A refrigerator full of food, a CPAP machine, a modem, and a few lights can make an outage feel manageable. Add a space heater, microwave, or central air conditioner, and even a large battery can drain far faster than expected. This battery backup runtime guide shows how to estimate real runtime before you buy, so your backup plan matches the loads that matter most.
Start With Watt-Hours, Not Just Battery Size
Battery runtime comes down to two numbers: how much energy your battery can store and how much power your equipment uses. Battery energy is measured in watt-hours (Wh), while the power draw of an appliance is measured in watts (W).
The basic calculation is straightforward:
Runtime in hours = usable battery watt-hours ÷ average load watts
A 2,000Wh battery backup system powering a steady 200W load has a theoretical runtime of 10 hours. In real use, however, the result will be lower because batteries and inverters are not 100% efficient.
For planning purposes, use this more realistic formula:
Runtime = battery capacity × usable capacity factor × inverter efficiency ÷ load watts
For a lithium iron phosphate battery, a usable capacity factor of 80% to 90% is often reasonable. For lead-acid deep cycle batteries, many owners plan around 50% depth of discharge to protect battery life. Inverter efficiency commonly falls around 85% to 95%, depending on the model and the size of the load.
That means a 2,000Wh lithium battery with 85% usable capacity and 90% inverter efficiency provides about 1,530Wh of usable AC power. At a 200W average load, expect roughly 7.5 hours, not 10.
Build a Backup Load List That Reflects Real Life
The most common runtime mistake is calculating every device in the house, then choosing a battery system that is too small for the job. During an outage, start with critical loads rather than convenience loads.
For many homes, critical loads include refrigeration, a Wi-Fi router and modem, phone charging, a few LED lights, medical equipment, a sump pump, and possibly a television or laptop. Your actual list depends on the season, your household, and whether you rely on well water, a septic pump, or electric garage access.
Check the label on each device for watts or amps. If the label only lists amps, multiply amps by volts to estimate watts. A device rated at 1.5 amps on a 120V outlet uses about 180W.
Appliance labels provide a starting point, but average draw matters more than maximum draw. A refrigerator may be rated around 150W to 250W while running, yet its compressor cycles on and off. Over a full day, its average energy use may be much lower than its running wattage suggests. A coffee maker, microwave, and hair dryer use high wattage, but usually only for a few minutes at a time.
A practical outage plan separates loads into three groups: always on, occasional use, and do not run on battery. This keeps energy available for the equipment you cannot easily replace with another option.
Account for Startup Surge and Inverter Limits
Runtime is only half the equation. Your battery backup must also produce enough power to start and run your appliances.
Motors, compressors, and pumps can draw a brief startup surge that is several times higher than their normal running watts. A refrigerator that runs at 200W may need 1,000W or more for a moment when the compressor starts. Sump pumps, chest freezers, power tools, and window air conditioners can have similar demands.
Check both ratings on a power station, inverter, or battery backup system: continuous output and surge output. Continuous output tells you what it can run steadily. Surge output tells you whether it can handle short startup demands.
For example, a 1,000W battery backup may have enough stored energy to run a refrigerator for hours, but it may still shut down if the compressor needs a surge above the inverter's limit. When a load is mission-critical, leave room between your appliance requirements and the system's rated output.
Estimate Runtime With a Realistic Example
Consider a homeowner using a 2,400Wh lithium battery backup system during a storm. After allowing for battery reserve and inverter losses, assume 1,850Wh is available to AC outlets.
Their critical loads look like this:
| Load | Average Power Draw | Estimated Use |
|---|---:|---:|
| Refrigerator | 90W average | Continuous cycling |
| Modem and router | 20W | Continuous |
| LED lights | 25W | Evening use |
| CPAP machine | 45W | 8 hours overnight |
| Phone charging | 15W | Intermittent |
During the day, the average load may be close to 130W once refrigerator cycling is considered. At that rate, 1,850Wh ÷ 130W equals about 14 hours. Overnight, adding the CPAP and more lighting may increase the load to 175W or more, reducing runtime.
This is why a single runtime claim can be misleading. A battery may run a small refrigerator for a day, but it may only run the refrigerator, networking equipment, lighting, and medical device for half that time. The answer depends on what is connected and when it runs.
Battery Type Changes the Planning Math
Lithium iron phosphate, often called LiFePO4, is a popular choice for backup power because it offers a high usable capacity, long cycle life, and consistent performance. It is generally better suited to frequent use and deeper discharge than traditional lead-acid batteries.
Lead-acid deep cycle batteries can still be a practical option for certain inverter setups, RVs, and budget-conscious installations. They usually require more conservative discharge planning, weigh more, and can lose usable capacity more noticeably as they age. A 100Ah battery is not automatically equal to another 100Ah battery unless voltage, chemistry, discharge rate, and usable depth of discharge are also considered.
To convert battery bank capacity into watt-hours, multiply volts by amp-hours. A 12V, 100Ah battery stores about 1,200Wh on paper. A 24V, 100Ah battery stores about 2,400Wh. If you are building a multi-battery system, make sure the inverter voltage and battery bank voltage match.
Plan for Longer Outages With Recharging
A battery backup is excellent for quiet, instant power, but stored energy is finite. For outages lasting more than a night, recharge options become part of the runtime calculation.
Solar can extend battery operation significantly, but daily production changes with panel size, weather, season, shading, and panel angle. A 200W solar panel will not reliably produce 200W all day. In cloudy conditions or after a hurricane, solar output may be limited when you need it most.
A generator can be the more dependable recharge source for multi-day outages, especially when running larger loads or replenishing batteries quickly. A hybrid setup often makes the most sense: use the battery for overnight power, quiet indoor-safe operation, and sensitive electronics, then use a generator during the day to recharge batteries and handle heavy loads.
GenVault shoppers comparing portable power stations, solar charging, and generators should think of these options as complementary rather than interchangeable. The right setup depends on your outage length, load size, noise tolerance, fuel access, and budget.
Avoid the Runtime Assumptions That Cause Problems
Do not size a system around the battery's largest advertised number alone. Verify usable capacity, AC output, surge rating, recharge speed, and the actual wattage of the items you plan to run. Also remember that batteries lose capacity over time, and very hot or cold conditions can affect performance.
Avoid plugging in resistance-heating appliances unless your system was specifically sized for them. Space heaters, electric kettles, toaster ovens, clothes irons, and portable air conditioners can consume 1,000W to 1,500W or more. They can drain a battery backup in a short period even when the system has enough output to run them.
For medical equipment, follow the device manufacturer's backup power guidance and maintain a second plan. A battery system can provide valuable protection, but life-safety equipment deserves extra margin and dependable monitoring.
The best backup system is not the one with the biggest number on the box. It is the one that keeps your specific essentials running long enough to get through the outage with fewer surprises.