Understanding Battery Backup Runtime: A Complete Guide
When the power goes out, your battery backup becomes your lifeline. But that "1000Wh" specification on the box doesn't tell the whole story. Understanding how to calculate realistic runtime ensures you're prepared—not surprised—when you need backup power most.
This guide explains the math behind battery runtime, helps you understand which appliances matter during an outage, and shows you how to prioritize power usage to extend your backup duration. Whether you live in an apartment with a small power station or a house with a whole-home battery system, these principles apply universally.
The Runtime Formula: What the Calculator Does
Battery runtime follows a straightforward formula, but real-world factors complicate it:
Runtime (hours) = (Battery Capacity × Efficiency) ÷ Total Load Where efficiency is typically 0.85 (85%) for inverter-based systems
For example, a 1000Wh battery powering a 100W constant load:
- Usable capacity: 1000 × 0.85 = 850Wh
- Runtime: 850 ÷ 100 = 8.5 hours
The 15% efficiency loss accounts for the inverter converting DC battery power to AC household power. Some high-end units achieve 90%+ efficiency, while older or cheaper units may be closer to 80%.
Why Advertised Capacity Doesn't Equal Usable Capacity
Every battery system loses power through:
- Inverter conversion: 10-20% loss converting DC to AC
- Temperature effects: Cold reduces capacity; heat accelerates degradation
- Depth of discharge: Many systems stop at 10-20% to protect battery longevity
- Age: Lithium batteries lose 2-3% capacity per year
Our calculator uses 85% as a conservative middle ground. If your power station is new and high-quality, you might see slightly better results. If it's older or operating in extreme temperatures, expect less.
Appliance Power Draw: Understanding Watts vs. Watt-Hours
Confusion between watts (W) and watt-hours (Wh) causes most runtime calculation errors. Here's the distinction:
Watts (W) measure instantaneous power draw—how much electricity a device uses at any given moment. A 60W light bulb draws 60 watts while it's on.
Watt-hours (Wh) measure energy consumption over time. That same 60W bulb running for 5 hours consumes 300Wh of energy (60 × 5 = 300).
Average vs. Peak Wattage
Many appliances don't draw constant power. Refrigerators cycle their compressors on and off. A fridge rated at "150W" might draw 150W when the compressor runs but 0W when it's coasting. Over 24 hours, average consumption is typically 50-80W—far less than the nameplate suggests.
Our calculator uses realistic average wattages, not peak ratings. This gives you practical runtime estimates rather than worst-case scenarios that rarely occur.
The High-Draw Problem: Space Heaters and AC Units
Space heaters (1500W), window air conditioners (1200W), and similar high-draw appliances present a fundamental challenge: they drain batteries extremely fast.
A 1500W space heater running from a 2000Wh battery:
- Usable capacity: 2000 × 0.85 = 1700Wh
- Runtime: 1700 ÷ 1500 = 1.1 hours
This isn't a battery failure—it's physics. High-wattage devices simply consume energy faster than batteries can reasonably supply for extended periods. For heating and cooling during extended outages, consider propane heaters, extra blankets, or generator backup instead of battery power.
Priority Power Planning
When battery capacity is limited, prioritization becomes critical. Not all power uses are equally important during an emergency.
Tier 1: Life Safety (Always Power)
- Medical devices: CPAP, oxygen concentrators, medication refrigeration
- Communication: Phone charging (at least one charged phone)
- Lighting: Minimal LED lighting for safety
Tier 2: Critical Comfort (Power If Possible)
- Refrigeration: Food preservation reduces waste and ensures nutrition
- WiFi router: Enables communication, information access, and work
- Fans: Low-power comfort in heat (30-50W vs. 1500W for AC)
Tier 3: Convenience (Power If Abundant)
- TV/Entertainment: Morale matters in extended outages
- Full lighting: Beyond minimum safety lighting
- Charging all devices: Multiple phones, tablets, etc.
By focusing Tier 1 needs first, you ensure critical systems remain powered even if the outage extends beyond your battery capacity.
Solar Integration: Extending Runtime Indefinitely
Solar panels transform your battery from a fixed-duration backup into a renewable power system. During daylight, panels recharge what you've used overnight, potentially providing unlimited runtime for moderate loads.
How Solar Production Works
A 200W solar panel doesn't produce 200 watts continuously. Production varies by:
- Peak sun hours: Most U.S. locations average 4-6 hours of "full sun equivalent" daily
- Weather: Clouds reduce output 50-90%
- Angle: Optimal orientation increases production 20-30%
- Temperature: Hot panels produce slightly less
A realistic estimate: 200W panel × 5 peak hours × 0.8 efficiency = 800Wh daily production.
Solar + Battery Math
If your daily consumption is 600Wh and your solar produces 800Wh, you're generating more than you use—sustainable indefinitely during sunny periods.
If consumption is 1200Wh and production is 800Wh, you're drawing 400Wh net from the battery daily. A 2000Wh battery would last about 4 additional days beyond the first day's solar contribution.
Our calculator incorporates solar input, assuming 5 peak sun hours, to show how panels extend your effective runtime.
Apartment vs. House Considerations
Living situation significantly impacts backup power planning.
Apartment Dwellers
Advantages:
- Smaller space = lower lighting needs
- No sump pump concerns
- Often connected to building emergency systems
- Neighbors may share resources
Challenges:
- Limited solar panel placement (balcony only, if allowed)
- May have HOA/lease restrictions on generators
- Shared building systems out of your control
Recommended minimum: 500-1000Wh power station for phone, lights, router, and one night of CPAP if needed.
House Dwellers
Advantages:
- Roof solar potential
- Generator options (distance from neighbors)
- Full control over systems
Challenges:
- Larger refrigeration needs (full-size fridge + freezer)
- Potential sump pump requirements
- More lighting area
- Well pump if not on municipal water
Recommended minimum: 2000Wh+ power station, or whole-home battery system (10-20kWh) for serious preparation.
Medical Device Prioritization
For households with medical equipment, power backup isn't convenience—it's potentially life-critical.
CPAP Machines
CPAP power draw varies significantly:
- Basic CPAP: 30-40W (sufficient for 25+ hours on 1000Wh)
- With heated humidifier: 50-70W
- With heated tubing: Add 20-30W
Tip: Many CPAPs can run from 12V DC directly, bypassing the inverter and gaining 15% efficiency. Check if your machine has a DC adapter option.
Oxygen Concentrators
Portable oxygen concentrators draw 100-300W depending on flow rate. Calculate carefully and maintain significant buffer capacity. Consider compressed oxygen backup for critical needs.
Medication Refrigeration
Insulin and other temperature-sensitive medications require consistent cooling. A small medical-grade cooler (40-60W) may be more efficient than running a full refrigerator just for medications.
Extending Battery Life
Beyond calculating runtime, simple strategies stretch your backup duration:
Reduce Phantom Loads
Devices draw power even when "off." Unplug chargers when not actively charging. Use power strips to fully disconnect non-essential items.
Refrigerator Optimization
- Keep fridge full (thermal mass maintains temperature longer)
- Don't open door unnecessarily (each opening loses cold air)
- A full freezer stays frozen 48 hours; half-full lasts 24 hours
- Consider running fridge only 15 minutes per hour during outages
LED Everything
If you're still using incandescent or CFL bulbs, switch to LED. A 10W LED produces light equivalent to a 60W incandescent—6x more efficient.
Time-Shift Usage
If you have solar, run high-draw appliances during peak sun hours when panels are producing. Charge devices midday. Save battery for overnight essential use.
When Battery Backup Isn't Enough
Battery backup excels for short outages and low-to-moderate loads. For extended outages or high-power needs, consider:
- Portable generator: 3000-5000W for essential circuits
- Whole-home generator: 10,000W+ with automatic transfer switch
- Hybrid approach: Generator for high-draw periods, battery for overnight quiet
Use our Generator Sizing Calculator if your power needs exceed what batteries can reasonably provide.
Next Steps
Once you understand your power needs, assess your overall preparedness with our Preparedness Score Calculator, then ensure you have adequate water storage to match your power backup duration.
Frequently Asked Questions
Multiply your battery capacity in watt-hours (Wh) by 0.85 (accounting for inverter efficiency), then divide by your total appliance wattage. For example, a 1000Wh battery powering a 100W load: (1000 × 0.85) ÷ 100 = 8.5 hours of runtime.
A typical household refrigerator uses 100-400 watts while running, but cycles on and off. Average daily consumption is 1-2 kWh (1000-2000 Wh). During a power outage, expect roughly 50-80 watts average when accounting for compressor cycling.
For basic needs (phone charging, lights, router), a 500Wh power station suffices. For running a refrigerator overnight, you need 1500-2000Wh minimum. For medical devices like CPAP plus essentials, plan for 1000Wh or more depending on your specific equipment.
Yes, but you need adequate capacity. A typical refrigerator needs 1000-1500Wh per day. A 2000Wh power station can run most fridges for 24-36 hours. Larger units (3000Wh+) provide 2-3 days of refrigerator runtime.
CPAP machines typically use 30-60 watts without a heated humidifier. With heated humidifier and heated tubing, consumption can reach 80-100 watts. A 500Wh battery can power a basic CPAP for 8-12 hours, sufficient for one night.
Yes. Solar panels recharge your battery during daylight hours, effectively extending runtime. A 200W solar panel in good conditions generates roughly 800-1000Wh per day, potentially doubling or tripling your effective runtime for moderate loads.
Watts (W) measure power draw at any moment—how much electricity a device uses while running. Watt-hours (Wh) measure energy capacity—total power over time. A 100W device running for 5 hours uses 500Wh of energy.
Inverter efficiency losses (typically 10-15%) reduce usable capacity. A "1000Wh" battery delivers roughly 850Wh of actual power. Additionally, batteries lose capacity in extreme temperatures and as they age. Always calculate with 85% usable capacity.