Battery Size Calculator - Lead-Acid and Lithium Capacity Sizing

A battery size calculator answers the one practical question that sits behind every UPS, inverter, and off-grid build: how many amp-hours do you need to keep a given load running for a given time, while still leaving a safe reserve at the end? Enter the appliance wattage, the pack's nominal voltage, the required runtime, and the percentage of charge to want left, and the battery size calculator returns the load current and the capacity the pack must hold. It works for flooded and AGM lead-acid banks, for LiFePO4 home storage, and for Li-ion cells in a UPS, camper, or portable power station.

• • Size a UPS for a desktop and monitor: Estimate the amp-hour capacity a 12 V lead-acid UPS needs to keep a 200 W desktop running long enough to save work during a 10 minute outage.
• • Plan off-grid solar runtime: Match a fridge, lights, and a router against a 24 V lead-acid or lithium bank to see how many amp-hours each night of autonomy really costs.
• • Size a camper or RV house bank: Pick a 12 V lithium pack that can run lights, a fridge, and a water pump for a weekend without falling below the safe depth of discharge.
• • Compare lead-acid and lithium-ion directly: Run the same load through both chemistries and see how much smaller the lithium-ion pack can be.

A battery size calculator sits one step upstream of capacity conversion. Once the load demands a known amp-hour figure, the Tools cluster handles watt-hours, C-rate, and charger matching.

Treat the result as the upper bound for a healthy pack at room temperature and derate by ten to twenty percent for cold weather, ageing cells, or inverter losses before buying hardware.

Once the size is known the next step is converting amp-hours into watt-hours and matching a C-rate, and that workflow is handled by the battery capacity calculator in the Tools cluster.

The math has two stages. First the tool turns appliance wattage and pack voltage into a steady load current. Then it feeds that current into the chemistry-correct capacity formula to return the recommended amp-hour size.

• P (loadWatts): Appliance load in watts.
• V (voltage): Nominal pack voltage in volts.
• I_L (loadCurrent): Steady current in amperes, equal to P divided by V.
• t (duration): Runtime in hours, converted from the chosen unit.
• Q (remainingCharge): Charge percentage to leave in the battery at the end.
• Battery type: Picks the formula: lead-acid uses the Peukert term; lithium-ion does not.

The lead-acid formula carries a Peukert-style correction (0.02t + 0.6). The 0.6 term is a baseline derating for cell losses and the 0.02 multiplied by duration adds an extra penalty as the run gets longer. Lithium-ion drops the correction term.

Switching the duration unit from minutes to hours triggers an internal conversion before the math runs.

According to Omni Calculator, the lead-acid battery size is B_Pb = (100 x I_L x t) / ((100 - Q)(0.02t + 0.6)) and the lithium-ion battery size is B_Li = (100 x I_L x t) / (100 - Q), with I_L equal to the appliance wattage divided by the battery voltage.

According to NIST Guide for the Use of the SI, the ampere is the SI base unit of electric current and the volt is the SI derived unit of potential difference, which together define ampere-hour and watt-hour as the standard units for battery charge and energy.

When the load is a multirotor pulling current at a fixed C-rate the same amp-hour rating feeds the drone flight time calculator to translate capacity into minutes in the air.

Four ideas drive every battery sizing decision.

Lead-acid and lithium-ion behave very differently under deep discharge, which is why the calculator carries two formulas. Increase the reserve if the pack will see high temperatures, fast charging, or daily deep cycling.

If the load is variable, average the wattage first, or rerun with the peak wattage to bracket the worst case.

For handheld electronics the Peukert correction is small and the same amp-hour and voltage inputs feed the battery life calculator to estimate hours of device runtime.

The form takes five inputs and returns three answers in real time.

1. 1 Enter the application load in watts: Type the wattage of the appliance or the combined load wattage.
2. 2 Pick the battery chemistry: Choose lead-acid for flooded, AGM, or gel packs and lithium-ion for LiFePO4 or NMC packs.
3. 3 Enter the pack voltage: Use 12 V for car or UPS, 24 V or 48 V for off-grid solar, 3.7 V per Li-ion cell.
4. 4 Choose the duration unit and enter the runtime: Pick hours for overnight planning and minutes for short UPS or test runs.
5. 5 Set the remaining-charge reserve: Use 50 percent for lead-acid and 80 percent for lithium-ion as the safe cycle-life default.
6. 6 Read the load current and amp-hour capacity: Use the load current to size wiring, fuses, and charge controllers, and the amp-hour capacity to choose the pack.

Sizing a UPS for a 200 W desktop computer: enter 200 W, lead-acid, 12 V, 10 minutes, 50 percent remaining. The calculator shows 16.67 A load current and 9.21 Ah battery size, matching the Omni Calculator worked example.

After sizing the pack, the next question is how long it takes to refill, and the battery charge time calculator handles that follow-up with capacity and charger current as inputs.

Six practical wins show up the first time you size a battery bank instead of guessing.

• • Avoid buying an oversized battery bank: The amp-hour figure is derived from the actual load and runtime, so you stop paying for capacity the load never uses.
• • Plan UPS and inverter runtime: Set the runtime to the minutes or hours you need during an outage and the calculator returns the capacity that buys you that window.
• • Compare lead-acid and lithium-ion directly: Running the same load through both chemistry options shows how much smaller the lithium-ion pack can be.
• • Size wiring and fuses from the load current: The load current in amperes is the number that drives cable gauge, fuse rating, and charge-controller sizing.
• • Build in a safe depth-of-discharge reserve: The remaining-charge field forces you to choose how empty the pack is allowed to go, the biggest factor in cycle life.
• • Plan variable-load systems: Average the wattage of a fridge compressor and lights, or use the peak wattage to bracket the worst case.

Pairing a load current with an amp-hour requirement also helps when you pick a solar charge controller, since charge current is usually capped at a fraction of battery capacity.

The watt-hour line is the cleanest comparison number when you weigh a 12 V pack against a 24 V pack for the same load, because it removes the voltage difference.

For starter batteries the same amp-hour and cold cranking amp logic is what the car battery sizing calculator uses to recommend a BCI group size for a given engine and climate.

Five factors shift the recommended battery size in real installations.

• • The (0.02t + 0.6) Peukert correction is a simplified approximation that fits 12 V to 48 V packs; very small Li-ion cells or large stationary banks may need manufacturer data.
• • The calculator assumes a steady DC load. Variable loads like a fridge compressor cycle need averaging or worst-case bracketing before you trust the answer.

Battery sizing is a planning step, not a perfect forecast. Buy a pack with at least twenty percent margin over the calculator output when the system will run unattended, and re-check the sizing after the first year of service.

For off-grid solar sizing, compare the required amp-hour capacity against the daily energy harvest from your panels, the same flow used by guides that quote 1.5 to 2 days of autonomy at 50 percent DoD.

According to Battery University, lead-acid batteries tolerate roughly 50 percent depth of discharge for routine cycling and lithium-ion cells last longer when kept above 20 percent depth of discharge.

The state-of-health factor above matters most for traction packs, where the EV battery degradation calculator tracks how much of the original capacity remains after years of cycling.