Battery Size Calculator finds the amp-hours and number of batteries needed to run a load for a set time, on lead-acid or lithium banks at 12, 24 or 48 V with inverter loss counted.
System voltage is the battery bank's DC voltage, not the inverter's 120 V. Remaining charge is what is left at the end.
Inverter Loss and Margin
Enter about 85 to 95% when an inverter feeds the load. IEEE 485 adds a 25% aging margin to standby lead-acid.
Battery Bank Layout
Battery Size by Runtime
| Runtime | Battery Size | Bank Energy |
|---|---|---|
| 30.00 min | 68.31 Ah | 819.7 Wh |
| 1.000 h | 134.4 Ah | 1.613 kWh |
| 2.000 h | 260.4 Ah | 3.125 kWh |
| 3 h | 378.8 Ah | 4.545 kWh |
| 4.000 h | 490.2 Ah | 5.882 kWh |
| 6.000 h | 694.4 Ah | 8.333 kWh |
| 8.000 h | 877.2 Ah | 10.53 kWh |
| 10.00 h | 1,042 Ah | 12.50 kWh |
| 12.00 h | 1,190 Ah | 14.29 kWh |
| 24.00 h | 2,000 Ah | 24.00 kWh |
Step-by-Step Working
- Energy the load uses: 500 W × 3 h = 1.500 kWh.
- From the battery: 1.500 kWh ÷ 1 efficiency = 1.500 kWh, which is 125.0 Ah at 12 V.
- Remaining charge: 50% left means 50% is usable, so 125.0 Ah ÷ 0.5 = 250.0 Ah.
- Lead-acid rate factor: 0.02 × 3 h + 0.6 = 0.66, so 250.0 Ah ÷ 0.66 = 378.8 Ah.
- Battery count: 378.8 Ah ÷ 100 Ah rounds up to 4 in parallel, × 1 in series for 12 V = 4 batteries.
Battery Size Calculator Formula for Amp-Hours
The Battery Size Calculator works out how many amp-hours a battery bank needs to carry a load for a set time and still hold some charge at the end. To calculate battery size by hand, start with the current the load pulls from the battery.
P is the load in watts, V is the battery bank voltage and the last term is the inverter efficiency as a decimal. A 500 W load on a 12 V bank draws 41.67 A with no inverter loss and 49.02 A through an inverter entered at 85%.
B is the battery size in amp-hours, t is the run time in hours and Q is the percent of charge that must remain when the run ends. The tool opens on 500 W for 3 hours at 12 V with 50% remaining.
With the 85% inverter the same sum gives 294.1 Ah. That is the figure the common watts times hours over volts, efficiency and depth of discharge formula returns, and it is correct as it stands for a lithium battery.
Load can also be typed in kilowatts or as amps at battery voltage. Amps are taken as measured at the battery, so no inverter loss is added to them. If you only have volts and ohms for a DC load, the Current Calculator gives the amps, and the Electrical Power Calculator gives the watts.
Why Lead-Acid Needs a Bigger Bank
A lead-acid battery is rated over a 20 hour discharge. Empty it faster and it gives up less than its label says. For Lead-Acid, AGM or Gel the Battery Size Calculator therefore divides by a rate factor that depends on the run time.
At 3 hours the factor is 0.66, so the 250 Ah becomes 378.8 Ah. The factor reaches 1 at 20 hours and stays there, which is why long runs match the plain formula. The note under the result shows both figures whenever the factor applies.
Used as a battery size calculator for lithium ion or LiFePO4, the factor is dropped and the remaining charge changes to 20%. The same 500 W for 3 hours then needs 156.3 Ah, shown on the fourth card next to the lead-acid answer.
| Run Time at 500 W, 12 V | Lead-Acid, 50% Left | Lithium, 20% Left |
|---|---|---|
| 30 minutes | 68.3 Ah | 26.0 Ah |
| 1 hour | 134.4 Ah | 52.1 Ah |
| 2 hours | 260.4 Ah | 104.2 Ah |
| 3 hours | 378.8 Ah | 156.3 Ah |
| 4 hours | 490.2 Ah | 208.3 Ah |
| 8 hours | 877.2 Ah | 416.7 Ah |
| 24 hours | 2,000 Ah | 1,250 Ah |
The 50% figure for lead-acid follows battery maker practice. Owners on the Northern Arizona Wind and Sun forum cite the Trojan manual for 50% as the working floor and warn against going under 20% at all. Change Remaining Charge if your battery's data sheet says otherwise.
Inverter, Solar and Home Battery Sizing
As an inverter battery size calculator, the two entries that matter are under Inverter Loss and Margin. Inverter Efficiency raises the battery current, and System Voltage must be the DC voltage of the bank and not the 120 V on the output side.
Aging and Design Margin adds spare capacity on top. IEEE 485, the sizing practice for standby lead-acid, uses an aging factor of 1.25 so the battery still carries the load when it has faded to 80% of rated capacity. The Whole Building Design Guide paper on it also treats a design margin of 10 to 15% as typical. A 25% margin lifts the 378.8 Ah example to 473.5 Ah.
For a battery size calculator solar or off-grid users can work with, switch Size From to Daily Energy Use. Enter the kilowatt-hours used per day and the Days of Autonomy the bank must cover without charging.
To choose battery capacity for a house, take a home that uses 10 kWh a day and wants 2 days of autonomy. It needs 22.22 kWh from the battery through a 90% inverter. On a 48 V system that is 925.9 Ah of lead-acid or 578.7 Ah of lithium. A home battery size calculator result like this is more practical to build at 48 V than at 12 V, where it would be four times the amp-hours.
Daily energy comes from a utility bill or from adding up each appliance, which the Energy Consumption Calculator can total. The Battery Size by Days of Autonomy table under the cards lists the bank for 1 to 7 days.
The fourth card also shows the same bank at another voltage. As a battery size calculator for 12V, 24V or 48V systems, it makes one point clear. The 378.8 Ah bank at 12 V is 189.4 Ah at 24 V, the same 4.545 kWh with half the current.
Battery Count, Series and Parallel
Under Battery Bank Layout in the Battery Size Calculator, enter the capacity and voltage of one battery. The third card turns the required amp-hours into a whole number of batteries, with the series and parallel split. The 378.8 Ah example needs 4 batteries of 100 Ah at 12 V, wired 1 in series by 4 in parallel.
Batteries in series add voltage and parallel strings add amp-hours. The same card gives the Runtime With This Bank, 3.186 hours here, because rounding up to whole batteries buys a little extra time.
The second card is the one to check against the battery data sheet. It shows the battery current, the discharge rate as a C figure and the current per string. The example bank supplies 41.67 A at 0.11C, or 10.42 A from each of the four strings.
That current also sizes the cables between the bank and the inverter, which the Wire Size Calculator handles on its DC setting. To go the other way, from a battery you already own to the hours it will run, the Battery Life Calculator is the battery run time calculator for the job.
150Ah and 200Ah Battery Questions
How long can a 150Ah battery run a load of 300 watts?
A 12 V 150Ah lead-acid battery held to 50% runs a 300 watt load for about 1.9 hours, and a 600 watt load for about 56 minutes. A 200Ah battery lasts about 2.6 hours and 1.25 hours on the same loads. These use the rate factor above with no inverter loss.
| Load at 12 V | 150 Ah Lead-Acid | 200 Ah Lead-Acid | 150 Ah Lithium | 200 Ah Lithium |
|---|---|---|---|---|
| 300 W | 1.92 h | 2.61 h | 4.8 h | 6.4 h |
| 600 W | 0.93 h | 1.25 h | 2.4 h | 3.2 h |
Through a 90% inverter the 150Ah battery backup drops to 1.71 hours at 300 W and the 200Ah battery to 2.33 hours. To see it in the Battery Size Calculator, enter 300 W with a 1 hour runtime, set Single Battery Capacity to 150 and read Runtime With This Bank.
Which battery is best, 150Ah or 200Ah?
Neither is better in itself. A 200Ah battery capacity means it can deliver 10 A for 20 hours, which is 2.4 kWh at 12 V, and a 150Ah battery stores 1.8 kWh. The difference in capacity is a third more energy, and a little more than a third more run time on lead-acid because the larger battery is discharged at a gentler rate.
The correct battery size is the one the Battery Size Calculator returns for your load. If the answer is 140 Ah, a 150Ah battery covers it, and at 180 Ah the 200Ah battery does. Which brand is the no. 1 inverter battery matters less than matching the amp-hours, the voltage and the discharge current rating.
What is the maximum load capacity of a 150Ah battery?
It depends on how long the load must run. On the lead-acid formula with 50% remaining, a 12 V 150Ah battery carries about 558 W for one hour and a 200Ah battery about 744 W. The maximum current at any instant is a separate limit, printed on the data sheet as the continuous discharge rating.
That also answers what size of inverter is good for a 200Ah battery. An inverter is chosen by the watts of the load, and the battery sets how long it lasts. A 1,000 W load through a 90% inverter pulls 92.6 A from a 12 V 200Ah battery and runs for about 40 minutes.
Can a 150Ah battery run a fridge or an AC?
A fridge is a light load to put through the Battery Size Calculator. Take one that uses 1.2 kWh a day on its energy label, fed through a 90% inverter. A 12 V 150Ah lead-acid battery runs it for about 14 hours and a 200Ah battery runs the fridge for about 21.6 hours.
An air conditioner is a different scale. A 1.5 ton AC at an EER of 10 draws 1,800 W, which pulls 166.7 A from a single 12 V battery and drains a 150Ah lead-acid unit in about 16 minutes. For 2 hours at 24 V the bank is 520.8 Ah, which is 8 batteries of 150 Ah. Whichever battery is best for AC is the bank that meets that size.
How many hours to charge a 200Ah battery?
Divide the amp-hours to put back by the charger current. A 200Ah battery at 50% needs 100 Ah, which is 5 hours of bulk charging at 20 A. Lead-acid then needs more time to finish, and a Trojan Battery article puts a full recharge from 50% depth of discharge at six to eight hours.
Standard Sizes and Car Batteries
What are the standard battery sizes?
Battery size has two meanings. For household cells the standard battery sizes are AAA, AA, C, D and 9V, with AA the normal battery in most devices and D the largest of the common round cells. In China the AAA cell is sold as a No. 7 battery, which is what type 7 batteries refers to.
For vehicles the size is a BCI group number that fixes the case dimensions and terminal layout. NAPA lists Group 24 at 10-3/4 by 6-3/4 by 9 inches and Group 27 at 12 inches long, and notes that the right size for a car is the one its maker designed it to use.
How do I find my battery size?
To identify or check a battery size, read the label. Capacity is printed in Ah or mAh, next to the voltage, and a vehicle battery also carries its group number. A car battery size calculator question is really a fitment question, so match the group size from the owner's manual first.
This tool works as a battery Ah calculator for what the vehicle battery must run with the engine off. Enter the accessory load and the hours, and set Remaining Charge high enough that the engine will still start afterward.