How Big is a Battery? Understanding Battery Size, Capacity, and Power

How big is a battery?

When people talk about battery size, they often mean how much energy it can store — but it’s a bit more complex than that. Building on our post about how you measure energy, let’s break down what battery size really means, from capacity and charge rate to power and beyond.

Energy Storage Capacity (kWh or MWh)

Battery energy storage capacity is the total amount of energy the battery can store, measured in kilowatt-hours (kWh) or megawatt-hours (MWh). Think of this as like the size of a water tank where you measure the water capacity in litres. The more energy stored, or more kilowatt-hours (kWh) or megawatt-hours (MWh), the longer the battery can supply power.

Power Rating or Power Capacity (kW or MW)

Power rating or power capacity is the maximum rate at which the battery can discharge or charge, measured in kilowatts (kW) or megawatts (MW). Its normally determined by the capacity the battery system’s power conversion equipment.

This is like the width of the pipe that water might flow through. The larger the pipe the faster we can fill or empty our water tank. And similarly the larger the power rating the faster we can charge or discharge our battery.

Storage Duration (hours)

Battery storage duration describes how long the battery can discharge at its rated power. It’s calculated: Energy Capacity (MWh) ÷ Power Rating (MW). A 4 MWh battery with a 1 MW power rating has a 4-hour duration. A 1 MWh battery with a 2 MW power rating has a 0.5-hour duration. We’ve written about storage duration in more detail here.

Charge Rate (C-rate)

C-rate or ‘charge rate’, is another way of defining how quickly a battery can be charged or discharged relative to its storage capacity.

A 1C rate means the battery can be fully charged or discharged in one hour. A 0.5C battery takes two hours; a 2C battery takes 30 minutes.

While we often think of the power rating as being a characteristic of the battery system’s power conversion equipment, we generally think of C-rate as being constrained by the battery’s chemistry and design.

Its import to note that C-rate applies to both charge and discharge, and, again, depending on the battery’s chemistry and design, the C-rate for charge and discharge can be different.

Lifetime Energy Throughput (Cycles or MWh)

Battery “size” also needs to be understood over time—how much energy the battery can deliver across its usable life. This is often described as lifetime energy throughput, measured in total MWh, or in terms of some number of full charge and discharge cycles.

A battery’s usable capacity typically degrades with cycling, temperature exposure, and calendar time. Battery OEMs often specify an end-of-life capacity threshold (e.g., 60% or 70% of original capacity), beyond which safe performance is no longer guaranteed.

Volume (Volumetric Energy Density Wh/L)

Volumetric energy density measures how much energy a battery stores per unit of volume. Higher volumetric density means more energy in a smaller footprint. This is important for space-constrained applications, like transport for example. Technologies like lithium-ion are good here, while alternatives like flow batteries or sodium-ion, or more exotic ‘gravity batteries’ tend to require more physical space.

Weight (Gravimetric Energy Density Wh/kg)

Another measure of the size of a battery is the energy stored per unit of weight. This is critical for for applications like aviation.

The standout for low gravimetric energy density might be the iron air batteries that are being developed for seasonal energy storage. This might sound uncharitable, but iron-air batteries are, at their core, little more than giant stacks of rusting and de-rusting iron.

Putting It Together

So, when someone asks, “How big is your battery?”, the answer really is: it depends. Are you asking how much energy it holds? How fast it can respond? How much space it takes up? How much it weighs? Or how much energy it can deliver over its life. Battery size depends on many factors — energy capacity, power rating, charge rate, and more.

Finding the right battery solution for your project application means considering all of these factors. Gridcog has the capabilities to simulate all the sizes: from the biggest utility-scale battery projects to distributed residential virtual power plants, to batteries in vehicles (and vessels), and more. Get in touch with the team to find out more.