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In an idealized calculation, 5 MWh of usable energy would supply a constant 50 MW load for 6 minutes: 5 MWh ÷ 50 MW = 0.1 hour, and 0.1 hour × 60 = 6 minutes. That is an energy-balance example, not a guarantee about a real battery. The system must also be able to deliver 50 MW, and the 5 MWh must be usable energy available to the load.
How the six-minute calculation works
Megawatt-hours (MWh) measure an amount of energy; megawatts (MW) measure the rate at which energy is delivered. Divide energy by power to calculate duration:
Duration = energy ÷ power
- Divide 5 MWh by 50 MW: 5 ÷ 50 = 0.1 hour.
- Convert hours to minutes: 0.1 × 60 = 6 minutes.
The result assumes the full 5 MWh is delivered to the load and the load stays at 50 MW throughout. NREL explains the same duration relationship with a different example: a battery with 1 MW of power capacity and 4 MWh of usable energy has a duration of four hours (NREL, Timescales of Energy Storage).
Why 5 MWh alone cannot establish outage runtime
The system needs enough power capacity
A 5 MWh energy rating describes stored energy, not the maximum rate at which the system can supply it. To serve a 50 MW load, the battery and its power-conversion equipment must be capable of delivering 50 MW. The hypothetical provides no such power rating, so it establishes a conditional duration calculation—not that an unspecified battery can actually serve the load. NREL distinguishes rated power, the maximum instantaneous discharge capability, from the amount of energy stored in its battery-storage FAQ.
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The energy must be usable at the delivery point
Installed or nameplate energy is not necessarily the same as energy available to a facility. The relevant figure is usable energy delivered at the system’s AC output, after accounting for operating limits and conversion. NREL’s duration analysis notes that economically usable discharge capacity can be below rated capacity (NREL, Timescales of Energy Storage).
If 5 MWh already means usable energy at the load, applying another efficiency haircut to the calculation would count losses twice. If it instead refers to a DC pack or another upstream rating, the deliverable amount cannot be determined without system-specific specifications.
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How load and operating assumptions change the result
With the same usable energy, a lower constant load lasts longer and a higher constant load lasts less time. For a varying load, runtime depends on energy consumed over time; a peak of 50 MW does not mean the site draws 50 MW continuously. Reserves, discharge limits, battery condition, and controls can also affect how much energy is available during an outage.
Round-trip efficiency is not automatically a reduction to outage runtime. It compares useful output with energy input across a charge-and-discharge cycle. NREL’s 2025 utility-scale model uses 85% as a representative round-trip efficiency assumption; that is a modeling input, not a universal one-way discharge factor for this hypothetical (NREL, 2025 Annual Technology Baseline).
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What the calculation does—and does not—say about an outage
The six-minute result is a mathematical illustration, not a resilience prediction or measured test result. It does not establish whether a real installation can island from the grid, serve the entire facility, or deliver all of its stated energy. NREL’s System Advisor Model documentation describes how its outage simulation switches a modeled battery to dispatch that prioritizes meeting load; that model behavior is not proof that an unspecified real system can do the same (NREL SAM help, Battery Dispatch BTM).
The U.S. Energy Information Administration likewise defines battery duration by dividing energy capacity by power capacity, while noting that grid-service and load-shifting applications may call for different durations (EIA, March 25, 2022). The ratio answers a duration question only when the relevant power and usable-energy figures, plus the operating assumptions, are known.
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What to check for a real facility estimate
Before using a battery specification to estimate outage support, confirm these items in the system documentation:
- Power capability: Can the battery and inverter supply the facility’s required instantaneous and sustained MW?
- Usable AC energy: How many MWh are available at the point of delivery, rather than merely installed or stored upstream?
- Load profile: What is the expected demand over the outage, not just the site’s peak load?
- Operating limits and reserves: What energy is held back or unavailable under the intended discharge conditions?
- Outage operation: Do the system’s islanding equipment and controls support the loads the facility intends to keep running?
Without those project-specific inputs, the 5 MWh ÷ 50 MW calculation remains a useful illustration, but not a reliable estimate for a particular building or site.
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