Independent energy/ CALCULATOR

Storage energy return from a matched cycle

Calculate returned energy percentage and the unreturned difference from two recorded kWh totals. Not a battery capacity, health, autonomy or certified efficiency test.

Recorded energy into storage EinkWhPositive kWh crossing the documented storage-system boundary over the matched cycle. Use gross input, not net grid import, peak kW, nominal capacity or Ah.

Recorded energy returned EoutkWhkWh leaving the SAME boundary over that cycle, with comparable initial/final stored state and no uncounted source. Zero is valid as a recorded output, not a battery-health verdict.

Energy returned / energy supplied

86.00 %

Unreturned energy on the recorded boundary

0.700 kWh

Unreturned share of supplied energy

14.00 %

Ein > 0 and 0 ≤ Eout ≤ Ein. One matched cycle, comparable initial/final stored energy, same energy basis and boundary, complete gross charge/discharge records and documented auxiliary loads. No state-of-charge correction, capacity, degradation, reserve, cost or test certification. Defaults are fictional. Equal displayed SOC alone does not establish equal stored energy.

FROM ESTIMATE TO ACTION

Use the result, not just the number.

Define the energy boundary first

Use existing manufacturer monitoring or documented metering records; do not open batteries, panels or live conductors, add instruments or change dispatch settings. Identify timestamps, system, register meanings and whether measurements are at AC system ports or DC battery terminals. Both totals must cross the same defined boundary. Do not mix AC charging with DC discharging, whole-home import/export or solar energy passing directly to a load. Record whether inverter and auxiliary consumption are inside the boundary; do not add a loss again if it is already included.

Check that the record closes the storage cycle

The NREL research procedure requires matched initial/final SOC for its round-trip calculation. This page is record interpretation, not an instruction to reproduce that test. Use only a documented comparable initial/final stored state. An identical rounded BMS percentage is not proof of identical stored energy. If the state differs, Ein − Eout includes that change: 0.7 kWh difference could be 0.4 kWh left in storage plus 0.3 kWh elsewhere, not 0.7 kWh of loss. Do not invent a correction from SOC × nameplate capacity or force a deep cycle to obtain a number.

Keep operating conditions and uncertainty visible

Retain cycle dates/duration, temperatures, load/rate conditions, initial/final state evidence, boundary and meter resolution. Standby time and auxiliary demand belong in the record. A difference near the measurement uncertainty cannot be interpreted as a precise loss or a fault diagnosis. For several comparable closed cycles sum energies first. A second 10 kWh in / 8 kWh out cycle gives 80%; pooled return is (4.3 + 8) / (5 + 10) × 100 = 82%, not (86 + 80) / 2 = 83%. Keep different boundaries or conditions separate. Do not alter BMS limits, discharge reserves, protections or manufacturer test procedures; this arithmetic does not approve an installation or warranty claim.

Inputs to verify

Recorded energy into storage Ein (kWh): Positive kWh crossing the documented storage-system boundary over the matched cycle. Use gross input, not net grid import, peak kW, nominal capacity or Ah.
Recorded energy returned Eout (kWh): kWh leaving the SAME boundary over that cycle, with comparable initial/final stored state and no uncounted source. Zero is valid as a recorded output, not a battery-health verdict.

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