Home Backup Runtime Guide
Runtime is the most practical metric for home battery backup — it answers the question everyone asks: how long will this keep my lights on? This guide provides runtime tables for common battery sizes, explains the physics behind the numbers, and shows how load management can dramatically extend your backup duration without adding hardware.
How Runtime Is Calculated
Battery runtime is the ratio of usable energy to load power. Usable energy equals total battery capacity multiplied by depth of discharge (DoD) and inverter efficiency. Load power is the total wattage of all devices running simultaneously. Dividing usable energy by load gives runtime in hours.
The calculation assumes a constant load. In practice, loads fluctuate — the refrigerator cycles on and off, lights are turned on and off, and devices are added or removed throughout the day. For a more accurate estimate, calculate runtime for average load over the full backup period rather than peak load at a single moment.
Runtime Formula
For a 10 kWh (10,000 Wh) LFP battery at 85% DoD with a 92% efficient inverter, the usable energy is 10,000 × 0.85 × 0.92 = 7,820 Wh. At a 1,000W load, runtime is 7.82 hours. At 2,000W, it drops to 3.91 hours.
Runtime by Battery Size and Load
The table below shows estimated runtime for four common battery sizes across five load levels. All values assume LFP chemistry (85% DoD) with a 92% efficient inverter. Use this as a quick reference when selecting a battery size for your backup needs.
| Battery Size | 500W | 1,000W | 2,000W | 3,000W | 5,000W |
|---|---|---|---|---|---|
| 5 kWh | 7.8 hrs | 3.9 hrs | 2.0 hrs | 1.3 hrs | 0.8 hrs |
| 10 kWh | 15.6 hrs | 7.8 hrs | 3.9 hrs | 2.6 hrs | 1.6 hrs |
| 15 kWh | 23.5 hrs | 11.7 hrs | 5.9 hrs | 3.9 hrs | 2.3 hrs |
| 20 kWh | 31.3 hrs | 15.6 hrs | 7.8 hrs | 5.2 hrs | 3.1 hrs |
These values represent a static load scenario. Real-world runtime will be slightly higher for loads that cycle off periodically (like refrigerators) and slightly lower for loads with high inrush currents (like motor-driven appliances).
Typical Home Load Profiles
Understanding your actual load profile is essential for predicting runtime. A home's power draw is not constant — it peaks during morning and evening hours when multiple appliances run simultaneously and drops to a baseline overnight. The following table shows typical load levels for a household with critical loads only during different periods of the day.
| Time Period | Active Load | Duration | Energy (Wh) |
|---|---|---|---|
| Morning (6am – 9am) | 1,500 – 2,500W | 3 hours | 4,500 – 7,500 |
| Daytime (9am – 5pm) | 500 – 1,000W | 8 hours | 4,000 – 8,000 |
| Evening (5pm – 10pm) | 2,000 – 3,500W | 5 hours | 10,000 – 17,500 |
| Overnight (10pm – 6am) | 200 – 500W | 8 hours | 1,600 – 4,000 |
The average daily consumption across all periods is the weighted mean of the energy column. A typical critical-only profile totals 20,000–37,000 Wh (20–37 kWh), though the peak-to-average ratio matters for inverter sizing even if average consumption determines battery capacity.
Worked Example: Runtime Estimation
Scenario: 15 kWh LFP battery, typical critical-load household.
Given:
- Battery: 15,000 Wh total
- DoD: 85%
- Inverter efficiency: 92%
- Average critical load: 1,200W
Step 1: Calculate usable energy:
Step 2: Calculate runtime at average load:
Step 3: Apply load profile for more accuracy:
Daytime (8h @ 800W): 6,400 Wh
Evening (5h @ 2,500W): 12,500 Wh
Overnight (8h @ 300W): 2,400 Wh
Total: 27,300 Wh → exceeds 11,730 Wh
Conclusion: A 15 kWh battery cannot sustain 24 hours of full critical-load profile for this household. It covers approximately 10 hours of daytime and evening loads. To achieve 24-hour coverage, increase battery capacity to approximately 27 kWh or reduce evening loads.
Factors That Reduce Runtime
Several real-world factors reduce runtime below the theoretical calculation. Understanding these factors prevents overestimating your backup duration and helps you design a more resilient system.
Temperature Derating
LFP batteries lose 10–20% of usable capacity below 0°C (32°F). If your battery is in an unheated garage or outdoor enclosure in winter, expect reduced runtime. Keep batteries above 10°C (50°F) for optimal performance.
Battery Age
LFP capacity degrades roughly 20% over 3,000 cycles. A 5-year-old battery used daily retains approximately 85–90% of original capacity. A 10-year-old battery may retain 75–80%. Factor this aging into your sizing if planning for long-term system life.
Inverter Light-Load Efficiency
Inverters are less efficient at very light loads (below 10% capacity). A 5 kW inverter running a 200W load may operate at only 80% efficiency instead of the rated 92%. This adds up over long overnight periods with low loads.
Inrush Currents
Motors in refrigerators, pumps, and compressors draw 3–6× their running wattage for 1–3 seconds during startup. These brief surges consume extra energy and must be within the inverter's surge rating, though their impact on total runtime is minimal.
Try It
Use the Home Backup Calculator to estimate runtime for your specific load profile and battery configuration.
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Calculate the exact runtime of any battery configuration at a specific load with the Battery Runtime Calculator.
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Frequently Asked Questions
How long will a 10kWh battery power my house?
A 10 kWh LFP battery at 85% DoD with 92% inverter efficiency provides approximately 7.8 kWh usable. At a 1,500W critical load, it runs for about 5.2 hours. At a 500W load, it lasts roughly 15.6 hours. The runtime scales linearly with load — halving the load doubles the runtime.
Does battery size directly determine backup runtime?
Battery size sets the upper limit, but runtime is determined by the ratio of usable energy to load. A 20 kWh battery powering a 2 kW load lasts about 7.8 hours. The same battery powering a 500W load lasts roughly 31 hours. Reducing load is often more cost-effective than adding battery capacity.
How do temperature and age affect battery runtime?
Cold temperatures reduce usable capacity — LFP batteries lose 10–20% capacity below 0°C (32°F). Battery capacity also degrades over time: LFP batteries retain approximately 80% capacity after 3,000 cycles (roughly 8–10 years of daily cycling). A 10-year-old battery delivers roughly 80% of its original runtime.
Can I extend battery runtime without adding more batteries?
Yes. Reduce the load by turning off non-essential devices, use higher-efficiency appliances, and ensure the inverter is properly sized for the load (inverters are more efficient at 40–80% capacity than at very light loads). Smart load management and scheduling high-draw activities during daylight hours (if solar-charged) also extends effective runtime.