How long will a Tesla Powerwall last during a power outage?

Two Tesla Powerwalls will give you 27 kWh of usable energy. For most users, this equates to 1-3 days of backup power, depending on how conservative you are using the electric loads that are being backed up. Residents who also own a solar panel system, which will recharge their battery during an outage, will receive backup power for a longer period of time—essentially unlimited in sunny conditions.