Home battery storage is moving from an exotic feature of the homes of the wealthy to a more mainstream option for any home with solar panels, incentivized by a new 30% federal tax credit. But If you think of batteries primarily as a backup technology, you're missing the main way to justify their cost: saving money on energy every day.

Hands-on with home batteries

ZDNet's recent roundup of the best home battery systems inspired me to visit a home battery installation in the San Francisco Bay Area where a Generac PWRcell system was being mounted inside the garage by northern California installer Rob Heckendorn of Future Energy Savers. The system was fed by the home's existing rooftop solar panels, something you can't do with Tesla Powerwall home batteries, which must be combined with its specific solar roof tech.

The Generac system is modular, allowing the homeowner to add batteries in 3kWh increments up to 18kWh in a single battery cabinet housing. In this case, they chose three modules for a total of 12kWh of capacity, a number Rob Heckedorn says is as close to an average as he sees and enough power to run three or four of the home's dozen circuits for a few days.

Think beyond blackouts

But thinking of batteries in terms of blackout coverage creates a blind spot: Yes, the US may have more power outages than other developed countries, but they remain rare. A better priority is to spec your battery system for "rate arbitrage" to save money and be more green every day.

The idea is simple: Store excess free energy from your rooftop in your batteries rather than selling it back to a utility that doesn't like the idea and offers a pittance. Instead, use that stored energy later in the day during peak times to avoid the highest electricity rates while also doing your part to take strain off the grid. It's the missing piece that makes residential rooftop solar a more comprehensive proposition.