Energy needed--
12V battery bank--
Home Backup Power Calculator
Estimate home backup watt-hours and 12V battery-bank capacity from essential watts, runtime, and efficiency.
Updated May 2026No signup requiredBuilt for mobile
Practical intro
Use this home backup power calculator to turn a short list of household essentials into an energy target. It is meant for refrigerators, modem and router loads, lights, fans, phone charging, small medical-device planning conversations, and other selected circuits rather than every appliance in the home.
When to use this calculator
- You want to compare a battery bank, portable power station, or inverter setup for a short outage.
- You are deciding which loads are essential enough to include in a backup plan.
- You need an approximate 12V battery-bank equivalent for early planning.
- You want to understand why a small battery may not cover high-wattage heating, cooking, or well-pump loads.
Input explanation
- Essential load is the combined average running watts of the devices you plan to support.
- Runtime needed is the number of hours those loads should operate.
- Efficiency adjusts for inverter conversion, wiring, battery reserve, and real-world losses.
- The 12V battery-bank result is an equivalent planning value, not a wiring diagram.
- For motor loads, check starting surge and dedicated circuit requirements separately.
Formula or estimation method
Energy needed = watts x hours / efficiency fraction.
12V amp-hours = adjusted watt-hours / 12.
If using another battery voltage, amp-hours = adjusted watt-hours / system voltage.
Practical capacity should be increased when manufacturer guidance limits depth of discharge.
Step-by-step worked example
A home emergency list averages 1,500 watts for 12 hours. Raw energy is 18,000 watt-hours. At 85% efficiency, the adjusted energy target is about 21,176 watt-hours. At 12 volts, that equals about 1,765 amp-hours before battery chemistry, temperature, reserve, and inverter limitations are considered.
Common mistakes
- Trying to include electric heat, ovens, dryers, or central air without understanding their large loads.
- Using breaker size as the actual watt load.
- Forgetting that a refrigerator, sump pump, or furnace blower may have a starting surge.
- Planning a battery bank without matching inverter, charger, fuse, cable, and ventilation requirements.
- Ignoring local code when connecting backup power to house wiring.
Planning notes and limitations
- Prioritize health, refrigeration, communications, lighting, and heating-system controls before comfort loads.
- A transfer switch or interlock may be required for connected generator or inverter circuits.
- Battery backup is usually best for quiet indoor loads; fuel generators are often better for long, high-energy outages when operated safely outdoors.
- This calculator does not replace an electrician's load calculation or equipment manual.
Backup power safety checklist
Before relying on the estimate, confirm the equipment manual, fuel or battery limits, ventilation needs, weather protection, cords, transfer method, and local requirements. Fuel-burning generators belong outdoors away from openings because carbon monoxide can be deadly. Electrical connections, transfer switches, high-current batteries, and permanent wiring should be handled under applicable code by qualified people.
Safety disclaimer
This calculator is for planning only. Follow local electrical code, transfer-switch rules, manufacturer instructions, carbon monoxide safety warnings, and licensed electrician guidance before connecting backup power to home circuits.
FAQ
Home Backup Power Calculator questions
Can I power my whole house from batteries?
Sometimes, but whole-house systems require much larger capacity, proper inverter equipment, code-compliant installation, and professional design.
Why does the 12V amp-hour number look so large?
Low-voltage systems need many amp-hours for large watt-hour loads. Higher-voltage battery systems may use fewer amp-hours but still need the same energy.
Should I include every appliance?
Start with essential loads. Add comfort loads only after you understand the extra energy and surge requirements.
Can I connect this to my breaker panel?
Only with approved transfer equipment and code-compliant installation. Never backfeed a home through a cord or outlet.
What if I use lithium batteries?
Lithium batteries often have more usable capacity than lead-acid, but follow the battery management system and manufacturer limits.
Before you buy equipment
Use this result as a screening estimate, then compare it with rated running watts, surge watts, usable watt-hours, fuel availability, recharge time, maintenance, and installation requirements. A backup plan is safer when it starts with fewer essential loads and leaves reserve instead of assuming every rating can be used at the same time.