A battery does not return every megawatt-hour it takes from the grid. Some energy becomes heat inside cells, some is lost in power conversion, and some keeps the storage plant cool, monitored, and ready. Round-trip efficiency measures the energy that comes back, but the measurement boundary decides what the number actually means. That distinction matters when comparing equipment quotations or estimating trading revenue. A high cell-level efficiency is not the same promise as net electricity delivered through a project's grid meter. This guide explains the difference, works through an explicitly hypothetical dispatch, and sets out the questions a storage buyer should ask. The essential equation RTE = energy out ÷ energy in For a complete cycle with the same starting and ending stored-energy condition, 100 MWh in and 85 MWh out means 85% round-trip efficiency. It does not mean the battery has lost 15% of its permanent storage capacity. 1. Start at the meter, not the marketing headline Pacific Northwest National Laboratory's GridPIQ documentation defines round-trip efficiency as total energy output divided by total energy input, measured at the point of connection. This is a useful starting point because a storage project is a system, not just a collection of cells. A DC-to-DC measurement at the battery terminals describes energy entering and leaving the battery on its direct-current side. An AC-to-AC measurement also captures the conversion between alternating current and direct current, but its exact scope still depends on where the AC meters sit. Measuring at a power conversion cabinet can omit a transformer and cable losses that a grid-connection measurement includes. Auxiliary equipment introduces another boundary question. Cooling, pumps, controls, communications, and other supporting loads may be supplied through the main connection or a separate circuit. A contract should say whether those loads count, how they are metered, and whether standby periods are included. Otherwise, two apparently comparable percentages may describe different pieces of the plant. For energy trading, the useful target is an accounting boundary that matches electricity purchases and sales. Record any separately supplied auxiliary electricity too. The highest available specification is not necessarily wrong; it may simply answer a narrower question than the owner's financial model. 2. Follow the energy through a complete cycle Consider a deliberately simplified plant that imports 100 MWh and later exports 85 MWh. Its net round-trip efficiency is 85% if both quantities cover the same cycle and the same metering boundary. The remaining 15 MWh was consumed or dissipated within that boundary. It is not available for sale later in the same accounting cycle. The starting and ending state of charge must be comparable. If an operator starts with a full battery, imports only a little energy, and then empties it, dividing exports by those imports can produce a misleading result. The extra exported energy was already stored before the measurement period. A sound test restores the initial condition or explicitly accounts for the change in stored energy. Energy efficiency is also different from power. A 100 MW inverter rating tells you an instantaneous operating limit, not how many MWh the plant can deliver across a full discharge. Duration depends on usable energy and discharge power. Our storage duration explainer covers that separate sizing question. Metric What it answers What it does not establish Round-trip efficiency How much input energy comes back? Annual profit or available capacity Capacity retention How much storage capability remains? Losses during each cycle Availability When can the system operate? Energy conversion performance Power rating How quickly can it charge or discharge? Total energy delivered 3. Why real operation differs from a single test Losses depend on operating conditions. Current passing through electrical resistance produces heat. Power converters have losses of their own. Thermal management consumes energy to keep equipment within its operating limits. Changing charge power, discharge power, temperature, and the time spent waiting between cycles changes the balance of those losses. A simple auxiliary-load example shows why utilization matters. Suppose a hypothetical facility draws an average 0.1 MW for supporting equipment over 24 hours. That is 2.4 MWh per day. Against 100 MWh of charging energy it is a modest quantity; against 10 MWh it is much larger. This example is arithmetic, not a claim that any particular plant has that auxiliary demand. It illustrates why a lightly used system can show a different net result from the same equipment cycling frequently. Historical fleet measurements and modeling assumptions are useful reference points, but neither is a product guarantee. The U.S. Energy Information Administration reported an average monthly round-trip efficiency of 82% for the U.S. utility-scale battery fleet in 2019. EIA published that analysis in February 2021 using reported electricity consumption and gross generation. It is historical fleet evidence, not a measurement of today's newest projects. The 2024 Annual Technology Baseline uses an 85% round-trip efficiency assumption for utility-scale battery storage. That figure is a modeling input, not proof that every system achieves 85%. The difference between an observed fleet average and a standardized model is precisely why buyers should request site-specific test conditions rather than treat either number as a universal benchmark. 4. Translate efficiency into trading economics Suppose electricity costs $30 per MWh when charging and sells for $70 per MWh when discharging. With 100 MWh imported and 85 MWh exported, charging costs $3,000 and discharge revenue is $5,950. The energy-only margin is therefore $2,950. These are hypothetical prices, not a market forecast. Ignoring losses would predict $4,000 of energy margin, overstating this example by $1,050. The correct energy-purchase cost per exported MWh is $30 divided by 0.85, or about $35.29. For positive charging prices, the energy-only break-even sale price is the charging price divided by round-trip efficiency, before every other project cost. Energy margin is not project profit The example excludes capital recovery, financing, network charges, market fees, maintenance, degradation, taxes, and any additional operating costs not already included in the metering boundary. Capacity and ancillary-service revenues are also excluded. Do not subtract the same auxiliary energy twice if it is already captured in net efficiency. For a fixed 85 MWh delivery obligation, a 90%-efficient plant would need about 94.44 MWh of charging electricity, compared with 100 MWh at 85%. At $30 per MWh, the difference is about $166.67 per such cycle. This is a different comparison from holding charging energy fixed and selling more output. State which quantity is fixed before assigning a dollar value to a percentage-point improvement. Efficiency alone does not identify the best investment. Equipment cost, storage duration, lifetime, dispatch restrictions, and revenue opportunities also matter. A system used mainly for rare reliability events may have different priorities from one that trades energy every day. Model the intended service, not an idealized cycle chosen only because it makes the equipment look good. 5. Write a useful efficiency requirement An efficiency guarantee should be a reproducible measurement agreement. Ask for the electrical single-line diagram with meter locations marked, the treatment of separate auxiliary supplies, and a clear definition of imported and exported energy. Confirm whether the guarantee describes commissioning performance, a later operating year, or a specified lifetime condition. • Boundary: Battery terminals, converter terminals, or the grid connection? Are transformers and cables included? • Duty cycle: What charge and discha