Quick answer
Battery Backup Duration is how long a charged home battery can keep your chosen loads running when the grid is down. It is found by dividing the stored energy available, in kWh, by the average power those loads draw, in kW.
It is not a fixed product spec. The same battery can last a few hours or a few days, depending on what you run, how full it was when the power failed, and whether solar can recharge it.
Quick facts
The key facts about battery backup duration, with sources:
- Basic formula
- Available kWh ÷ average load in kW = hours
- Refrigerator rule of thumb
- Runs at full wattage about one-third of the time it is plugged in 1
- US average time without power, 2024
- About 11 hours per customer 5
- South Carolina, 2024
- Nearly 53 hours, the longest of any state 5
- Calculator
- HyreSolar battery sizing tool
Key takeaways
- Runtime = energy available ÷ average load. Both numbers are yours to shape.
- Start from the charge level when the outage begins, not from the label.
- Fridges cycle, so their average draw is far below their nameplate watts.
- Solar can stretch runtime by recharging on sunny days, but plan for cloudy ones.
- Size for the outages you actually get. SC averaged nearly 53 hours in 2024.
Duration is energy divided by demand
Think of a battery as a water tank and your home as a tap. Duration is how long the tank lasts with the tap open a certain amount. Open it wider and it runs dry sooner.
In battery terms, the tank is the energy you can use, in kilowatt-hours (kWh). The tap is the average power your loads draw, in kilowatts (kW). Divide the first by the second and you get hours.
The wider topic of backup, and how a battery takes over when the grid fails, is on the battery backup page. This page is only the runtime maths and what changes it.
The runtime calculation, step by step
Each step uses numbers you can find on a data sheet, an appliance label or your app.
- List the loads you want running in an outage. Note each one’s watts from its label or the maker’s sheet.
- Estimate hours per day each load runs. For a fridge, divide the hours it is plugged in by three, because it cycles on and off 1.
- Work out daily kWh: watts × hours ÷ 1,000, then add them up 1.
- Find the energy on hand: the battery’s usable kWh × the charge level when the outage starts.
- Divide energy on hand by daily kWh to get days, then × 24 for hours.
- Check the power limit: add up watts that could run at the same moment and compare with the battery’s continuous output.
- Leave a margin for conversion losses, heat or cold, and loads you forgot.
Step 6 is often skipped. A battery can have plenty of energy and still trip if too much starts at once. The power rating page explains that limit.
Four runtime figures you may be shown
| Figure | How it is worked out | What it is good for |
|---|---|---|
| Full-output runtime | Energy ÷ the battery’s maximum continuous kW | The worst case: a little over an hour on most single units |
| Load-list runtime | Energy ÷ the average draw of your chosen loads | The figure to plan around |
| Solar-assisted runtime | Load-list runtime plus daily recharge from panels | Best case on sunny days; can stretch to many days |
| App “time left” | Live charge ÷ live load, updated every few seconds | Moment-to-moment; swings as loads switch on and off |
Example 1: building a daily load
An example only. Wattages are Virginia Cooperative Extension nameplate figures; the hours are assumptions for an outage day, and the lighting line is an assumed LED total.
| Load | Watts × hours | kWh per day |
|---|---|---|
| Refrigerator, frost-free, 16 cu ft (725 W) 1 | 725 × (24 ÷ 3) = 725 × 8 | 5.80 |
| Ceiling fan, top of range (175 W) 1 | 175 × 8 | 1.40 |
| Flat-screen TV (120 W) 1 | 120 × 4 | 0.48 |
| Laptop (50 W) 1 | 50 × 8 | 0.40 |
| LED lights, several rooms (60 W, assumed) | 60 × 6 | 0.36 |
| Total | 8.44 kWh per day (about 0.35 kW average) |
If all five ran at once, the peak would be about 1,130 W. That is within the continuous output of each battery in Example 2. A well pump, electric range or central air would change the picture a lot; see electrical load for how designers sort loads.
Example 2: runtime on three real batteries
Uses the 8.44 kWh day from Example 1. Battery energy and output figures are from current data sheets. Solar recharge is left out, as on a cloudy day.
| Battery | Energy, output | Starting full | Starting at 80% |
|---|---|---|---|
| Enphase IQ Battery 5P, one unit | 5.0 kWh usable, 3.84 kVA 2 | 5.0 ÷ 8.44 × 24 ≈ 14 h | 4.0 ÷ 8.44 × 24 ≈ 11 h |
| Tesla Powerwall 3, one unit | 13.5 kWh nominal AC, 11.5 kW 3 | 13.5 ÷ 8.44 × 24 ≈ 38 h | 10.8 ÷ 8.44 × 24 ≈ 31 h |
| FranklinWH aPower 2, one unit | 15 kWh usable AC, 10 kW 4 | 15 ÷ 8.44 × 24 ≈ 43 h | 12 ÷ 8.44 × 24 ≈ 34 h |
These are upper estimates. Real runtime is a little shorter after losses. Tesla gives no separate usable figure, so its 13.5 kWh is the rated number 3.
Example 3: working backwards from an outage length
Same 8.44 kWh day. Target: South Carolina’s 2024 average of nearly 53 hours without power 5. No solar recharge assumed.
| Step | Math | Result |
|---|---|---|
| Energy needed | 8.44 × 53 ÷ 24 | About 18.6 kWh |
| Enphase IQ Battery 5P units | 18.6 ÷ 5.0, rounded up | 4 units (20 kWh) |
| Powerwall 3 units | 18.6 ÷ 13.5, rounded up | 2 units (27 kWh) |
| FranklinWH aPower 2 units | 18.6 ÷ 15, rounded up | 2 units (30 kWh) |
An average hides the long tail: some homes were out for far longer. With working solar and some sun, fewer units can cover more hours. The battery sizing calculator runs this with your own loads.
Where the duration numbers come from
- Appliance labels: watts or volts × amps on the back or base 1.
- Your bill or smart-meter data: daily kWh for the whole home, a ceiling for a whole-home plan.
- The battery data sheet: usable or nominal kWh and continuous kW.
- The app: current charge level and live load, often with an estimated time left.
- The installer’s design: which circuits are backed up, which sets the loads you can count.
Why work it out, and where the maths falls short
Doing the maths helps you
- Pick a battery size from your needs, not a sales pitch.
- Decide which circuits to back up.
- Set a reserve that matches your outage risk.
- Spot an over-sized or under-sized quote.
The maths cannot capture
- How your family will really use power in a blackout.
- Weather that blocks solar recharge for days.
- Motor start surges that trip a battery.
- Capacity lost to age over the years.
What shortens real runtime
Starting charge. A battery spending its energy on bill savings may be half full when the grid drops. Your backup reserve sets the floor.
Conversion losses. Energy passes through electronics on its way out. Enphase lists a 90% AC round-trip efficiency 2.
Temperature. Tesla notes Powerwall 3 output may be reduced at high heat 3. Cold slows charging.
Heavy loads. Heating, cooling, cooking and well pumps draw kilowatts. One central air unit can use more in an hour than the whole Example 1 list.
Age. Capacity fades with years, so runtime does too. See usable battery capacity for warranty floors.
How duration drives cost
Each extra hour of backup at the same load means more kWh, and more kWh usually means more units. Cutting the load list is the cheapest way to add hours.
Duration also shapes the rest of the job: more units can mean a new mounting spot, a larger gateway or a panel change. We do not publish installed prices; see what drives solar and storage cost.
How an installer sizes for duration
- Agree the goal: hours of backup, which loads, and whether solar recharge is counted.
- Collect load data from your bills, meter data or a site walk-through.
- Pick partial (critical-load) or whole-home backup.
- Choose the units and check both kWh and kW against the loads.
- Submit the design for permit; codes cap kWh by location in the home.
- Commission the system and set the reserve that protects your target hours.
All wiring and panel work is for a licensed electrician or installer.
Keeping your runtime estimate honest
- After any outage, compare the hours you got with the plan.
- Re-run the maths when you add a big load such as an EV charger or heat pump.
- Check the app’s full-charge kWh once a year as the battery ages.
- Raise the reserve before named storms so the outage starts with a fuller battery.
Warning signs and when to call a pro
- Backup ends hours sooner than the same loads gave last year.
- The system trips when the fridge or a pump starts.
- The app’s time-left estimate swings wildly with steady loads.
- The battery will not recharge from solar during an outage.
Safety rule
Never stretch runtime by back-feeding a generator into the house or moving circuits yourself. A licensed electrician must make any change to backed-up circuits.
Rules that cap how much duration you can install
No rule sets a minimum backup time for homes. But codes cap stored energy, which caps duration. UL’s summary of the 2021 IRC R328 and NFPA 855 allows up to 20 kWh per unit, 40 kWh in utility rooms and basements, and 80 kWh in garages, on outside walls or outdoors 6.
States adopt code editions on their own schedules, and some amend them. Your local permit office applies the edition in force where you live, and it will check the total kWh on your plans.
Sources: [6]
Backup duration vs related terms
| Term | Unit | What it tells you |
|---|---|---|
| Backup duration | Hours | How long your loads can run |
| Battery capacity | kWh | How much energy is stored |
| Power rating | kW | How much can run at once |
| Electrical load | W or kW | What your home is drawing |
| Backup reserve | % | How much is kept back for outages |
Misconceptions
- Myth A 13.5 kWh battery lasts 13.5 hours.
- Reality Only at a steady 1 kW. At the Example 1 load it is closer to 38 hours; with central air it could be a few.
- Myth Fridges use their label watts all day.
- Reality They cycle; Virginia Tech advises dividing plugged-in hours by three 1.
- Myth Solar will always recharge the battery.
- Reality Storms bring cloud. Plan for at least the first day without much sun.
- Myth More batteries always means more hours.
- Reality Only if the loads stay the same. Adding circuits to backup can eat the gain.
Outage length in SC, GA and VA
The EIA reports US customers averaged about 11 hours without power in 2024, nearly twice the decade before.
South Carolina averaged nearly 53 hours, the most of any state, after Hurricane Helene cut power to at least 1.2 million SC customers 5.
In Georgia and Virginia, check your own utility’s outage history and size for the outages your area actually gets.
Choosing your target duration
- Look up how long your outages usually last, and the longest you remember.
- Write the short list of loads you truly need.
- Run the numbers in the calculator, starting at 80% charge, not 100%.
- Ask each installer to show their runtime maths on the quote.
- Decide whether a generator, partial backup or more storage closes any gap.
Questions about battery backup duration
How long will a home battery last in a power outage?
It depends on the energy stored and what you run. Divide the kWh available by your average load in kW. In our example, a 13.5 kWh battery running a fridge, fan, TV, laptop and lights (8.44 kWh a day) lasts about 38 hours from full, or about 31 hours from 80%.
How do I calculate battery runtime?
Add up each load’s watts × hours used ÷ 1,000 to get kWh per day. Multiply the battery’s usable kWh by the charge level when the outage starts. Divide that energy by daily kWh and multiply by 24 for hours. Then check that the loads running together stay under the battery’s continuous output.
How long can a battery run a refrigerator?
Longer than the label suggests, because fridges cycle. Virginia Tech advises dividing plugged-in hours by three. A 725 W fridge then uses about 5.8 kWh a day, so a full 13.5 kWh battery could run it alone for a little over two days, before losses.
Can a battery run central air conditioning?
Some can, but not for long. Central air draws several kilowatts and needs a big start surge, so the battery must be rated for it and runtime drops sharply. Many backup plans leave central air off the list or add units. Ask your installer to size for it explicitly.
Does solar extend battery backup time?
Yes, if the system is set up to recharge during an outage and the sun is out. On a clear day the panels can refill some or all of what you used overnight. On stormy days they may add little, so size the battery for at least one day without much sun.
How many batteries do I need for a 2-day outage?
Multiply your daily kWh by two and divide by one battery’s usable kWh, then round up. At 8.44 kWh a day, two days needs about 17 kWh: four 5 kWh units, or two 13.5 or 15 kWh units. Your own load list changes the answer.
Why does my battery run out faster than the estimate?
Common reasons are a lower charge when the outage began, loads you did not count, heating or cooling cycling on, heat or cold, and conversion losses. An ageing battery also holds less. Compare the app’s load history with your plan to find the gap.
How long do power outages last in South Carolina?
In 2024, South Carolina customers averaged nearly 53 hours without power, the longest of any state, according to the EIA. Hurricane Helene caused most of it. In more typical years outages are much shorter, so many homes size for a day or two and shed loads in a big storm.
Is it better to back up a few circuits or the whole house?
For duration, a few circuits wins: a short list of loads makes the same battery last far longer. Whole-home backup adds comfort but uses energy faster. The whole-home battery backup page compares the two designs in detail.
Sources
- Virginia Cooperative Extension (Virginia Tech), Estimating Appliance and Home Electronic Energy Use, pub. 2901-9014, retrieved .
- Enphase Energy, IQ Battery 5P data sheet (DSH-00010-11.0-EN-2025-05-06), retrieved .
- Tesla, Powerwall 3 Datasheet (2024), retrieved .
- FranklinWH, aPower 2 data sheet (2025), retrieved .
- US EIA, Today in Energy: Hurricanes in 2024 led to the most hours without power in 10 years, retrieved .
- UL Solutions, ESS separation and maximum quantities: residential requirements (2021 IRC R328 / NFPA 855), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Load formula and appliance wattages from Virginia Cooperative Extension; battery energy and output from Enphase, Tesla and FranklinWH data sheets; outage durations from EIA; code limits from UL’s summary. Hours of use, the LED lighting total and starting charge levels are assumptions, labelled as such.
Suggest a correction. We fix errors and say what changed.