Comparing BESS Efficiency: Metering Boundaries and Auxiliary Loads

A percentage needs a measurement boundary

Two BESS efficiency figures are comparable only when their measurement boundaries and test conditions match. Ask whether the claim describes a cell, a DC battery subsystem, a converter or the complete AC installation. A converter efficiency figure cannot by itself establish the net energy returned by a complete charge-discharge cycle.

Define round-trip efficiency

For a defined test boundary, round-trip efficiency is discharged energy divided by charging energy over the agreed cycle. An illustrative test that records 100 kWh entering and 88 kWh leaving the same boundary gives 88%. These numbers are arithmetic examples, not product ratings. Record the starting and finishing state of charge so that a change in stored energy is not mistaken for a conversion loss or gain.

Ask how auxiliary demand is counted

Cooling, heating, controls and other supporting equipment may draw energy during operation or standby. Identify where each auxiliary circuit is metered and whether its energy is already included in the reported result. If an AC meter already measures total system import and export including auxiliaries, subtracting auxiliary energy again would double count it. Require the supplier to state the treatment explicitly.

Match the duty cycle

Ask for charge and discharge power, rest periods, ambient conditions, state-of-charge limits and the equipment configuration used in the test. Compare results under the duty cycle relevant to the project. A short cycling test and a cycle with a long idle period can include different amounts of supporting energy consumption; do not infer a universal site result from one headline figure.

Use dispatch inputs carefully

When preparing a site model, keep one-way conversion assumptions separate from round-trip results. Define the operating SOC window and usable-energy boundary consistently. The SAM documentation treats conversion performance, operating SOC constraints and battery losses as model inputs. Its defaults are simulation assumptions and do not verify any product offered on this website.

Build a comparable supplier response

Ask each bidder to fill the same fields: equipment model, AC/DC boundary, auxiliary supply arrangement, power profile, duration, temperature, SOC endpoints, measured import/export and test uncertainty. Keep unspecified fields marked To be confirmed. Assess site economics using the load profile and tariff separately; efficiency alone does not establish a savings guarantee.

FAQ: Is the highest stated efficiency always the best offer?

No single percentage establishes project suitability. First make the figures comparable, then assess the required power, usable energy, operating restrictions, documented warranty and integration scope. Product-specific performance remains subject to the selected model documents and project acceptance terms.

Sources and scope

• SAM documentation: Battery Storage model inputs
• PNNL: Glacier battery technical performance assessment