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Calculator · Updated August 2026
Battery backup runtime calculator
Pick a capacity and what you want to keep running. See the honest runtime.
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The calculator
How long would a battery last?
Draw figures are typical averages, not your house. A well pump cycling hard or a heat pump in a cold snap will beat these numbers badly.
This is an estimate, not a savings guarantee.
It uses published utility rates, NREL regional sun data and a standard system derate. It does not know your roof orientation, your shading, your usage pattern or what your utility will charge next year. No solar company can honestly guarantee savings, and any that does is describing a model rather than making a promise.
The thing to understand first
Solar panels alone do not work during a blackout. A grid-tied inverter is required to disconnect when the grid goes down, so line workers are not repairing a circuit your roof is energizing. Without storage and the correct switching, a sunny roof produces nothing you can use during an outage. If a salesperson has let you believe otherwise, revisit that before signing.
Capacity is not the same as usable capacity
Batteries are marketed on nominal capacity and deliver usable capacity, which is lower. Manufacturers reserve a portion to protect cell life. Use the usable figure from the spec sheet, not the headline number, and note that usable capacity declines over the warranty period.
What actually drains a battery
Resistance heating
Electric ranges, ovens, resistance water heaters and space heaters are the fastest way to empty storage. A single oven cycle can consume more than a refrigerator uses in a day.
Air conditioning
Compressor loads are large and run for hours. Backing up whole-house cooling requires either much more storage or accepting a short runtime.
Well pumps
Short but heavy draws. Manageable on average, but the starting surge sizes the inverter rather than the battery, which is a separate constraint people miss.
EV charging
Do not back it up. A single charge session exceeds most residential batteries entirely.
Sizing from your real outage pattern
Look up how often and how long your power actually fails rather than working from a brochure. Two four-hour outages a year is a completely different problem from a rural line that fails for three days after every ice storm. Size around the loads you genuinely need for the duration you genuinely experience, not around a product tier.
Solar changes the arithmetic
A battery alone is a fixed reservoir that empties. A battery paired with panels refills each day, which converts an overnight device into something that can carry critical loads through a multi-day event, provided you ration heavy loads. That is the configuration worth having if long outages are your concern.
Whether it pays financially depends on your state
In Massachusetts and Connecticut, grid-event programs pay battery owners directly, which changes the maths. Where net metering is one-for-one at retail, storage does not create financial value because an exported kilowatt-hour is already worth a retail one — there the case is resilience. Where export credit is well below retail, storing production for evening use captures the difference.
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Who you would be working with
SolarAmerica
We design your system and manage the project; installation is performed by vetted licensed partners in your area. SolarAmerica is not itself the installing contractor.
Questions
The things people actually ask
How long will a home battery last in a power outage?
On essential loads only, a 13.5 kWh battery typically runs for a day or more. Add a heat pump zone and it falls to roughly half a day. Add air conditioning and it can be a few hours. Paired with solar it recharges daily, which extends it substantially.
Do solar panels work in a blackout without a battery?
No. Grid-tied inverters must disconnect when the grid fails, for line-worker safety.
Can a battery run my whole house?
Usually not. Most residential installations back up a critical loads panel rather than the entire service. Whole-home backup needs much more capacity or active load management.
Is a battery worth it financially?
It depends on your state's export credit and on whether a grid-services program pays you. Where net metering is one-for-one at retail, storage is a resilience purchase rather than a financial one.
Get the number for your actual roof
About 60 seconds. No cost, no obligation, and we will tell you if solar does not make sense for your home.
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