Battery Backup Sizing, Runtime & Solar Pairing FAQ

Phase 1

How Long Will My Battery Actually Last?

This is a math question, not a marketing claim. A single Powerwall 3 holds 13.5 kWh of usable energy. A typical Western Washington home running essential loads only, refrigerator, some lighting, a few outlets, internet, and a well pump if you have one, draws roughly 1 to 2 kWh per hour. That puts a single battery at roughly 6 to 12 hours of essential backup on stored charge alone, longer if you’re conservative with usage or have solar recharging it during the day. “Days of backup power” without qualification is a simplification. Real multi-day coverage takes either solar recharge, a second unit, or active load management, and we’ll walk through the actual math for your home before you buy.

Phase 2

How Many Batteries Do I Need?

It depends on what you’re actually trying to back up. For most homes wanting to keep the refrigerator, some lights, and internet running during an outage, one battery is enough. For whole-home backup that includes things like an electric range, a well pump, or central AC running simultaneously, most homeowners need two to three units. Enphase’s modular design makes it easy to start with one and add more later; Tesla’s single large unit means you’re deciding your capacity ceiling up front, though a second Powerwall can be added.

• One unit typically covers refrigeration, lighting, and internet
• Two to three units typically needed for whole-home coverage, including major appliances
• We size your system to your actual circuit panel and usage, not a generic recommendation

Phase 3

Can I Add a Battery to My Existing Solar?

In most cases, yes. Both Tesla Powerwall and Enphase IQ Battery systems can be retrofitted onto existing solar installations, though compatibility depends on your current inverter and panel setup. Pairing a new battery with existing solar also keeps you eligible for Washington’s sales tax exemption on the battery itself, since the exemption requires the battery to be principally powered by a qualifying solar system. We’ll check your existing equipment as part of any retrofit quote.

More Battery Backup Questions

How long will a Tesla Powerwall run my house during an outage?

On stored charge alone, a single Powerwall 3 typically runs essential loads for 6 to 12 hours. With solar recharging it during daylight hours, or with load management, that stretches well beyond a single charge cycle. Whole-home, unmanaged coverage generally requires a second unit.

Do I need special wiring to add a battery?

Yes, a battery installation includes a transfer switch or load center that determines which circuits it backs up during an outage. This is standard part of every installation and is included in your quote, not an add-on surprise.

Will my battery charge from solar or from the grid?

Both, if you have solar. A battery paired with solar charges from your panels during the day and can also top off from the grid if needed. A standalone battery without solar charges from the grid only, which also means it doesn’t qualify for Washington’s sales tax exemption.

Can a battery power my whole house, including AC and an electric range?

It can, but usually requires two or more units and careful circuit planning, since large appliances draw significant power. A single unit is generally sized for essential circuits, not simultaneous whole-home load including major appliances.

How long does battery installation take?

A straightforward single-unit installation on a panel that doesn’t need upgrading typically takes one to two days, including inspection scheduling. A project requiring a panel upgrade or multiple units takes longer, and we’ll give you a realistic timeline as part of your quote.