What this calculator does: You enter your battery bank’s capacity, its chemistry, your system’s efficiency, the temperature it will sit at, its condition and the load it will carry. The calculator estimates how much usable energy the bank holds and how many hours or days it can run that load with no sun. It can also work backward and tell you how big a bank you need for a target backup time.
Who it’s for: Homesteaders and cabin owners with a battery bank, or planning one, who want to know whether it can ride through a cloudy stretch or an outage, or who are comparing lithium against lead-acid.
The answer it produces: Usable battery capacity in kWh, runtime in hours and days at your load, optionally the days of backup at your average daily energy use, and optionally the minimum rated battery bank size for a backup time you choose. When you enter a cold temperature or a worn battery, it shows the adjustments it applied, and it prints a warning for very heavy loads, fast lead-acid discharge and below-freezing temperatures.
Important limitation: This is a planning estimate. It assumes a constant load, a full battery at the start and no solar input, and it uses fixed usable-depth values (about 90% for lithium, 50% for lead-acid). It reduces capacity for the temperature and battery condition you enter, but the temperature adjustment is one straight-line estimate used for both chemistries, not your battery’s own curve. It does not model the extra capacity lost to heavy discharge rates beyond printing a warning, so real runtime can be shorter than the number you see.
Battery Runtime and Sizing Calculator
Battery Bank Runtime Calculator
How the Calculation Works
The calculator checks your entries and then uses four short formulas. It is plain arithmetic, and the math runs in your browser.
- Usable capacity. Battery capacity (kWh) × 1,000 × depth-of-discharge factor × system efficiency × temperature factor × battery condition = usable watt-hours (Wh). The depth-of-discharge factor is 0.9 for lithium (LiFePO4) and 0.5 for lead-acid. Efficiency defaults to 85%. The temperature factor is 1 at 80°F and above, 0.5 at 0°F and below, and a straight line between, which is 0.5 + 0.5 × (temperature ÷ 80). Battery condition is the percentage of rated capacity you enter, divided by 100.
- Runtime. Usable Wh ÷ continuous load (W) = hours. Divide the hours by 24 for days.
- Days of backup (optional). If you enter your average daily energy use, days of backup = usable Wh ÷ (daily kWh × 1,000).
- Battery bank size for a target (optional). Required Wh = load (W) × desired hours. Recommended capacity (kWh) = required Wh ÷ (depth-of-discharge factor × efficiency × temperature factor × battery condition) ÷ 1,000. This is the rated capacity you would shop for.
Because runtime is simply energy divided by power, doubling the load in this tool exactly halves the runtime. Real batteries, especially lead-acid, do slightly worse than that at heavy loads, which is one reason to treat the result as a best case. The tool does not shorten the runtime for heavy loads. It prints a warning instead:
- Very heavy load. If the load would drain the full rated capacity in under an hour, it tells you to check the battery’s maximum continuous discharge rating.
- Fast discharge. For lead-acid only, if the rated capacity would last less than 20 hours at your load, it warns that the bank can deliver less than its rating and that the runtime is an upper bound.
- Below freezing. If you enter a temperature under 32°F, it warns that lithium batteries generally should not be charged below freezing without heaters or low-temperature protection, and that a discharged lead-acid battery can freeze.
Inputs and Assumptions
These are the fields you fill in:
| Input | What to enter | Notes |
|---|---|---|
| Battery bank capacity (kWh) | The rated (nameplate) capacity of the whole bank | Enter rated capacity, not usable capacity, because the calculator applies the usable percentage itself. To convert from amp-hours, use volts × amp-hours ÷ 1,000. For example, 48 V × 200 Ah = 9.6 kWh. |
| Battery type | Lithium (LiFePO4) or lead-acid (AGM or flooded) | Sets the usable depth of discharge: about 90% for lithium, about 50% for lead-acid. The tool starts on lead-acid. |
| System efficiency (%) | Combined inverter and wiring efficiency, default 85 | Accepts 1 to 100. A blank field falls back to 85, and a value outside 1 to 100 shows an error. |
| Battery temperature (°F) | Where the bank sits, default 80 | Accepts −40 to 140. At 80 or above there is no cold reduction. Heat is not modeled. |
| Battery condition (%) | Percent of rated capacity remaining, default 100 | Accepts 10 to 100. Use 100 for a new bank, or a tested figure for an older one. |
| Continuous load (W) | The average power the bank will supply | Must be greater than zero, or the tool shows an error. To convert a daily energy total to an average load, divide watt-hours by 24. |
| Average daily energy use (kWh) | Optional | Adds a days-of-backup result based on your daily total. |
| Desired backup time (hours) | Optional | Adds a recommended battery bank size for that duration at your load. Optional fields cannot be negative. |
And these are the assumptions built into the tool:
| Assumption | Value used |
|---|---|
| Usable depth of discharge | 90% lithium, 50% lead-acid (two fixed choices) |
| Load profile | Constant, at the wattage you enter |
| Starting charge | Bank is full |
| Solar or generator input | None during the backup period |
| Efficiency | One combined percentage applied once |
| Cold weather | 100% capacity at 80°F, 50% at 0°F, straight line between, for both battery types |
| Battery age | Only the condition percentage you enter |
| Not included | Discharge-rate effects (warnings only), heat, inverter idle draw, battery power limits |
What the Result Means
- Usable battery capacity (kWh). The energy you can realistically pull from the bank after applying the depth-of-discharge limit and system losses. It is always well below the rated number on the battery label. When your temperature or battery condition reduces it, a line under the result says which adjustments were applied.
- Estimated runtime. How long the bank can carry your load with no sun, in hours and in days. Treat it as a best case.
- Days of backup at your average daily use. Shown only if you enter a daily kWh figure. This is the number to compare against how many cloudy days you need to survive.
- Recommended battery bank size. Shown only if you enter a desired backup time. It is the minimum rated capacity, with depth of discharge, efficiency, temperature and battery condition already built in, so it is comparable to the kWh on a battery’s datasheet.
- Warnings. Yellow boxes under the result flag a very heavy load, fast lead-acid discharge or a below-freezing temperature. They do not change the numbers, they tell you what to check.
Example Calculation
Take a 10 kWh (rated) battery bank, the tool’s 85% default efficiency, a battery at 80°F in 100% condition and a 300 W continuous load, which are the tool’s starting values. The only thing that changes below is the battery type.
| Result | Lead-acid (50%) | Lithium (90%) |
|---|---|---|
| Usable capacity | 10,000 × 0.5 × 0.85 = 4.25 kWh | 10,000 × 0.9 × 0.85 = 7.65 kWh |
| Runtime at 300 W | 4,250 ÷ 300 = 14.2 hours (0.6 days) | 7,650 ÷ 300 = 25.5 hours (1.1 days) |
| Days of backup at 2.9 kWh a day | 4,250 ÷ 2,900 = 1.5 days | 7,650 ÷ 2,900 = 2.6 days |
| Bank needed for 24 hours at 300 W | 7,200 ÷ (0.5 × 0.85) = 16.94 kWh | 7,200 ÷ (0.9 × 0.85) = 9.41 kWh |
Adjusting the example for the real world. The same 10 kWh of rated capacity gives lithium about 80% more runtime than lead-acid, because of the usable-depth difference alone. Three things then pull the result down. First, efficiency: for the lead-acid bank above, runtime is 11.7 hours at 70% efficiency and 15.8 hours at 95%, so check your inverter’s actual efficiency instead of trusting the default. Second, temperature. Change the battery temperature and the tool applies its straight-line factor:
| Battery temperature | Capacity factor | Lead-acid usable / runtime | Lithium usable / runtime |
|---|---|---|---|
| 80°F or warmer | 100% | 4.25 kWh / 14.2 h | 7.65 kWh / 25.5 h |
| 40°F | 75% | 3.19 kWh / 10.6 h | 5.74 kWh / 19.1 h |
| 20°F | 62.5% | 2.66 kWh / 8.9 h | 4.78 kWh / 15.9 h |
| 0°F or colder | 50% | 2.13 kWh / 7.1 h | 3.83 kWh / 12.8 h |
Third, age: a lead-acid bank that a capacity test shows is down to 80% of its rating, entered as 80 in the battery condition field, delivers 3.40 kWh and 11.3 hours at 80°F instead of 4.25 kWh and 14.2 hours. The cold figures use one line for both chemistries, taken from two points on the Battery University reference, so check your own battery’s datasheet for its low-temperature performance.
Using it with the solar calculator. In the worked example on our off-grid solar calculator page, a small cabin uses 2.898 kWh a day. Two days without sun is 5.796 kWh, which is about 121 W of average load for 48 hours. Enter 121 W and 48 hours in this tool and, at the default 80°F and full condition, it recommends a rated bank of about 7.59 kWh for lithium, or about 13.67 kWh for lead-acid. If that bank sits at 20°F, the same target needs about 12.15 kWh of lithium or 21.87 kWh of lead-acid. The solar calculator’s own battery figure for the same cabin, 7.16 kWh of lithium, is a little lower because it assumes a 90% inverter efficiency where this tool starts at 85%. Use this tool to test that figure against your own efficiency, temperature and battery condition.
How to Use the Result in a Real System
- Start from measured loads. Get daily energy use from the solar calculator or from a plug-in watt meter, then divide by 24 for the continuous load. Runtime depends on total energy, so an average works for the runtime figure.
- Enter rated capacity from the datasheet. Convert amp-hours to kWh with volts × amp-hours ÷ 1,000, and use the whole bank, not one battery.
- Compare chemistries with the sizing feature. Run the same target backup time for both battery types. In the example above, lead-acid needs about 80% more rated capacity than lithium for the same 24 hours.
- Enter the temperature and condition of your bank. Use the coldest temperature the bank will sit at when you need it, and a tested capacity for an older bank. Then add margin for the things the tool still leaves out, such as heavy discharge rates and inverter idle draw, and round the recommended size up rather than down.
- Check that the battery can deliver the power, not just the energy. A bank can hold plenty of kWh and still be limited in how many watts it can output at once. Compare your peak load against the battery’s continuous discharge rating and against the inverter’s limits.
- Make sure the array can refill it. The tool assumes no sun, but the bank has to recharge afterward. Use the winter panel figure from the solar calculator to confirm your panels can recover the energy you draw down.
- Get the wiring and protection right. Battery banks can deliver very high fault currents. Cable sizes, fuses and disconnects must follow the National Electrical Code and your local rules, so have a qualified electrician or installer review the design.
Important Variables That Can Change the Result
- Battery chemistry and datasheet limits. The tool offers two fixed usable depths. Your battery’s manufacturer may allow more or less, and warranty terms often depend on how deeply you discharge.
- Temperature. Cold reduces the capacity a battery can deliver, and the tool applies a straight-line estimate for it. Heat shortens a battery’s life, which the tool does not model. Where the bank lives matters as much as what it is.
- Discharge rate. Pulling a battery hard can reduce the capacity you actually get, and the effect is stronger in lead-acid. The tool treats runtime as a straight division and only warns you when the load is heavy.
- Battery age and cycle count. A worn bank holds less than its label says. Enter its tested percentage in the battery condition field.
- Inverter efficiency and idle draw. Efficiency varies with load, and many inverters use some power just by being on. Add that idle draw to your load if it matters at your scale.
- Load pattern. A steady 300 W and a load that spikes to 2,000 W and idles the rest of the time can have the same average and very different results in practice.
- Starting charge. The tool assumes a full bank. If the bank is only 70% charged when the outage starts, runtime drops accordingly.
- Battery management limits. Lithium batteries have built-in protection that can shut them off at low charge, high current or extreme temperature.
- Solar or generator input. Any recharging during the backup period extends runtime, which is why the estimate is a worst case.
Common Mistakes
- Entering usable capacity instead of rated. The calculator already applies the usable percentage, so entering an already-reduced number counts the loss twice.
- Mixing up amp-hours and kilowatt-hours. A 12 V, 100 Ah battery holds 1.2 kWh, not 100.
- Using peak wattage as the continuous load. A device’s maximum draw is not its average. Using it will make the bank look much weaker than it is.
- Forgetting standby loads. Inverters, routers, chargers and controllers draw power around the clock, and they quietly shorten real runtime.
- Setting efficiency to 100%. Inverters and wiring always lose some energy, so 100% will overstate runtime.
- Treating lead-acid and lithium as interchangeable. A 10 kWh lead-acid bank and a 10 kWh lithium bank do not give the same usable energy.
- Assuming a full battery. The estimate starts from 100% charge. After several cloudy days the bank may already be partly drained.
- Mixing old and new batteries in one bank. The weakest battery tends to limit the rest.
- Leaving the temperature at 80°F for a cold location. A bank in an unheated shed will deliver less in winter, exactly when the sun is scarce.
- Guessing the battery condition. Entering 100 for a bank of unknown age flatters the result. Test the capacity first.
When This Calculator Should NOT Be Used
- For life-safety or medical equipment. Anything that must never lose power needs professional design, redundancy and testing.
- For batteries other than LiFePO4 or lead-acid. Other chemistries have different usable depths, and the tool only offers these two.
- For loads dominated by large surges. Well pumps, compressors and power tools can be limited by the battery’s power rating long before its energy runs out.
- For very cold or very hot battery locations without checking your battery’s own data. The cold adjustment is a straight line between two points, and heat is not modeled, so use manufacturer data for extreme temperatures.
- For old or unknown-condition batteries without a capacity test. The condition field only helps if you enter a real, tested figure.
- For systems with significant generation during the outage. The tool assumes zero input, so it cannot model a solar-plus-storage day.
- As a final purchase specification. Confirm capacity, discharge limits and warranty terms on the manufacturer’s datasheet, and have the system reviewed by a qualified installer.
Related Honey Hen Calculators
- Off-Grid Solar Calculator: size your panels, inverter and daily energy budget before you size the bank.
- Off-Grid Budget Calculator: estimate the total cost once your bank is sized.
- Off-Grid Heating Calculator: estimate heating fuel so heat does not drain your batteries.
- Off-Grid Home Cooling: DC heat pump and earth tube options that pair with a battery bank.
- All Homestead Calculators: the full set of planning tools in one place.
Related Honey Hen Articles
- Solar Battery Backup for Off-Grid Homesteads: A Guide
- DIY Off-Grid Solar Installation: A Step-by-Step Guide
- Hidden Costs Solar Installation: What to Avoid
- Energy-Efficient Off-Grid Appliances: 10 Picks for Homesteads
- Energy Independence Off-Grid Living: Power Options
Authoritative Sources
- U.S. Department of Energy: Solar Energy and Storage Basics. Explains how batteries store solar energy, the main chemistries, and why conversion losses mean storage is never 100% efficient.
- U.S. Department of Energy: Energy Storage Technology and Cost Characterization Report (2019). Rates battery cycle life at a stated depth of discharge, which shows why discharge depth matters. Its figures are grid-scale reference values, not specific consumer batteries.
- Sandia National Laboratories: Temperature Effects on Sealed Lead Acid Batteries and Charging Techniques to Prolong Cycle Life (2004). Documents how heat shortens lead-acid battery life and how cold reduces performance.
- Battery University: Discharging at High and Low Temperatures. A widely used reference that explains how cold lowers available capacity in lead-acid and lithium batteries. It is an industry educational resource rather than a government publication.
- Sandia National Laboratories: Stand-Alone Photovoltaic Systems, A Handbook of Recommended Design Practices. A 1987 design handbook that covers battery selection and sizing alongside the rest of a stand-alone system. It is dated, so confirm current equipment specifications.
Battery Bank Runtime Calculator FAQ
How big of a battery bank do I need off-grid?
It depends on your average load, how many hours or days of backup you want, and your battery chemistry. Enter your load and desired backup time and the calculator returns the minimum rated capacity. With the tool’s built-in values, a lead-acid bank needs about 1.8 times the rated capacity of a lithium bank for the same usable runtime.
Should I enter rated capacity or usable capacity?
Enter rated capacity, the kWh on the battery’s label or datasheet. The calculator applies the usable percentage and your efficiency for you, and it shows the resulting usable figure in the results.
How do I convert amp-hours to kilowatt-hours?
Multiply volts by amp-hours and divide by 1,000. A 12 V, 100 Ah battery is 1.2 kWh, and four of them wired for 48 V and 100 Ah hold 4.8 kWh. Add up every battery in the bank.
Lithium or lead-acid: which is better for off-grid?
Lithium (LiFePO4) costs more upfront but allows much deeper usable discharge, and it is lighter. Lead-acid is cheaper initially but needs a larger rated capacity and more careful maintenance. Which is better depends on your budget and how you weigh upfront cost against usable capacity. The sizing feature above shows the capacity difference for your own numbers.
Why might my real runtime be shorter than the calculator says?
The tool leaves out several things that reduce real-world runtime: heavy discharge rates, inverter idle draw, heat, a bank that is not fully charged and any difference between its cold-weather curve and your battery’s. Cold weather and battery age are included only through the temperature and condition values you enter, so if you leave those at their defaults for a cold location or an older bank, the result will be too high. If your result is close to the runtime you need, add margin.
Does cold weather affect battery runtime?
Yes. Cold lowers the capacity a battery can deliver, and the effect can be large. One widely used battery reference notes that a battery giving full capacity at room temperature may deliver only about half at 0°F (−18°C). The calculator has a battery temperature field that applies a straight line from full capacity at 80°F to half at 0°F. If you live somewhere cold, keep the bank in a conditioned space where you can, enter the coldest temperature it will see, and size with extra margin.
What do the yellow warnings mean?
They flag conditions where the simple division of energy by load is least reliable. A very heavy load drains the rated capacity in under an hour, so check the battery’s maximum continuous discharge rating. A fast-discharge warning for lead-acid means the load would use up the rating in under 20 hours, so the runtime is an upper bound. A below-freezing warning reminds you that lithium generally should not be charged in the cold without heaters or protection, and that a discharged lead-acid battery can freeze.
Will solar recharging extend my runtime beyond this estimate?
Yes. This calculator assumes no solar input during the backup period, which represents a worst case like a stretch of cloudy days. Any recharging during that time will extend your actual runtime beyond the estimate.
How do I calculate battery backup time by hand?
Multiply battery voltage by amp-hours to get watt-hours, multiply by the fraction of that capacity you can safely use, then divide by your load in watts. Battery backup time (hours) = battery volts × amp-hours × usable fraction ÷ load in watts. Illustrative example with assumed numbers, not a measurement: a 12 V, 100 Ah battery is 1,200 Wh; at an assumed 80% usable that is 960 Wh; running a steady 200 W load, 960 ÷ 200 = 4.8 hours. The calculator above does the same math and adds the warnings that catch the cases where this simple division is optimistic.
How do I calculate inverter battery backup time?
Use the same formula, but size the load at the inverter’s AC output and account for the inverter’s own losses, since it draws more power from the battery than it delivers to your appliances. Check your inverter’s datasheet for its efficiency and its idle (no-load) draw, and for how much load it can carry continuously. Then reduce the usable battery energy by that efficiency before dividing by the load. If you would rather size the battery bank for a target number of hours, the calculator can work backward from your load and backup time.
This calculator provides general planning estimates, not an engineering design. Actual runtime depends on battery age and condition, temperature, wiring, inverter efficiency, and manufacturer specifications — consult a qualified solar installer or electrician before purchasing equipment.
