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Energy in and efficiency
Battery & storage
Battery round trip efficiency loss calculator
Enter the energy going into your battery and its round trip efficiency rating. See exactly how much you get back out, and how much is lost.
How it works
Three steps to seeing your efficiency loss
Step 1
Know the energy going in
This is the kWh sent to the battery during charging, before any conversion loss, similar to the input side of the charge time from solar tool.
Step 2
Check your battery's rated efficiency
This figure is published on most battery spec sheets, typically 88 to 96% for modern lithium systems.
Step 3
See the real delivered energy
The gap between input and output is energy permanently lost to heat during conversion. The depth of discharge tool covers the other major usable-capacity factor.
Understanding round trip efficiency loss
What round trip efficiency actually measures
Round trip efficiency is the percentage of energy that comes back out of a battery compared to what went in during charging, capturing losses from both the charge and discharge halves of the full cycle combined, a concept the Department of Energy's inverter basics guide covers as part of how conversion equipment affects overall system performance. A 92% round trip efficiency rating means 8% of every kWh sent into the battery is lost somewhere along the way, never available for use later. This is a genuinely separate factor from depth of discharge, covered directly in the depth of discharge impact tool, which limits how much of the rated capacity can be used at all.
Where the lost energy physically goes
Round trip efficiency losses occur primarily as heat, generated during the DC to AC and AC to DC power conversions an inverter performs, a process the inverter efficiency loss calculator in our Inverters & Equipment category isolates on its own, and from internal electrical resistance within the battery cells themselves during both charging and discharging, a mechanism detailed in NREL's battery storage performance research. This heat isn't recoverable as usable electricity, it simply dissipates, which is why round trip efficiency is treated as an unavoidable loss, distinct from the coupling losses covered in the AC vs DC coupled sizing tool.
How efficiency changes as a battery ages
Round trip efficiency can decline modestly over a battery's lifespan alongside its overall capacity degradation, though the exact rate depends on the specific chemistry and how heavily the battery is cycled. A battery rated at 92% efficiency when new might perform closer to 88 to 90% after several years of regular daily cycling, which is worth factoring into long-term self-consumption calculations more than one-off backup power planning. The inverter efficiency loss calculator checks whether your inverter can handle this result.
What to do with this number next
This efficiency figure feeds directly into the usable vs nameplate capacity calculator, which combines it with depth of discharge for a complete real-world usable energy figure to use in backup and sizing decisions. For the chemistry-level efficiency comparison behind this figure, the battery round trip efficiency comparison tool in our Battery Chemistry & Storage category compares this by chemistry type.
Questions
Frequently asked
Charging a battery with 10 kWh of input energy at 92% round trip efficiency delivers back about 9.2 kWh, a loss of 0.8 kWh. At 88% efficiency, that same 10 kWh input delivers only 8.8 kWh, a loss of 1.2 kWh.
Round trip efficiency losses occur mainly as heat, generated during the DC to AC and AC to DC conversions the inverter performs, and from internal resistance within the battery cells themselves during charging and discharging. This heat is simply lost energy that doesn't come back out as usable power.
Yes, round trip efficiency can decline modestly as a battery ages, alongside overall capacity degradation, though the rate varies by chemistry and usage pattern. A battery rated at 92% new might perform closer to 88 to 90% after several years of regular cycling.
It applies equally to both, but the financial impact differs. For daily solar self-consumption, efficiency losses reduce the value captured from every cycle over the battery's life, while for backup power, the impact is smaller since efficiency losses simply reduce available energy during a single event.