Understanding your runtime estimate
Why this number is an estimate, not a guarantee
This calculator assumes a constant, steady load for the full runtime, which real households rarely have. A refrigerator cycles on and off, and startup surges from motors and compressors briefly draw more than their running wattage. Use this number as a planning baseline, not an exact countdown clock. The outage frequency profile tool models a more realistic, cycling load pattern if you want a closer estimate.
Why load size matters more than most people expect
Runtime scales directly and inversely with load: halving your active load roughly doubles your runtime. This is the entire reason backup systems isolate critical circuits onto a subpanel instead of backing up the whole electrical panel, an approach Ready.gov's power outage preparedness guidance recommends prioritizing. A 13.5 kWh battery running an 800 watt critical loads circuit lasts about 14 hours, but the same battery running a 3,000 watt whole-home load lasts under 4 hours. The whole home backup tool covers this tradeoff directly.
How depth of discharge and efficiency stack together
These two factors multiply, not add. A battery with 90% depth of discharge and 92% round trip efficiency delivers 82.8% of its nameplate rating as usable energy, not 90% or 92% individually, consistent with the compounding assumptions in NREL's Annual Technology Baseline for residential battery performance. The depth of discharge calculator and round trip efficiency calculator both isolate one factor at a time if you want to see each contribution separately, and the quick financing comparison tool in our Financing category helps translate a shortfall into what a larger system would cost.
What solar recharging changes about this math
If your battery stays connected to solar panels through a backup gateway during an outage, daytime production adds energy back into the system while the sun is up, which can substantially extend your total runtime across a multi-day outage. The solar recharge sizing tool models this scenario specifically instead of assuming a single, uninterrupted discharge. If this number changes your budget, the quick financing comparison tool shows what the payments look like.
What to do with this number next
If your runtime comes up short of what you need, the battery expansion planning tool shows how adding a second unit changes the outcome, or the critical loads calculator can help you trim your load instead. If backup runtime needs to stretch across a home EV charger too, the EV solar battery backup priority tool in our Solar + EV Integration category covers that overlap.