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Battery & storage
Usable vs nameplate capacity calculator
Enter your battery's nameplate rating, depth of discharge, and round trip efficiency. See the real usable energy number, combined into one figure.
How it works
Three steps to your real usable number
Step 1
Start with nameplate capacity
This is the marketed, headline figure printed on every spec sheet and box, the same starting point used in the depth of discharge tool.
Step 2
Apply both loss factors together
Depth of discharge and round trip efficiency multiply, not add. This tool combines them into one result automatically.
Step 3
Use the usable figure for all sizing
Every backup planning decision should use this number, not the nameplate rating. The battery storage size calculator builds this in automatically.
Understanding usable vs nameplate capacity
Why nameplate capacity is a marketing figure, not a planning figure
Every battery is sold and marketed by its nameplate rating, the theoretical maximum capacity under ideal conditions. Real-world usable capacity is always somewhat lower once depth of discharge limits and round trip efficiency losses are applied, typically by 10 to 20% combined for modern lithium batteries, a gap the Department of Energy's inverter basics guide notes applies to conversion equipment generally, not just batteries. Using the nameplate figure directly for backup planning will overstate how much energy is actually available, which is why every sizing tool on this site works from usable capacity instead.
Why the two loss factors multiply, not add
Depth of discharge and round trip efficiency each apply to what remains after the other factor has already reduced the total, which is how sequential percentage reductions combine mathematically. A 10% loss from depth of discharge followed by an 8% loss from efficiency removes about 17.2% of the total, not a simple 18%, since the second loss applies to an already-reduced amount. This distinction matters more as loss percentages grow larger, consistent with the compounding-loss math described in NREL's battery performance research.
Why this gap varies by battery brand and chemistry
Newer lithium iron phosphate battery chemistry generally supports higher depth of discharge and round trip efficiency than older or budget lithium-ion alternatives, meaning the usable-to-nameplate gap isn't a fixed industry standard. Checking the specific published specifications for the battery you're considering, rather than assuming a generic figure, gives a more accurate usable capacity number for your actual system. The cost per kWh comparison tool is worth checking alongside this, since usable capacity affects the real cost per usable kWh, not just nameplate kWh, and the inverter efficiency loss calculator in our Inverters & Equipment category covers the parallel gap on the inverter side.
What to do with this number next
Take your usable capacity figure directly into the battery storage size calculator or backup runtime estimator, both of which are built around this same usable-first approach to sizing. Usable capacity also depends on chemistry-specific degradation over time; the degradation-adjusted capacity over time calculator in our Battery Chemistry & Storage category models that directly.
Questions
Frequently asked
A 13.5 kWh battery at 90% depth of discharge and 92% round trip efficiency delivers about 11.2 kWh of usable energy, roughly 17% less than the nameplate figure. Both factors combine multiplicatively, not by simple addition.
Each factor applies to what's left after the other has already been applied, which is how multiplication works for sequential percentage reductions. A 10% loss followed by an 8% loss removes about 17.2% total, not 18%, since the second loss applies to an already-reduced amount.
No, it varies by manufacturer and battery chemistry. Newer lithium iron phosphate batteries often support higher depth of discharge and efficiency than older or budget models, so checking the specific published specs for your battery is more accurate than assuming a standard figure.
Always size based on usable capacity, since that's the energy actually available to run your loads. Sizing based on nameplate capacity alone will leave you short of your actual backup target once real-world losses are applied.