For example, battery energy is commonly related through the simple expression below. A 100Ah battery at 12.8V has 1,280Wh of nominal energy. The calculation is useful, but it does not tell you the battery's maximum discharge current, charge profile, temperature limits, or allowed series/parallel configuration.
Nominal battery energy (Wh) = battery capacity (Ah) × nominal voltage (V)
Runtime adds more assumptions. If some capacity is intentionally reserved, or power is converted through an inverter, the load may receive less energy than the nominal Wh figure suggests. That is why a transparent runtime tool should expose inputs such as usable depth of discharge and efficiency instead of silently assuming perfect conditions.
Estimated runtime (hours) = delivered usable energy (Wh) ÷ average load power (W)
These relationships are educational models, not equipment approvals. A compressor, pump, refrigerator, motor, or inverter can have startup surges and changing efficiency that a steady-load equation does not capture. This is why calculator pages pair results with explanatory content, limitations, examples, and links to related tools.
Other published tools let readers focus on more specific tasks. The LiFePO4 Battery Calculator concentrates on lithium iron phosphate planning, the Battery Charge Time Calculator explores charging duration, and the Series-Parallel Battery Calculator helps explain how voltage and amp-hour capacity change when compatible batteries are combined.