The same chemistry, a different box
A lithium ion battery is a rechargeable cell that stores electricity through a chemical reaction and gives it back later. A national laboratory chapter written for the United States Department of Energy says lithium ion is the dominant technology in consumer products, electric vehicles and grid scale storage alike, built by wiring together as many factory made boxes as a site needs. A Tesla Powerwall 3, the wall mounted box that stores solar or backup power in a house, holds 13.5 kilowatt hours, a kilowatt hour being the same unit of stored electricity that shows up on a household electric bill. EnergySage calls that amount typical for a home that wants to keep its essential devices running during an outage. Top home battery brands range from 9.5 to 17.5 kilowatt hours.
EnergySage calls 13.5 kilowatt hours typical for a home that wants to keep its essential devices running during an outage.
Show the numbers
| Smallest top brand | 9.5 |
| Tesla Powerwall 3 | 13.5 |
| Largest top brand | 17.5 |
What a grid sized box looks like
Hillman Energy Center, LLC has filed to build a 125 megawatt battery storage project in Tewksbury, Massachusetts, made of 169 separate enclosed units the company calls a containerized system. Each unit holds lithium ion batteries, an inverter that turns stored power into the current the grid uses, a thermal management system that keeps the cells from overheating, and a battery management system, the same 3 things inside a home battery box, built to ship pre assembled at up to 93,700 pounds. A megawatt measures how fast a system can push power onto the grid, not how much electricity it stores, so 125 megawatts does not say how many kilowatt hours the project holds. Tewksbury sits at the small end of proposed grid projects, below 3 others the Energy Information Administration lists.
The Tewksbury figure comes from source 3, the project filing itself. The other 3 come from the Energy Information Administration, source 1.
Show the numbers
| Tewksbury, Massachusetts, proposed | 125 |
| Gemini Solar Hybrid, Nevada | 380 |
| Manatee Solar Energy Center, Florida | 409 |
| Bellefield Solar and Energy Storage Farm, California | 500 |
Turning power into a size comparison
Only 1 of the 5 sources states a duration in hours for Tewksbury, inside a sentence about a tax payment, not a design specification. The signed host community agreement sets its minimum town payment of not less than 2,000,000 dollars a year on the assumption that the project holds a 4 hour duration. Multiplying that duration by 125 megawatts gives about 500 megawatt hours for the whole project, a calculation for this article, not a figure any source states directly. Spreading that evenly across 169 units gives each one about 2,960 kilowatt hours, a second layer of assumption, since no source says the duration applies evenly per unit. Dividing that by the 13.5 kilowatt hours in a Powerwall puts one grid container at about 219 times the electricity in one home battery.
Why the cells inside are not identical
The same laboratory chapter says the 2 sides tune the same chemistry differently. A grid project can use cells with lower energy density, meaning less stored energy for a given size or weight, because none of its containers has to fit inside a house or ride on a truck. That has pushed grid projects toward lithium iron phosphate cells, which the laboratory calls safer than the higher density cells used in cars and home batteries. United States utility scale battery storage has grown by an average of 70% a year over the last 3 years, reaching nearly 52 gigawatts by the middle of 2026, so a fenced yard like the one proposed for Tewksbury is becoming a common sight near a substation, not a one time event.