Flow Battery Sizing Calculator
Flow batteries store energy in liquid electrolyte held in external tanks, decoupling power (set by the electrochemical stack) from energy capacity (set by tank volume) in a way lithium-ion cannot. This calculator takes a desired power rating, discharge duration, and electrolyte energy density, then reports the system's energy capacity and the volume of electrolyte required to deliver it -- a direct measure of the physical footprint flow batteries trade for their long-duration scalability. It pairs naturally with our Utility-Scale BESS Sizing Calculator for a lithium-ion comparison, and our planned Flow Battery LCOS Calculator for translating these physical quantities into levelized cost of storage.
Continuous power the system must deliver during discharge.
Flow batteries are typically most cost-competitive at 4-12+ hour durations, where lithium-ion's cost per kWh becomes comparatively expensive.
Vanadium redox flow batteries (the most commercially mature flow chemistry) typically achieve 15-35 Wh/L, depending on electrolyte concentration and system generation.
desired power rating (kW) × desired discharge duration (hours)
(energy capacity (kWh) × 1000) ÷ electrolyte energy density (Wh/L)
required electrolyte volume (Liters) × 0.264172
This planning estimate sizes energy capacity and electrolyte volume only. Stack power sizing, pump and plumbing design, state-of-charge operating range, shunt currents, thermal management, and site footprint require vendor-specific engineering.
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Flow batteries decouple power and energy in a way lithium-ion can't -- want more energy? Just add bigger tanks. In this example, an 8-hour, 1 MW system needs 320,000 liters (about 84,500 gallons) of electrolyte -- a genuinely large footprint compared to lithium-ion's compact cells, but that's exactly the tradeoff: flow batteries excel at long-duration storage precisely because scaling energy capacity doesn't require adding more expensive electrochemical stacks, just more (relatively cheap) electrolyte.
How flow battery sizing is calculated
This calculator sizes a flow battery for a target continuous power output and discharge duration, then converts the resulting energy capacity into the volume of electrolyte required to store it, given an electrolyte energy density. Three quantities tie the calculation together.
Energy Capacity (kWh) = Desired Power Rating (kW) × Desired Discharge Duration (hours). Multiplying the continuous power the system must deliver by the number of hours it must sustain that output gives the total energy the battery must hold. At the defaults (1,000 kW and 8 hours), that is 1,000 × 8 = 8,000 kWh.
Required Electrolyte Volume (Liters) = (Energy Capacity (kWh) × 1000) ÷ Electrolyte Energy Density (Wh/L). Converting the energy capacity from kilowatt-hours to watt-hours (× 1000) and dividing by the electrolyte's energy density -- the amount of energy stored per liter of electrolyte -- gives the total electrolyte volume the tanks must hold. At the defaults (8,000 kWh and 25 Wh/L), that is (8,000 × 1000) ÷ 25 = 320,000 Liters.
Required Electrolyte Volume (Gallons) = Required Electrolyte Volume (Liters) × 0.264172. Converting the liter figure to U.S. gallons using the standard 0.264172 gallons-per-liter factor gives the same volume in more familiar units for U.S. project planning. At the defaults, that is 320,000 × 0.264172 = 84,535.0 Gallons.
Two notes on the model. First, the electrolyte energy density is the single most consequential input, and it varies meaningfully by chemistry and system generation: vanadium redox flow batteries (the most commercially mature flow chemistry) typically achieve 15-35 Wh/L depending on electrolyte concentration and system generation, so the editable field lets you substitute a vendor-specific value. Second, this calculator isolates the energy-capacity and electrolyte-volume question and excludes stack power sizing, round-trip efficiency, depth of discharge, cycle life, pumping losses, thermal management, and project economics, all of which matter for real flow battery project evaluation. Data sources: Flow battery architecture and vanadium redox flow battery (VRFB) energy density ranges from DOE (Department of Energy), NREL (National Renewable Energy Laboratory), and IRENA (International Renewable Energy Agency) energy storage technology assessments; VRFB commercial deployment data from industry reports and operator documentation; liter-to-gallon conversion using the standard 0.264172 U.S. gallons per liter factor. Verification: with defaults (1,000 kW, 8 hours, 25 Wh/L), Energy Capacity = 8,000 kWh, Required Electrolyte Volume = 320,000 Liters, Required Electrolyte Volume = 84,535.0 Gallons.