MW vs MWh
Definition
A megawatt (MW) is a unit of power: how fast electricity can be delivered at a given moment. A megawatt-hour (MWh) is a unit of energy: the amount delivered when one megawatt flows for one hour. EIA describes a storage system's power capacity as 'the maximum instantaneous amount of electric power that can be generated on a continuous basis' and its energy capacity as 'the total amount of energy that can be stored in or discharged from the storage system.' Dividing energy by power gives a battery's duration.
Why it matters
Battery projects are often described by one number, usually MW, which hides how much energy they can deliver. Two projects of the same MW can differ fourfold in MWh, in cost and in what they can earn. Investors should read both ratings, check whether MWh is usable or nameplate energy, and see which one each revenue line is paid on.
Common misconceptions
- •MW and MWh are not interchangeable. One is a rate, the other an amount.
- •A higher MW rating does not mean more stored energy.
- •Nameplate MWh is not always usable MWh; operating limits and degradation reduce it.
- •The label is only a starting point. Investor outcomes usually depend on the governing documents, cash-flow priority, valuation policy, disclosure quality, and manager discretion surrounding the term, not just the shorthand definition used in marketing materials.
Technical details
Power-by-duration examples
Energy Vault, Cross Trails (Texas): 57 MW / 114 MWh, two hours.
Ormat, Lower Rio: 60 MW / 120 MWh, two hours.
Ormat, Arrowleaf: 35 MW / 140 MWh, four hours.
SB Energy, Pelican's Jaw battery (California): 238 MWac / 954 MWh, about four hours.
Fleet scale
EIA's table for 2022 shows U.S. utility-scale batteries at 8,842 MW of power capacity and 11,105 MWh of energy capacity.
Regional mix differs: in 2019 EIA found PJM batteries averaged 45 minutes of duration and CAISO batteries 4.0 hours.
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- What is the difference between battery power and energy capacity?
- Power capacity (MW) limits how much the battery can push to the grid at once; energy capacity (MWh) limits how long it can keep doing so. Energy Vault's Cross Trails battery in Snyder, Texas is 57 MW / 114 MWh: it can discharge 57 MW for about two hours.
- Why do two batteries with the same MWh behave differently?
- EIA's example: two systems each with 2 MWh of energy capacity, where one 'produces 2 MW for short periods of time' and the other 'produces less than 1 MW consistently for 4 hours.' The first suits fast grid services; the second suits shifting energy across the day.
- Which number do revenues depend on?
- Both. Capacity payments and fast grid services are usually paid on MW; energy sales and arbitrage are paid on MWh delivered. A contract for a four-hour product requires enough MWh to hold the MW for four hours.