Power Budget by Operating Mode

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.

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.