Make an electrical budget

A simple direct-current — open definition circuit joins a source and load through a complete conducting path. The source transfers energy while charges circulate; the load does not normally consume electric charge — open definition. In steady operation the rate of charge entering a component equals the rate leaving it. An open switch breaks the conducting path and stops steady current through that branch in this simple model.

Resistance — open definition relates voltage — open definition across a component to current through it. An ideal ohmic resistor follows V = IR with a constant resistance under the assumed conditions. Real electronics can behave differently, and even ordinary resistance changes with temperature. A calculator result based on a fixed resistor should not be presented as the operating law of every motor, battery or instrument.

Electrical power delivered to a load is P = VI under the stated direct-current conditions. For an ideal resistor it can also be written I²R. Energy over a constant-power interval is power multiplied by duration. A watt-hour — open definition is an energy unit: one watt delivered for one hour, or 3,600 joules. A watt and a watt-hour are not alternate names for the same quantity.

A source must cover useful loads and conversion or wiring losses. It also has limits on the instantaneous power it can deliver. A large energy inventory does not guarantee that a battery can safely supply an arbitrarily large current. For a mission plan, check both the energy available over an interval and the peak operating demand rather than allowing one total to stand in for both.

An invented probe has 120 watt-hours of usable battery energy at the start of an eclipse. Its instrument needs 24 watts and supporting electronics need 6 watts. Ignore all other loads and losses for this first model. The total is 30 watts, giving four hours before that usable inventory is exhausted. This is an arithmetic bound under explicit assumptions, not an operational reserve recommendation.

Now suppose a 90%-efficient converter supplies the full 30-watt load. The battery must provide 30/0.90, or about 33.3 watts, reducing the estimate to 3.6 hours. Multiplying 30 by 0.90 would run the energy account backward: inefficiency requires more input for the same output. A real schedule would further include startup loads, temperatures, battery aging and a mission-specific reserve.

Follow electrical input, output and loss

In an ideal parallel circuit, branches share the same terminal voltage while their currents add at the source. If two 24-volt branches draw 0.5 and 0.25 ampere, their total load is 18 watts. In an ideal series circuit, the same current passes through successive components, with voltage drops adding around the loop. These are different arrangements, not two ways to label the same wiring.

The International Space Station Solar Array Wings connect sunlight collection to a much larger power system. The photograph cannot establish the load schedule or battery state. A useful design review would request a circuit diagram, load timeline and energy account. If an estimate disagrees with telemetry, test the assumptions and measurement timing before blaming conservation of charge or energy.