Home Power Outage Battery Backup Calculator
Calculate what battery size, inverter size, and backup power system you need to run essential appliances during a power outage, storm, hurricane, wildfire outage, or emergency grid failure.
Appliance Loads
Select appliances to add them to your backup plan.
Backup Settings
How long you need backup power during the outage.
12V for small portable setups, 24V for mid-size, 48V for whole-home.
LiFePO4 recommended: safest, longest life, deepest discharge.
Auto-set by chemistry. LiFePO4: 80%, Lead-Acid: 50%. You can override.
Modern pure sine wave inverters: 90–95%. Use 90% for conservative sizing.
Extra capacity buffer for temperature, aging, and unexpected loads.
Total wattage of solar panels connected to charge controller.
Average daily peak sun hours for your location. U.S. average: 4–6 hours.
MPPT controllers: 95–98%. PWM controllers: 80–90%.
Backup System Results
Suggested Battery Configurations
Solar Recharge Estimate
Planning Notes
Add appliances above to calculate your backup power requirements. Results update instantly as you adjust inputs.
Mathematical Formulas
The calculation follows a standard energy-chain approach from load to battery bank:
Surge is calculated as the highest single appliance startup surge plus the running load of all other appliances—not all surges stacked simultaneously.
Formulas & Worked Examples
Example 1: Essential Loads — 8 Hour Outage
Keep Wi-Fi, phone charging, a few lights, and a laptop running through an overnight outage (8 hours).
Given Values
- Wi-Fi Router: 12W
- Laptop: 60W
- LED Lights ×2: 20W
- Phone Charger: 15W
- Backup Duration: 8 hours
Step 1: Total running load
= 107W
Step 2: Energy for 8 hours
= 856 Wh
Step 3: After inverter efficiency (90%)
= 951 Wh
Step 4: With 20% margin
= 1,141 Wh
Step 5: Battery (12V LFP, 80% DoD)
= 119 Ah → 12V 120Ah LFP
Final Answer
A 12V 120Ah LiFePO4 battery (1,536 Wh) covers essential overnight loads with comfortable margin.
Light essential loads are affordable to back up — a single 12V battery handles them easily.
Example 2: Full Home Backup — 24 Hour Outage
Keep fridge, freezer, Wi-Fi, lights, sump pump, and phone charging running for a full day (24 hours).
Given Values
- Fridge + Freezer: 270W (cycling avg ~110W)
- Wi-Fi Router: 12W
- LED Lights ×4: 40W
- Sump Pump (intermittent): 800W (avg ~50W)
- Phone + Laptop: 75W
- Backup Duration: 24 hours
Step 1: Average running load
= 287W average
Step 2: Energy for 24 hours
= 6,888 Wh
Step 3: After inverter efficiency (90%)
= 7,653 Wh
Step 4: With 20% margin
= 9,184 Wh
Step 5: Battery (48V LFP, 80% DoD)
= 239 Ah → 48V 250Ah LFP
Final Answer
A 48V 250Ah LiFePO4 battery bank (12 kWh) covers full-home essential loads for 24 hours.
Whole-day backup requires serious capacity — 48V systems are most efficient for loads this size.
Example 3: Critical Medical — 48 Hour Outage
Power a CPAP machine, oxygen concentrator, phone, Wi-Fi, and a few lights for 2 full days.
Given Values
- CPAP Machine: 40W
- Oxygen Concentrator: 300W
- Wi-Fi Router: 12W
- LED Lights ×2: 20W
- Phone Charger: 15W
- Backup Duration: 48 hours
Step 1: Total running load
= 387W
Step 2: Energy for 48 hours
= 18,576 Wh
Step 3: After inverter efficiency (90%)
= 20,640 Wh
Step 4: With 20% margin
= 24,768 Wh
Step 5: Battery (48V LFP, 80% DoD)
= 645 Ah → 48V 650Ah LFP
Final Answer
A 48V 650Ah LiFePO4 battery bank (31.2 kWh) powers critical medical equipment for 48 hours.
Medical equipment demands serious capacity — solar recharge is essential for multi-day medical backup.
U.S. Power Outage Planning Examples
Basic Internet Backup
Keep your home connected during an outage.
- • Wi-Fi Router: 12W
- • Laptop: 60W
- • LED Lights ×2: 20W
Total: 92W for 8 hours = 736 Wh → 12V 80Ah LFP battery
Food Protection Backup
Keep refrigerator and freezer cold during extended outages.
- • Refrigerator: 150W
- • Freezer: 120W
- • Wi-Fi Router: 12W
Total: 282W for 12 hours = 3,760 Wh → 24V 200Ah LFP battery
Medical Device Backup
Power CPAP and essential communication overnight.
- • CPAP Machine: 40W
- • Wi-Fi Router: 12W
- • Phone Charger: 15W
- • LED Light: 10W
Total: 77W for 10 hours = 963 Wh → 12V 100Ah LFP battery
Storm Emergency Backup
Comprehensive backup for severe weather events.
- • Refrigerator: 150W
- • Freezer: 120W
- • Sump Pump: 800W
- • LED Lights ×3: 30W
- • Wi-Fi Router: 12W
Total: 1,112W for 12 hours = 16,680 Wh → 48V 440Ah LFP system
Appliance Wattage Reference Table
| Appliance | Running Watts | Surge Watts | Notes |
|---|---|---|---|
| Refrigerator | 150 W | 600 W | Cycles 30–50% of time |
| Freezer | 120 W | 500 W | Cycles 30–40% of time |
| Wi-Fi Router | 12 W | 12 W | Continuous draw |
| LED Bulb | 10 W | 10 W | Per bulb |
| Laptop | 60 W | 90 W | Charging + running |
| TV (LED/LCD) | 100 W | 150 W | 55" class typical |
| CPAP Machine | 40 W | 80 W | With humidifier |
| Box Fan | 50 W | 75 W | 20" box fan |
| Sump Pump | 800 W | 2,000 W | ½ HP typical |
| Microwave | 1,000 W | 1,500 W | 1,000W model |
| Phone Charger | 15 W | 15 W | USB-C fast charge |
| Coffee Maker | 900 W | 1,200 W | Drip 12-cup |
LiFePO4 vs Lead-Acid for Backup Power
| Feature | LiFePO4 | AGM / Sealed | Flooded Lead-Acid |
|---|---|---|---|
| Usable Depth of Discharge | 80–100% | 50% | 50% |
| Cycle Life | 3,000–6,000 | 300–500 | 200–400 |
| Weight | Lightest | Heavy | Heaviest |
| Maintenance | None | None | Water refill |
| Cold Weather Performance | Good | Poor | Poor |
| Indoor Installation | Safe | Ventilation needed | Ventilation required |
| Upfront Cost | Higher | Lower | Lowest |
| Lifetime Cost per Cycle | Lowest | Higher | Highest |
Safety Notes
⚠ Never connect a portable generator or battery system to your home wiring without a proper transfer switch. Backfeed can electrocute utility workers restoring power.
⚠ Lithium batteries should be installed away from living spaces per local fire codes. LiFePO4 is the safest lithium chemistry but always follow manufacturer installation guidelines.
⚠ Do not exceed the continuous or surge rating of your inverter. Overloaded inverters can overheat, shut down, or cause fire. Add 25% headroom to your continuous load estimate.
⚠ Lead-acid batteries emit hydrogen gas during charging. Install in well-ventilated areas away from ignition sources. Never charge a frozen lead-acid battery.
Home Backup Power System Flow
Grid or solar charges the battery bank, which feeds the inverter to power your essential home circuits via a transfer switch.
Home Backup Outage Scenarios Compared
| Scenario | Key Loads | Duration | Avg Draw | Battery Size | Est. Cost |
|---|---|---|---|---|---|
| Essential 8h | Wi-Fi, lights, phone, laptop | 8 hours | ~110W | 12V 120Ah (1.5 kWh) | $800–$1,200 |
| Food Protection 24h | Fridge, freezer, Wi-Fi, lights | 24 hours | ~290W | 24V 200Ah (4.8 kWh) | $2,500–$3,500 |
| Medical Critical 48h | CPAP, O₂ concentrator, Wi-Fi, lights | 48 hours | ~390W | 48V 650Ah (31 kWh) | $12,000–$18,000 |
| Storm Emergency 12h | Fridge, sump pump, Wi-Fi, lights | 12 hours | ~350W | 24V 250Ah (6 kWh) | $3,500–$5,000 |
| Full Home 24h | All essential circuits | 24 hours | ~1,500W | 48V 500Ah (24 kWh) | $10,000–$15,000 |
| Multi-Day + Solar | Essentials with daily recharge | 72+ hours | ~300W | 48V 200Ah + 1,200W solar | $8,000–$12,000 |
Costs are approximate for LiFePO4 battery systems including inverter. Prices vary significantly by brand and installation complexity.
Frequently Asked Questions
What size battery do I need for a power outage?
For basic internet and phone backup (Wi-Fi router + laptop + lights for 8 hours), you need approximately 300–500 Wh. For food protection (refrigerator + freezer for 12 hours), plan for 2,000–4,000 Wh. For comprehensive emergency backup with medical devices, 3,000–8,000 Wh covers most critical household loads for 24 hours.
How long will a 100Ah battery run a refrigerator?
A 12V 100Ah LiFePO4 battery (1,280 Wh) can run a standard refrigerator for approximately 8–12 hours. The refrigerator compressor cycles on and off, so actual runtime depends on ambient temperature, fridge efficiency, and how often the door is opened. In warm conditions with frequent door openings, expect closer to 6–8 hours.
Is LiFePO4 better than lead-acid for home backup?
Yes, for emergency backup power. LiFePO4 provides 80–100% usable depth of discharge vs 50% for lead-acid, lasts 3,000–6,000 cycles vs 300–500 for lead-acid, charges faster, weighs 60% less, and requires zero maintenance. The upfront cost is higher but the lifetime cost per cycle is significantly lower.
What size inverter do I need for a refrigerator?
A standard refrigerator needs a 1,500–2,000W continuous inverter with at least 3,000W surge capacity. The running load is typically 150W, but the compressor startup surge can reach 600W or more. If you're running additional appliances, add their running watts and the largest single surge load to size your inverter.
Can a portable power station run a fridge?
Yes, a portable power station rated at 1,500W+ continuous with 3,000W+ surge can run a standard refrigerator. You'll need a unit with at least 1,000–1,500 Wh capacity for 8–12 hours of backup. Popular options include the EcoFlow Delta 2 (1,024 Wh), Bluetti AC200MAX (2,048 Wh), and Goal Zero Yeti 1500X (1,516 Wh).
How much battery backup do I need for a CPAP machine?
A typical CPAP machine draws 30–60W with humidifier (10–30W without). For 8 hours of sleep, you need 240–480 Wh. A 12V 50Ah LiFePO4 battery (640 Wh) provides a full night's CPAP power. Without the humidifier, a 12V 20Ah battery (256 Wh) can last one night.
Should I use 12V, 24V, or 48V for home backup?
For home backup systems over 1,000W, use 24V or 48V. Higher voltage reduces current, allowing thinner wires and lower losses. 48V is ideal for systems over 3,000W and is the standard for modern home battery systems. 12V is practical only for small portable setups under 1,000W.
Can solar panels recharge my battery during an outage?
Yes, if you have a hybrid inverter with battery input. A 400W solar panel array in 4–5 peak sun hours can generate 1,600–2,000 Wh daily—enough to partially recharge a home battery or extend backup runtime significantly. This is the most effective strategy for multi-day outage survival.
Why does my battery not last as long as the calculator says?
Common reasons: battery age and degradation (capacity drops 20–30% over years), cold weather reducing capacity 10–30%, higher actual surge draws than rated, wiring losses, inverter standby power draw, and appliance cycling patterns that differ from assumptions. This calculator provides conservative estimates—actual results vary.
What appliances should I avoid running on battery backup?
Avoid high-draw resistive loads: air conditioners (1,500–5,000W), electric stoves (2,000–5,000W), electric water heaters (4,500W), clothes dryers (2,500–5,000W), and space heaters (1,500W). These drain batteries rapidly and require impractically large systems. Focus on critical loads: refrigeration, lighting, medical devices, communication, and small electronics.
What Is Home Power Outage Battery Backup?
Why This Calculation Matters
→ U.S. power outages have increased 67% over the past decade—storms, wildfires, and grid strain make backup power a practical necessity in many regions.
→ Undersized battery systems leave you without refrigeration or medical device power during multi-day outages. A full fridge costs $300–$600 to restock if food spoils.
→ Oversized systems waste money—every unnecessary 5 kWh of LFP capacity adds $2,000–$4,000 in cost. Right-sizing saves thousands.
→ Ignoring surge loads from appliances like sump pumps and refrigerators can trip your inverter at the worst possible moment—proper surge sizing prevents mid-outage failures.
→ Not planning for temperature extremes means battery capacity drops 10–30% in cold weather, leaving you short when you need backup most during winter storms.
Practical Applications
Hurricane & Storm Preparedness
Size a battery system to maintain critical loads through 24–72 hour outages common during hurricanes, nor'easters, and severe thunderstorms.
Wildfire Public Safety Power Shutoffs
Prepare for planned grid shutoffs during high fire danger. California and western U.S. utilities regularly de-energize lines to prevent ignition.
Medical Equipment Backup Power
Ensure sufficient battery runtime for CPAP machines, oxygen concentrators, nebulizers, and other life-sustaining medical devices.
Winter Storm & Ice Storm Backup
Keep heating controls, sump pumps, refrigeration, and communication equipment running during ice storms and blizzards that knock out power for days.
Portable Power Station Sizing
Use this calculator to match a portable power station or solar generator to your specific emergency load requirements before purchasing.
Fridge & Freezer Protection
Calculate exactly how long a battery can keep your refrigerator and freezer running to prevent food spoilage during extended outages.
Common Mistakes to Avoid
✗ Undersizing the inverter for surge loads — sump pumps, refrigerators, and microwaves draw 2–3× running wattage during startup, tripping cheap inverters.
✗ Ignoring inverter efficiency losses — a 10–15% DC-to-AC conversion loss means a 1,000Wh battery delivers only 850–900Wh of usable AC energy.
✗ Sizing for running wattage without accounting for appliance duty cycles — refrigerators cycle 30–50% of the time, so continuous running estimates are conservative.
✗ Using lead-acid batteries at full rated capacity — lead-acid should not exceed 50% DoD, requiring double the名义 capacity versus LiFePO4.
✗ Forgetting safety margin for battery aging and temperature — cold weather reduces capacity 10–30%, and capacity degrades 20–30% over battery lifetime.
✗ Not planning for transfer switch installation — connecting a battery system to home wiring without a transfer switch can electrocute utility workers.
✗ Oversizing the battery bank without considering recharge strategy — a 10 kWh battery without solar or generator recharge is a one-time-use system.
✗ Running high-draw resistive loads from battery — air conditioners, electric stoves, and water heaters (1,500–5,000W) drain batteries in hours.
Why Trust These Calculations?
This calculator uses standard electrical engineering formulas for battery system sizing, aligned with UL 1741 inverter standards and IEEE 1547 interconnection guidelines. All calculations are transparent, derived from publicly available appliance wattage data and manufacturer specifications. The math is fully verifiable.
View our methodology and formula derivations →Fridge Battery Backup Calculator
Battery sizing specifically for refrigerator backup during outages.
CPAP Battery Backup Calculator
Size batteries for overnight CPAP machine power.
Sump Pump Battery Backup Calculator
Keep your sump pump running during storm outages.
Portable Power Station Runtime Calculator
See how long a portable power station runs your loads.
Solar Generator Sizing Calculator
Size a complete solar backup system with panels and battery.
Battery Runtime Calculator
Estimate discharge time from capacity and load.
Battery Sizing Calculator
Full battery bank sizing with temperature derating.
Solar Battery Sizing Calculator
Size batteries paired with solar panel arrays.
Energy Conversion Calculator
Convert between Ah, Wh, and kWh at any voltage.
Charging Time Calculator
Estimate charge time from solar or grid.
Voltage Drop Calculator
Size wire runs to minimize voltage drop.
DC Cable Loss Calculator
Calculate wiring losses in DC connections.
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