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FOGHORN
Batteries do not replace generators. They are a different 
class of tool entirely, and the operators who get the most 
out of them are the ones who stop trying to make the sub-
stitution work and start asking a more useful question: how 
are we going to use the battery? What are we trying to do?
What follows is a practical framework for working through 
those useful questions, drawn from years of designing bat-
tery-integrated power systems across the maritime industry.
START WITH WHAT A BATTERY ACTUALLY IS
Two very important terms get conflated constantly in the 
media when reporting on batteries: power and energy. 
They are different quantities, and the difference matters.
Power is a rate, measured in kilowatts (kW). Power is familiar, 
like horsepower in a car. It is how fast something is happening. 
Energy is a lot less familiar. Energy is a quantity, measured 
in kilowatt-hours (kWh), kilowatts multiplied by hours. Five 
kWh can be five kW for one hour, or one kW for five hours. 
Energy is the total power used over a period of time. 
A diesel engine makes a clear analogy when paired with a 
fuel tank: the engine is the power; the fuel tank is the en-
ergy. With a battery, those two are not independent—they 
are linked by a single piece of hardware. A bigger battery 
has both more energy and more power. Different battery 
designs trade one against the other. A battery built for high 
power tends to have lower energy density; one built for en-
ergy density typically has lower power capability.
Power capability is very relative for a battery. For instance, 
a requirement for 500 kW. Is that a lot? Well, it depends 
on the battery. The metric that captures this relationship 
is C-rate—the ratio of power to energy. A 500-kW output 
from a 50-kWh battery is 10C; the same 500 kW from a 
500-kWh battery is 1C. So energy is key for grounding our 
sizing for batteries. C-rate tells you whether a given amount 
of battery kWh can produce the required amount of pow-
er. Understanding these relationships gives great insight 
and familiarity to the real capabilities and trade-offs that 
exist between technologies.
Three things are worth understanding under the hood. First, 
the battery management system, or BMS, is the software 
brain that monitors and protects the cells—it is often misun-
derstood as some sort of safety disconnect, but in reality, it 
is a monitoring system that is keeping the battery in balance 
FOGHORN FOCUS
HIGH ENERGY
(Low C-Rate)
HIGH POWER
(High C-Rate)
kW
kWh
CHART: SPOC ENERGY

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