Batteries, Solar Power, and the Meaning of “Power”

VISTA Community Garden operates using an off-grid solar power system. Unlike an on-grid

system—which is connected to the utility grid and can send or receive electricity—an off-grid

system operates independently and relies on batteries to store energy. This allows the garden

to function even when utility power is unavailable or unreliable.

Our irrigation pump is powered by two lithium-ion batteries with a combined energy capacity of approximately 5,000 watt-hours.

Solar panels supply electricity to keep these batteries charged, allowing us to water the garden even when clouds temporarily block the sun.

Although the batteries may look like large car batteries, they are very different. Car batteries

are made with lead and acid and are designed to deliver a large burst of power for a very short time, such as starting an engine. They are rated in cold cranking amps (CCA). Solar batteries, by contrast, are deep-cycle batteries, designed to deliver energy steadily over long periods and tobe charged and discharged thousands of times. Our system uses lithium iron phosphate(LiFePO₄) batteries, which are much safer and more stable than other lithium-ion chemistriesand are well suited for solar energy storage.

In solar battery systems, energy refers to the total amount of electricity a battery can store or

deliver over time (measured in watt-hours), while power refers to how quickly that energy is

delivered at any moment (measured in watts).

The concept of power measurement has an interesting history. Long before electricity was

common, water had to be pumped out of deep mines so workers could reach coal and ore. In

the late 1700s, Scottish engineer James Watt greatly improved early steam engines used for

this purpose, replacing teams of horses that had previously powered the pumps.

Rather than selling his engines outright, Watt and his business partner Matthew Boulton

licensed them. To help mine owners understand the value of the new machines, Watt

compared their output to the work done by horses—a familiar reference at the time. This led to the term “horsepower,” which is still used today. Both horsepower and watts describe

power—the rate at which work is done—but horsepower is a historical unit, while the watt is

the modern scientific unit. One horsepower is approximately equal to 746 watts.

So, the next time we pay our electric bill, we can quietly thank Boulton and Watt for inventing this clever way to sell energy rather than selling a separate motor to every home—a strategy that made modern electricity and energy billing possible.