubeSat preset carries a 28 W array against a 12 W average load, the imaging smallsat 90 W against ~44 W. Once the by-mode budget closes, turn it into hardware — solar-array area, battery capacity, and power-system mass — with the spacecraft power budget calculator.
References
- // Wertz, J. R., Everett, D. F., Puschell, J. J. (2011). Space Mission Engineering: The New SMAD, Ch. 11 — Power Subsystem.
- // Larson, W. J., Wertz, J. R. (1999). Space Mission Analysis and Design, 3rd ed., Ch. 11 — the eclipse / sunlight energy-balance method.
- // Patel, M. R. (2005). Spacecraft Power Systems. CRC Press — battery depth-of-discharge vs. cycle life.
- // Vallado, D. A. (2013). Fundamentals of Astrodynamics and Applications, 4th ed. — circular-orbit period (Kepler's third law).
