Home Battery Sizing Guide
A home battery backup system keeps essential circuits running during grid outages. Unlike a UPS that provides minutes of runtime, a properly sized home battery can sustain critical loads for hours or even days. This guide walks through the process of identifying critical loads, calculating energy needs, and selecting the right battery capacity.
Step 1: Identify Critical Loads
Not every circuit in your home needs backup. Identifying critical loads keeps the battery system affordable and manageable. Critical loads are the devices and circuits that must remain powered during an outage — either for safety, health, or basic habitability.
| Load Category | Typical Power | Hours/Day | Energy (Wh) |
|---|---|---|---|
| Refrigerator | 150W | 8h (40% duty) | 1,200 |
| LED lighting (5 rooms) | 50W | 6h | 300 |
| Wi-Fi router + modem | 20W | 24h | 480 |
| Medical equipment (CPAP, oxygen) | 100W | 8h | 800 |
| Phone / laptop charging | 60W | 3h | 180 |
| Well pump (if applicable) | 750W | 1h | 750 |
| Garage door opener | 350W | 0.1h | 35 |
Step 2: Calculate Total Critical Load
Sum the energy consumption for all critical loads. This gives you the daily energy requirement in watt-hours. For backup planning, you also need to decide how many hours of coverage you want — overnight (12h), a full day (24h), or multiple days (48–72h).
Home Backup Sizing Formula
System efficiency for a home battery with inverter is typically 85–90%. DoD depends on chemistry: 80% for LFP, 50% for lead-acid. Coverage hours is how long you want the battery to sustain the loads without any recharge input.
Step 3: Select System Voltage
Home battery systems typically operate at 48V for systems above 3 kW. Lower voltages (12V or 24V) are possible for small systems but require thick cables and produce higher conduction losses. Most residential battery products (Tesla Powerwall, Enphase, SolarEdge) operate at 400V DC internally with a built-in inverter.
| System Voltage | Max Practical Load | Best For |
|---|---|---|
| 12V DC | ~1.5 kW | Small cabin, RV, very limited backup |
| 24V DC | ~5 kW | Small home, essential circuits only |
| 48V DC | ~15 kW | Standard residential, most home backup |
| 400V DC | 100+ kW | Integrated battery-inverter systems |
Worked Example: Essential Loads Overnight
Scenario: Size a battery to run essential household loads for 14 hours overnight during a winter power outage.
Given critical loads:
- Refrigerator: 150W × 8h (cycling) = 1,200 Wh
- LED lighting: 50W × 6h = 300 Wh
- Wi-Fi + modem: 20W × 14h = 280 Wh
- CPAP machine: 100W × 8h = 800 Wh
- Phone charging: 15W × 3h = 45 Wh
- Gas furnace fan: 300W × 14h = 4,200 Wh
Total critical load:
Step 1: Apply system parameters:
- Coverage: 14 hours overnight
- DoD limit: 80% (LFP)
- System efficiency: 88% (inverter + wiring)
Step 2: Required battery capacity:
Step 3: At 48V system voltage:
Step 4: Select battery. A 48V 200Ah LFP battery (9.6 kWh) meets the requirement almost exactly. For margin against capacity fade and colder temperatures, a 48V 280Ah battery (13.4 kWh) provides 38% headroom — recommended for systems that must be reliable through harsh winters.
Critical vs Full-Home Comparison
The difference between critical-load backup and whole-home backup is dramatic in both capacity and cost. Understanding this tradeoff helps set realistic expectations.
| Parameter | Critical Loads | Full Home |
|---|---|---|
| Daily energy | 3–8 kWh | 20–60 kWh |
| Battery capacity needed | 10–20 kWh | 40–120 kWh |
| 1-day autonomy cost (est.) | $5,000–$10,000 | $20,000–$60,000 |
| Weight (LFP) | 80–160 kg | 320–960 kg |
| Installation complexity | Subpanel, critical loads panel | Whole-home transfer switch |
Solar Integration
Pairing a home battery with solar panels transforms the system from a finite energy store into a replenishable one. During a daytime outage, solar panels recharge the battery while it simultaneously powers loads. This dramatically extends effective runtime — from hours to potentially unlimited days, weather permitting.
With Solar (5 kW array)
A 5 kW solar array generates 15–25 kWh/day depending on location. If daily critical loads are 7 kWh, solar produces a 2–3× surplus during daylight. The battery only needs to cover overnight loads (3–5 kWh), allowing a much smaller and cheaper battery bank.
Without Solar
Without solar, the battery must carry the full load for the entire outage duration. A 3-day outage at 7 kWh/day requires 21 kWh of usable battery capacity — a 26 kWh installed bank at 80% DoD. This is significantly more expensive than the solar-supplemented approach.
Try It
Use the Home Backup Calculator to input your critical loads and get a battery recommendation.
Open Home Backup CalculatorGeneral Sizing
For a complete sizing calculation with all parameters, use the Battery Sizing Calculator.
Open Battery Sizing CalculatorRelated Articles
What Is Battery Sizing?
Overview of the sizing process, why it matters, and key design considerations.
Read Guide →How Many kWh Do I Need for My House?
Understanding whole-home energy consumption and what it means for battery sizing.
Read Guide →Frequently Asked Questions
How big of a battery do I need for home backup?
For essential loads only (refrigerator, lights, router, medical devices), a 10–15 kWh battery covers 1–2 days. For whole-home backup including HVAC and appliances, you need 20–40+ kWh. The right size depends on your daily consumption, how many days of backup you want, and whether you have solar to recharge during the outage.
What is the difference between critical loads and full-home backup?
Critical loads are essential circuits — refrigerator, medical equipment, lighting, communication devices, well pump. These typically total 3–8 kWh/day and can be backed up with a moderate battery. Full-home backup includes HVAC, electric range, dryer, EV charging — pushing daily consumption to 20–60 kWh and requiring a much larger (and more expensive) battery system.
How long will a home battery last during an outage?
A 10 kWh battery running 5 kWh of critical loads lasts about 20 hours (at 80% usable capacity). The same battery running 15 kWh of full-home loads lasts about 3.2 hours. Runtime scales linearly with load — reducing consumption extends duration proportionally.
Should I pair my home battery with solar panels?
Solar panels extend battery runtime by recharging during daylight hours. A 5 kW solar array can generate 15–25 kWh per day depending on location and season. With solar, a smaller battery can sustain loads through multi-day outages because it is being replenished each day. Without solar, the battery must carry the entire load from its stored capacity alone.