Why Round-Trip Efficiency matters for your battery selection

Battery efficiency has a direct impact on operating costs

When evaluating marine battery systems, factors such as initial cost, energy capacity, safety, and cycle life typically receive the most attention. Yet another parameter can have a substantial impact on the total cost of ownership: Round-Trip Efficiency (RTE).

RTE measures how much of the electricity used to charge a battery can be recovered during discharge. Every battery loses some energy during charging and discharging, primarily as heat. The higher these losses, the more electricity must be purchased to deliver the same amount of usable energy.

For vessels operating frequent charging cycles, these losses accumulate over time, making efficiency an important factor in long-term operating costs.

What is Round-Trip Efficiency?

Round-Trip Efficiency is the ratio between the energy delivered by a battery during discharge and the electricity consumed during charging.

For example, if a system uses 100 kWh to charge but delivers only 95 kWh, its RTE is 95%. The remaining 5 kWh has been lost, mainly as heat.

Depending on the chemistry, system characteristics, and operating conditions, this figure can range from around 90% to 98%. An eight-percentage-point difference may not sound dramatic. But when multiplied by thousands of charging cycles over a vessel's operating life, the financial impact can become substantial.

Battery chemistry makes a difference

A second case involves a large tanker with three 1.4 MW auxiliary generators and an average electrical demand of 0.6 MW. An 830 kWh Echandia battery system cycling between charging and discharging allows the generator to run at 80–90% load while charging. It also allows the generator to reduce load or shut down while the battery supplies demand. This gives fuel savings of about 8% and a return on investment of less than three years. Under the stated cycling regime, it also gives an estimated 25-year battery life.

Why lifetime efficiency matters

RTE changes throughout a battery's service life. As batteries age, their internal resistance generally increases, leading to greater energy losses. The extent of this change depends on the battery chemistry and the vessel's operating profile.

This makes it important to look beyond the efficiency figures typically quoted by battery suppliers. Values obtained under laboratory conditions may not reflect actual onboard performance, where charging patterns and power demands differ between applications.

A reliable comparison therefore requires project-specific RTE data at both beginning of life and end of life. Assessing efficiency over the full operating lifetime provides a more accurate basis for evaluating long-term energy costs and comparing battery systems.

In our white paper, we examine the financial implications through practical calculations, compare the performance of different battery chemistries and outline the key considerations for procurement.