Home Battery Size for a 3-Day Outage

Surviving a 3-day outage on batteries takes far more storage than most buyers expect: typically 30 to 60 kWh, or two to four Powerwall-sized units. The math is unforgiving because multi-day backup means covering nights, cloudy stretches, and real household loads. This guide shows the calculation and the cheaper alternatives.

The short answer

For three days of essentials-only backup with no solar recharge, budget 30 to 45 kWh of usable storage (two to three Powerwall 3s, or six to nine Enphase 5Ps). With a decent solar array recharging daily, 15 to 25 kWh (one to two Powerwalls) handles three days in fair weather, because the panels refill the tank each day. Without solar, or with heavy loads like central AC, the number climbs fast and the price climbs faster.

Most buyers overestimate what one battery does and underestimate what three days require. Work the math before you shop. Our battery comparison covers the leading options, and the runtime guide shows what a single 13.5 kWh unit does hour by hour.

Step 1: your critical load math

List what must run, with realistic wattage and daily hours. Be honest: “essentials” for three days includes more than you think.

About 12.5 kWh per day for genuine essentials, or 37.5 kWh over three days. Add a 20 percent margin for inverter losses and the unexpected, and you need roughly 45 kWh of usable storage with zero solar help. That is three-plus Powerwall 3s. Now add central AC (25 to 35 kWh per day in summer) and you see why whole-home multi-day backup is a $50,000-plus proposition.

The single biggest lever is load discipline. Dropping the TV, halving the lights, and skipping the microwave saves 1.5 kWh per day. Running the well pump on a strict schedule saves more. Every kWh shed is roughly $700 to $1,000 of battery you do not have to buy.

Step 2: factor in solar recharge

Solar changes everything. A 8 kW array in decent sun produces 25 to 40 kWh per day, which covers the 12.5 kWh essentials load and recharges the battery simultaneously. In this scenario, the battery only needs to cover the nighttime gap (roughly 5 to 7 kWh for essentials) plus a cloudy-day buffer.

With solar, a single 13.5 kWh battery plus an 8 kW array carries essentials through three days of mixed weather in most climates. Two batteries (27 kWh) add real cloudy-day resilience: even with one bad solar day, you ride through. This is the configuration most resilience-focused homeowners actually buy, and it costs half what the no-solar math demands.

The catch: storm outages often come with cloudy weather. Hurricane and wildfire outages can mean days of poor solar production. Size your cloudy-day buffer for your region’s reality: the Gulf Coast in hurricane season is not the Southwest in a heat wave. If your threat model is storms, lean toward more storage; if it is heat-wave grid stress with clear skies, lean on the solar. Ready.gov’s power outage guide is the federal checklist worth working through before you size anything.

Three realistic scenarios

Scenario A: Essentials, no solar, 3 days. 45 kWh usable. Three Powerwall 3s or nine Enphase 5Ps. Cost: $35,000 to $55,000 installed. Rarely the right answer unless solar is impossible.

Scenario B: Essentials, 8 kW solar, 3 days mixed weather. 13.5 to 27 kWh usable. One to two Powerwall 3s. Cost: $11,000 to $28,000 for batteries plus the solar array. The sweet spot for most resilience buyers.

Scenario C: Whole home with AC, solar, 3 days. 40 to 60 kWh usable. Three to four Powerwall 3s. Cost: $40,000 to $60,000+ for batteries alone. Justifiable for medical needs or off-grid-grade resilience; overkill for most.

Most homeowners land in Scenario B and supplement with a portable generator for the true worst case. That hybrid is covered next. Whichever scenario fits, write down your must-run loads before talking to installers; the list is the single most useful thing you can bring to a battery consultation.

What it costs

Battery-only costs for 3-day configurations (installed, 2026): two Powerwall 3s run $20,000 to $28,000; three run $30,000 to $40,000. Equivalent Enphase 5P stacks cost more at these sizes because you need six to nine units. The DSIRE database lists state and utility storage incentives that can offset 10 to 30 percent in some markets. Remember the federal 25D credit ended for 2026 installs, so there is no 30 percent federal credit on these numbers.

Do not forget the electrical work scales too: multi-battery installs often need panel upgrades, larger gateways, and more complex wiring. Get the full installed scope in writing. If you already have panels, read adding a battery to an existing system for retrofit specifics, and compare the FranklinWH alternative before committing to a stack.

Cheaper alternatives

Battery plus portable generator: one Powerwall 3 ($11,000 to $16,000) plus a $1,000 to $2,500 dual-fuel portable generator covers essentials for days: the battery handles nights silently, the generator tops it up during the day if solar underperforms. Total cost is roughly half of a three-battery setup, and the generator covers the true worst case better than any affordable battery stack.

Smaller battery plus load discipline: a single 13.5 kWh unit with a strict essentials plan and solar recharge handles most real outages, which historically last hours, not days. Size for the probable, not the apocalyptic.

Partial-home backup design: wire only critical circuits to the battery instead of the whole panel. This stretches any battery further and can cut the required storage by a third. Your installer designs this with a critical-loads subpanel.