Outage Planning
12-Hour Outage Backup Plan
What should you actually keep running for half a day, and how much battery capacity does that require?
Starting values are hypothetical placeholders, not your appliances: replace them with your labels or watt-meter readings. Equivalent runtime means hours at full running watts within the 12-hour outage. 90% system efficiency is a planning assumption and 20% is Zaron's sizing margin; neither is a universal constant.
Calculate my full backup planQuick reference scenarios
Three Example 12-Hour Plans.
Planning examples built on hypothetical watts and equivalent hours shown on each card. They are not universal household values or product recommendations. Replace them with your own numbers.
Refrigerator + Wi-Fi + lights + phone charging
Refrigerator 150 W × 4 h · Wi-Fi 10 W × 12 h · Lights 40 W × 6 h · Phones 10 W × 2 h
≈ 1.31 kWh980 Wh raw ÷ 0.90 × 1.20Refrigerator + freezer + internet + lights + CPAP + limited kitchen use
Refrigerator 150 W × 4 h · Freezer 100 W × 4 h · Wi-Fi 10 W × 12 h · Lights 40 W × 6 h · CPAP 40 W × 8 h · Microwave 1000 W × 0.25 h
≈ 2.57 kWh1,930 Wh raw ÷ 0.90 × 1.20Core loads + fans + more lighting + home office
Refrigerator 150 W × 4 h · Freezer 100 W × 4 h · Wi-Fi 10 W × 12 h · Lights 60 W × 6 h · CPAP 40 W × 8 h · Microwave 1000 W × 0.25 h · Fans 50 W × 12 h · Home office 120 W × 6 h
≈ 4.49 kWh3,370 Wh raw ÷ 0.90 × 1.20The key concept
12 Hours Does Not Mean Every Appliance Runs for 12 Hours.
Refrigerators and freezers cycle. Sump and well pumps are intermittent. A microwave is high power but short duration. Wi-Fi and CPAP can be continuous. Heating or cooling support may dominate depending on the home. Zaron calls this equivalent runtime: the hours a load would run at full watts to use the same energy.
Hypothetical examples. Pumps are intermittent: measure yours.
× 12 Hvs. HIGH W × 5 MIN
12 Hours × Low Watts Can Matter More Than 5 Minutes × High Watts.
100 W for 12 hours is 1,200 Wh. A 1,500 W load for 5 minutes is 125 Wh. kWh is how much energy the battery stores; W or kW is how much power the inverter can deliver. You need enough of both.
What changes the result
Ten Things That Move the Number.
Climate
Hot or cold weather shifts cooling and heating demand.
Refrigerator / freezer duty cycle
Cycling, door openings and room temperature change energy use.
Heating or cooling needs
Can dominate the energy budget depending on the home.
Medical equipment
Device-specific watts and runtime; verify with the manufacturer.
Water pumps
Intermittent, motor-driven, startup matters.
Number of occupants
More people means more lights, charging and cooking.
Cooking strategy
A microwave is high power but short duration.
Charging other devices
Phones and laptops add up over 12 hours.
Inverter efficiency
Conversion losses are why Zaron divides by 0.90.
Appliance startup requirements
Verify from manufacturer specifications.
Outage priority strategy
Decide What Stays On.
Must stay on
Medical equipment, refrigeration, critical communications, water where applicable.
Strongly preferred
Internet, lights, fans, furnace blower, home office.
Optional high draw
Microwave, kettle, toaster, space heater, window AC.
Reducing optional high-power loads can dramatically reduce the system size you need.
Methodology
A Worked 12-Hour Example.
Hypothetical values for illustration. The refrigerator's 4 equivalent hours come from DOE's divide-by-three rule of thumb applied to 12 hours.
| Load | Watts | Equivalent runtime | Wh |
|---|---|---|---|
| Router / modem | 10 W | 12 h | 120 Wh |
| LED lighting | 40 W | 6 h | 240 Wh |
| Refrigerator | 150 W | 4 h | 600 Wh |
| CPAP (example) | 40 W | 8 h | 320 Wh |
| Phone charging | 10 W | 2 h | 20 Wh |
÷ 1,000 = 1.30 kWh raw
÷ 0.90 = 1.44 kWh
× 1.20 = 1.73 kWh recommended battery
Formula: raw Wh = Σ(watts × equivalent runtime hours). 90% efficiency and a 20% margin are Zaron planning assumptions, not universal constants.
1 kWh vs 2 kWh vs 5 kWh
What Different Battery Sizes Mean Over 12 Hours.
Arithmetic only: planning energy is nominal kWh × 0.90, divided by 12 hours. It is not a runtime claim for any product. Compare it with your own loads.
| Battery | Planning energy (90%) | Average constant load over 12 h |
|---|---|---|
| 1 kWh | 900 Wh | 75 W |
| 2 kWh | 1,800 Wh | 150 W |
| 5 kWh | 4,500 Wh | 375 W |
May cover only a lean plan. Our hypothetical minimal example needs about 1.31 kWh with margin.
More flexibility, depending on your loads.
May support a broader household plan, still limited by the inverter's output and voltage.
Battery vs generator
For 12 Hours, Batteries Can Be Practical.
Battery systems can be very practical for quiet essential loads over 12 hours. Heavy heating or cooling, or repeated high-power loads, may change the decision. Read the Battery Backup vs Generator guide →
Safety / technical cautions
Before You Rely on the Number.
- Zaron does not invent surge numbers. Verify startup requirements from manufacturer specifications.
- Do not confuse an appliance's circuit rating with its actual continuous draw.
- Do not backfeed a home electrical system. Connection and transfer equipment must follow manufacturer instructions and applicable electrical requirements.
- 120V vs 240V capability must match the equipment being powered. See the guide.
Evidence / sources
Where the Numbers Come From.
U.S. Department of Energy
Watts × hours method and the refrigerator divide-by-three rule of thumb.
ENERGY.GOV →FDA and USDA
An unopened refrigerator keeps food cold about 4 hours; a full freezer about 48 hours (24 if half full).
FDA → USDA FSIS →ENERGY STAR and manufacturers
Energy use for certified products, plus nameplates and manuals for watts, startup and voltage.
ENERGY STAR →FAQ
Straight Answers.
How much battery do I need for a 12-hour outage?
It depends on your loads. Multiply each load's watts by its equivalent runtime in hours, add them up, then divide the total Wh by 1,000, by 0.90 and multiply by 1.20 (Zaron planning assumptions). Use the calculator on this page.
Is 1 kWh enough for 12 hours?
Only for a lean plan. 1 kWh at 90% planning efficiency is about 900 Wh, or roughly 75 W of constant load over 12 hours. Add the 20% margin and your loads must be even lower. Verify your own watts.
Is 2 kWh enough?
It gives more flexibility. At 90% planning efficiency, 2 kWh is about 1,800 Wh, or roughly 150 W of constant load over 12 hours. Whether that covers your plan depends on your loads and on the inverter's output.
Can I run a refrigerator for 12 hours?
Often yes if the battery and inverter fit, because refrigerators cycle. DOE guidance is to divide plugged-in time by three to estimate hours at maximum wattage, so 12 hours gives about 4. Verify startup requirements with the manufacturer. The FDA says an unopened refrigerator keeps food cold about four hours.
Can I use a microwave during an outage?
Possibly, in short bursts. A microwave is a high-power, short-duration load. Check its label, the inverter's continuous and surge ratings and the voltage.
What appliances should I prioritize?
Start with Tier 1: medical equipment, refrigeration, critical communications and water where applicable. Then Tier 2: internet, lights, fans, furnace blower and office. Treat microwave, kettle, toaster, space heater and window AC as optional high-draw loads.
Do I need 240V backup?
Only if a load you need requires 240V. Many essentials are 120V, but verify each nameplate. See the 120V vs 240V guide.
Should I use a battery or a generator?
For a 12-hour outage, battery systems can be practical for quiet essential loads. Heavy heating or cooling or repeated high-power loads may change the decision. See the Battery Backup vs Generator guide.
How do I account for refrigerator cycling?
Use the refrigerator's label (annual kWh) or a plug-in watt meter to estimate equivalent runtime. DOE's rule of dividing plugged-in time by three is a starting point, not a guarantee.
What if I have a CPAP or medical device?
This page is electrical planning only, not medical advice. Check the device's power requirements with its manufacturer and discuss backup needs with your healthcare provider. Do not rely on a planning estimate for critical equipment.
Keep sizing
Size Each Load.
Plan ahead
Build the Backup Plan
Before the Power Goes Out.
Combine loads, runtime, voltage and startup checks in one place.
Calculate my full backup plan →