Outage duration
48hours
2 days selected.
Home Outage Backup Planner
Plan 24, 48, 72, or custom-hour backup needs for critical home loads with battery and generator sizing outputs.
Use this planner to estimate power for selected critical devices during a home outage. It focuses on watt-hours, running watts, and starting surge; it does not provide wiring instructions, fuel planning, food safety guarantees, or medical advice.
Outage planning inputs
Choose a common planning window or enter custom hours.
All modes still show both energy and surge sizing outputs.
Uses the largest single extra starting surge.
%
Default 85% accounts for common conversion losses.
%
Battery capacity intentionally left unused for planning margin.
Results
Daily energy need
1,920Wh
Energy use for one day of the selected critical loads.
Outage energy need
3,840Wh
Daily energy multiplied by the selected outage duration.
Recommended battery
5,315Wh
Energy need adjusted for efficiency and reserve.
Peak starting watts
1,275W
Staggered surge mode applied.
Recommended generator
1,600W
Peak starting estimate with 20% headroom, rounded up.
Running load
225W
Sum of running watts for selected critical loads.
Backup type
Battery
Used to frame the result labels, not to hide other outputs.
Critical load breakdown
| Item | Value | Notes |
|---|---|---|
| Refrigerator (1) | 1,200 Wh/day, 150 W running | 150 W x 1 x 8 hours/day; starting 1,200 W each; extra surge 1,050 W total. |
| Wi-Fi router (2) | 360 Wh/day, 15 W running | 15 W x 1 x 24 hours/day; starting 15 W each; extra surge 0 W total. |
| LED light bulb (3) | 360 Wh/day, 60 W running | 10 W x 6 x 6 hours/day; starting 10 W each; extra surge 0 W total. |
Planning breakdown
| Item | Value | Notes |
|---|---|---|
| Outage duration | 48 hours | Selected hours divided by 24 for the energy calculation. |
| Battery capacity | 5,315 Wh | Total outage energy divided by efficiency and usable battery fraction. |
| Surge mode | Staggered | Adds only the largest extra starting load to total running watts. |
| Generator size | 1,600 W | Peak starting watts x 1.2, rounded up to the nearest 100 W. |
Top energy users
| Item | Value | Notes |
|---|---|---|
| 1. Refrigerator | 1,200 Wh/day | These loads drive battery capacity more than starting surge. |
| 2. Wi-Fi router | 360 Wh/day | These loads drive battery capacity more than starting surge. |
| 3. LED light bulb | 360 Wh/day | These loads drive battery capacity more than starting surge. |
Formula
outageDays = outageHours / 24; totalWh = dailyWh x outageDays; batteryWhNeeded = totalWh / efficiency / (1 - reserve); peakStartingWatts = runningWatts + startingExtra; recommendedGeneratorWatts = roundUpToNearestHundred(peakStartingWatts x 1.2)Battery capacity is driven by energy over the outage duration. Generator sizing is driven by running load plus starting surge, so the planner shows both outputs.
- dailyWh
- Sum of each critical device watts x quantity x hours per day.
- batteryWhNeeded
- Total outage energy adjusted for conversion efficiency and reserve.
- runningWatts
- Sum of each critical device running watts multiplied by quantity.
- startingExtra
- Staggered mode uses the largest extra starting load; conservative mode adds all extra starting loads.
- recommendedGeneratorWatts
- Peak starting watts with 20% headroom, rounded up to the nearest 100 W.
Example
Conservative surge mode would add every extra starting load together, which can increase the generator estimate when several motor loads may start at the same time.
- A refrigerator uses 150 W for 8 hours per day, or 1,200 Wh/day.
- A router uses 15 W for 24 hours per day, or 360 Wh/day.
- Daily energy is 1,560 Wh; over 48 hours, total outage energy is 3,120 Wh.
- With 85% efficiency and 15% reserve, recommended battery capacity is about 4,318 Wh.
- Running load is 165 W. In staggered mode, refrigerator extra starting surge adds 1,050 W, so peak starting watts are 1,215 W.
- With 20% headroom and rounding, recommended generator size is 1,500 W.
How to use this planner
- Select 24, 48, 72, or custom outage hours.
- Add only the critical devices you want to support.
- Enter each device watts, quantity, hours per day, and starting watts when available.
- Choose a backup type and surge mode that matches your planning assumption.
- Review both the battery watt-hour estimate and generator wattage estimate before comparing equipment specifications.
Input guide
- Outage duration: planning window in hours; common presets are 24, 48, and 72 hours.
- Critical devices: loads you actually need during an outage, not every device in the home.
- Hours per day: expected daily runtime for each device, up to 24 hours.
- Starting watts: short startup demand for motors and compressors; leave blank only when unknown.
- Efficiency and reserve: planning assumptions for conversion loss and unused battery margin.
- Surge mode: staggered assumes only one large extra starting load at a time; conservative adds all extra starting loads.
Common mistakes
- Including noncritical loads and oversizing the emergency plan.
- Ignoring starting watts for refrigerators, freezers, and pumps.
- Assuming battery watt-hours and generator watts are the same kind of rating.
- Planning for 24 hours when local outage risk may require longer coverage.
- Treating a power estimate as a wiring, fuel, food safety, or medical readiness plan.
Limitations
This planner uses fixed wattage and hours-per-day assumptions. It does not model changing appliance duty cycles, battery age, temperature, fuel availability, solar weather variability, code requirements, medical device requirements, or food temperature history.
FAQ
Which devices should I include?
Start with critical loads such as refrigeration, communications, medical power needs, lighting, and any device required for basic safety.
Why estimate battery and generator needs separately?
Battery sizing is driven by energy over time, while generator sizing must also account for running load and starting surge.
Why can the same load need different battery and generator sizes?
Battery planning is based on energy over time. Generator planning also needs running watts and starting surge because motors and compressors can briefly need more power when starting.
Does this planner include fuel, wiring, or installation requirements?
No. It only estimates electrical load size. Fuel storage, ventilation, code compliance, and electrical connections require manufacturer guidance and qualified professionals.
Related calculators
Use these tools to refine generator sizing, refrigerator backup time, UPS runtime, and CPAP power estimates.
- Generator Wattage CalculatorEstimate running watts, starting surge, and a practical generator size for selected backup loads.
- Refrigerator Backup Time CalculatorEstimate refrigerator or freezer backup time using battery capacity, duty cycle, and inverter efficiency.
- UPS Runtime CalculatorEstimate UPS runtime for routers, computers, NAS devices, and other small electronics.
- CPAP Battery Backup CalculatorEstimate CPAP backup runtime and nights from battery capacity, humidifier use, reserve, and efficiency.
Methodology and disclaimer
This planner provides estimates for backup power sizing. It does not provide wiring instructions, fuel safety instructions, food safety guarantees, or medical advice.
The planner sums critical-load watt-hours per day, scales that by outage duration, and adjusts battery capacity for efficiency and reserve. Generator wattage uses the same surge logic as the calculation core: staggered mode adds the largest extra starting load, while conservative mode adds all extra starting loads, then applies 20% headroom and rounds up to the nearest 100 watts.