Make the numbers describe the operating mode

A mass budget sums the relevant components, consumables and allowances for a defined configuration — open definition. A power budget describes electrical demand and supply in particular operating conditions. An instrument may be installed throughout the mission but switched on only during selected modes. Adding every peak power as though it occurs simultaneously may be too conservative; omitting a real coincidence may be dangerously optimistic.

Define the configuration and mode before doing arithmetic. A launch mass, a dry mass and a mass after consuming propellant refer to different states. Likewise, average power, peak power and energy over an interval are distinct quantities.

In a fictional 120-kilogram payload allocation, the current design totals 96 kilograms and an explicitly protected reserve — open definition is 18 kilograms. Six kilograms remain unallocated. Calling the full 24-kilogram difference “free” would spend the reserve as though it had no purpose.

A reserve does not guarantee that uncertainty will stay within it. It makes the allowance visible so design changes can be judged honestly. The appropriate allocation depends on project knowledge and rules; these teaching numbers are not a recommended flight margin.

For a fictional observation mode, a 40-watt instrument operates for half an hour. It uses 20 watt-hours. A 10-watt heater running for the full hour adds 10 watt-hours, giving 30 watt-hours for those two loads. Conversion losses and other systems must be added if they are not already included.

A battery with that nominal stored energy does not necessarily supply all of it to the loads. Usable capacity, efficiency, temperature and permitted operating range matter. A budget must identify which side of a converter a number describes and which losses have been counted.

Keep allocated reserve visible

A design may fit the average power budget but fail during a simultaneous heater, transmitter and actuator demand. Evaluate the modes and transitions that create peaks. Compare supply capability, stored energy and the duration of the event rather than relying on one mission-wide average.

When a new instrument is proposed, follow its indirect costs: mounting, thermal control, cables, computation, storage and downlink. A payload’s listed mass or power rarely captures every system consequence. A complete budget makes those dependencies visible before the mission reaches testing.