Home Battery Backup Sizer

The average US home uses about 30 kWh a day, but almost nobody backs up the whole house with a battery. Essentials (fridge, freezer, furnace blower, Wi-Fi, lights, phones, a sump pump) run 4 to 7 kWh a day. Add a well pump and a window AC and you are at 10 to 12 kWh a day. Multiply by your outage days, add 20% reserve and divide by 0.9 for inverter losses and you have the battery size.

A whole-home class unit (6 to 12 kW with 240V output) can run every circuit in a typical house at once except central AC, an electric water heater, an electric range or an EV charger. Those four loads each draw 3.5 to 10 kW and would drain a 10 kWh battery in an hour or two. What most people do instead is put in a transfer switch that backs up 6 to 12 chosen circuits and leave the big resistive loads off.

The F3800 Plus is one 137 lb box with 3.84 kWh, 6 kW and 240V, and you add 3.84 kWh packs up to 26.9 kWh. The DELTA Pro Ultra is a separate inverter and battery (164 lb together) with 6.1 kWh, 7.2 kW and 240V, expandable to 30.7 kWh per inverter. Per kWh the DELTA Pro Ultra extra battery is cheaper than the BP3800 at current prices, so above about 8 kWh the EcoFlow stack usually costs less. Under 8 kWh and for a unit you will also roll out to the driveway, the Anker is simpler.

Only if you want hardwired loads to run. A fridge, router, lamps and a plug-in sump pump can run on extension cords. A furnace blower, well pump, central AC, water heater or anything 240V is wired into your panel and needs either a manual transfer switch with an inlet (about $300 to $900 in parts plus electrician time) or a smart panel ($1,400 to $1,800) that switches circuits automatically. Never backfeed through a wall outlet. It can kill a line worker.

Yes, if the battery supplies 240V split phase and its surge rating covers the pump start. A 1/2 HP pump draws about 1,000 W running and 3,000 W to start, and a 1 HP pump draws 2,000 W and 6,000 W. The F3800 Plus, DELTA Pro Ultra, DELTA Pro 3, Apex 300 and Explorer 5000 Plus all qualify. The Elite 200 V2 and a single HomePower 3600 Plus are 120V only. Pumps only run a few minutes per hour, so they barely dent the kWh. Surge and voltage are the whole problem.

Divide usable energy by your average load. 10 kWh at 90% inverter efficiency is 9 kWh usable. An essentials load that averages 180 W (4.3 kWh a day) runs about 50 hours. Add a window AC for 8 hours a day and the average climbs to 470 W, which cuts it to about 19 hours. The tool shows this number for every pick.

LiFePO4 (LFP). It is rated for 3,000 to 6,000 full cycles against about 500 to 1,000 for NMC, it tolerates sitting at full charge for months, and it is far harder to push into thermal runaway. Every system in this tool is LFP. NMC still shows up in cheap, light camping units where weight matters more than lifespan.

Up front, no. A 7 kW dual-fuel inverter generator with a transfer switch is $1,500 to $2,500. The same output from a battery with 10 kWh of storage is $6,000 to $10,000. The battery wins on silence, zero fuel storage, no carbon monoxide, no maintenance and instant switchover, and it earns its keep year round on time-of-use rates or with solar. For outages longer than two days, the cheapest reliable setup is a mid-size battery plus a small inverter generator to recharge it.

Yes. Every unit here has MPPT solar inputs from 1,000 W (Elite 200 V2) to 5,600 W (DELTA Pro Ultra). Portable panels can be bought later and plugged in. What does not work automatically is a rooftop grid-tied array, which shuts down in an outage. It needs a hybrid inverter or an installed battery system to keep producing when the grid is down.

The whole-home units switch in 10 to 20 ms, which keeps computers, routers and most appliances running. The Explorer 5000 Plus advertises 0 ms in online UPS mode. That is fast enough for a home office but not guaranteed for sensitive lab or medical gear, which should stay on a dedicated double-conversion UPS plugged into the battery.