1.6 Battery fundamentals

1.6.1 The basics

• Battery capacity is measured in kWh (energy), not kW (power). A 15 kWh battery can typically run an average household for 10–15 hours.
• 99% of residential batteries today are LFP (lithium iron phosphate) — safer than NMC, longer cycle life (6,000+ cycles), 10–15 year service life. The fire risk is lower than the phone in your hand.
• Key specs to read: Usable Capacity, Continuous Power (kW), Peak Power, Cycle Life, Depth of Discharge (DoD).

1.6.2 Battery form factors

• Stackable > vertical / wall-mounted > visible-wiring chains.
• Stackable installs are simplest but take up more floor area.
• Wall-mounted vertical units take less area, but verify it is a current-generation product. Older models still sold in the U.S. squeeze only one-third of the capacity into the same footprint as today's batteries.
• Open-wiring chains loosen and can short over time — rarely approved for residential installs in cities.

1.6.3 Battery safety

A common mistake is to lump residential storage batteries together with EV traction batteries. Every time someone sees an EV fire on the news, they get nervous about putting storage in their garage. This is a serious misconception.

In the U.S., every residential battery must pass strict safety certification, including UL9540 and UL9540A tests. Each unit ships with over-temperature, over-charge, and short-circuit protection, plus a fire-resistant enclosure.

And every modern residential battery uses LFP chemistry, which is extremely stable. In a stationary install under normal use, the fire risk is vanishingly low. That is why you almost never see news stories about residential storage batteries catching fire. A solar battery in your garage is safer than the laptop in your lap or the phone on your nightstand.

So as long as you have a compliant product installed by a compliant installer, you don't need to worry about battery safety.