Battery Backup Calculator

Size your home battery backup system in Kilowatt-hours (kWh) and Amp-Hours (Ah). Accurately calculate runtime for critical home appliances, emergency power outages, LiFePO4 vs Lead-Acid, inverter sizing, and solar recharge requirements.

Quick Outage Sizing Presets:
Battery Sizing Inputs (* = Required) Real-time Calculation

Select Essential Home Appliances to Backup *

Check items to include in your emergency backup bank:

Appliance
Running W
Run Hrs / Day
Continuous Running:
650 W
Max Starting Surge:
1,800 W
Daily Energy Used:
6,420 Wh/day
Backup Duration & Battery System Architecture
24 Hours (1 Day)

LiFePO4 offers 4,000+ cycles; Lead-Acid degrades quickly below 50% discharge.

Higher voltage reduces wire gauge thickness and inverter I²R heat losses.

Battery Storage Sizing Results

Engineered for US residential & off-grid backup standards (NEC 706)

48V LiFePO4 System
Recommended Gross Battery Capacity:
8.2 kWh
= 171 Amp-Hours (Ah) @ 48V DC
Net Usable Energy Storage: 7.4 kWh (7,380 Wh)
Min. Continuous Inverter Size: 1.0 kW (1,000 W)
Min. Peak Surge Inverter Size: 2.2 kW (2,200 W)
Real-World Battery Pack Equivalent Sizing:
48V Server-Rack LiFePO4 (5.12 kWh)
2 x Batteries
e.g. EG4-LL, Ruixu, SOK 48V 100Ah
Whole-Home ESS (13.5 kWh)
1 x Powerwall
e.g. Powerwall 3 / FranklinWH / Enphase
12V 100Ah Deep-Cycle Units
8 x Batteries
4 in series x 2 parallel
Electrical & Energy Metrics
Daily AC Load Consumption: 6.42 kWh/day
Backup Autonomy Target: 24.0 Hours
Inverter Inefficiency Loss: ~713 Wh (10%)
Continuous Battery DC Current: 15.0 Amps DC
Max Surge Battery DC Current: 41.7 Amps DC
Solar & Generator Replenishment
Min. Solar Array Size (4.5 Peak Sun Hrs): 2,050 Watts PV
Typical Solar Panels Needed: 5 x 400W Solar Panels
Generator Full Recharge Time (at 50A): ~3.5 Hours
Estimated Battery Lifespan: 10–15 Years (LiFePO4)
Recommended DC Fuse / Breaker: 60A – 80A Class T
Live Sizing Formula Calculation:
Gross kWh = (6,420 Wh) ÷ (0.90 Inverter Eff × 0.90 DoD × 1.00 Temp) × 1.15 Buffer = 8.2 kWh (171 Ah @ 48V)
NEC 706 & IEEE 485 Compliant Sizing

How to Calculate Battery Backup Sizing (Formulas & Steps)

Calculating battery storage requirements for power outages requires more than simply multiplying continuous watts by run hours. Real-world systems lose power in the DC-to-AC inverter, cannot safely discharge 100% of their chemical capacity without damage, and experience reduced output in cold garage or outdoor environments.

Step 1: Calculate Total AC Energy (Wh)
Energy (Wh) = Continuous Load (Watts) × Autonomy Hours

For multiple appliances, sum each appliance's running wattage multiplied by its daily operating hours.

Step 2: Adjust for Inverter & Depth of Discharge
Gross Wh = Energy (Wh) ÷ (Inverter Eff × DoD × Temp Factor)

Inverters lose 8%–12% as heat. Lithium batteries allow 90% DoD, while Lead-Acid batteries allow 50% DoD.

Step 3: Convert to Amp-Hours (Ah)
Amp-Hours (Ah) = Gross Watt-Hours (Wh) ÷ Bank Voltage (V)

Calculates total amp-hour rating at your system's DC bus voltage (12V, 24V, or 48V DC).

Step 4: Size Inverter Continuous & Surge
Inverter Cont. ≥ Peak Simultaneous Load × 1.25

Ensure the pure sine wave inverter can deliver locked-rotor starting current for refrigerators and sump pumps.

Battery Chemistry Comparison for US Home Energy Storage

Comparing common battery storage technologies for home backup, emergency UPS, and off-grid solar systems:

Battery Chemistry Usable DoD Cycle Life Round-Trip Efficiency Weight per 10kWh Best Use Case
Lithium Iron Phosphate (LiFePO4) 90% – 95% 4,000 – 6,000 95% ~220 lbs Modern home backup, server-rack ESS, daily solar cycling
Lithium Nickel Manganese (NMC) 85% – 90% 2,500 – 3,500 92% ~180 lbs High-energy density wall packs (e.g. Tesla Powerwall 2)
Sealed AGM Lead-Acid 50% 500 – 800 80% – 85% ~650 lbs Budget emergency backup, infrequent grid outage float service
Gel Cell Deep Cycle 50% 600 – 1,000 82% ~660 lbs Deep discharge marine, RV, remote off-grid telecommunications
Flooded Lead-Acid (FLA) 50% 400 – 700 75% – 80% ~700 lbs Lowest upfront cost; requires regular distilled water topping

Typical US Household Appliance Power Ratings & Surge Demands

Average running and surge wattage for standard 120V household loads during an electrical blackout:

Refrigerator / Freezer
150W Running | 800W Surge
~1.2 – 1.8 kWh / day
Wi-Fi Router & Modem
15W Running | 15W Surge
~0.36 kWh / day (24h)
CPAP Machine (Heated)
40W – 75W Running | 100W Surge
~0.4 – 0.6 kWh / 8hr night
1/2 HP Sump Pump
800W Running | 2,200W Surge
~1.6 kWh / day (intermittent)
Gas Furnace Blower Fan
400W Running | 1,200W Surge
~2.4 kWh / day (winter duty)
55" LED Television
100W Running | 120W Surge
~0.5 kWh / day (5h)

Frequently Asked Questions (FAQ)

How many kWh of battery backup do I need for a whole house?

An average US single-family home consumes roughly 25 to 30 kWh of electricity per day. For a full 24-hour blackout without utility power or solar panels, you need between 20 kWh and 30 kWh of usable battery capacity (or two 13.5 kWh home battery packs like the Tesla Powerwall or FranklinWH). If you only power critical circuits (refrigeration, lights, router, gas furnace blower), 5 kWh to 10 kWh is usually sufficient.

Why is 48V DC preferred over 12V DC for home battery backup systems?

To deliver 3,000 Watts through an inverter on a 12V system requires 250 Amps DC, demanding extremely heavy and expensive 4/0 AWG copper cables and creating severe heat dissipation. On a 48V system, that same 3,000 Watts requires only 62.5 Amps DC, allowing thinner #4 AWG wire, safer circuit breakers, and significantly higher round-trip efficiency.

Can I recharge my battery backup bank with a portable gas or dual-fuel generator?

Yes! Modern hybrid inverter-chargers (such as Sol-Ark, Schneider, EG4, or Victron MultiPlus) feature dedicated generator AC inputs. A 4 kW to 7.5 kW generator running for just 2 to 4 hours per day can completely replenish a 10 kWh to 15 kWh battery bank, allowing you to turn off the noisy generator at night while maintaining 24/7 continuous silent battery power.