Battery Calculator

Use this Battery Calculator guide to estimate battery capacity, runtime, charging time, inverter load, solar backup size and usable energy for common battery systems. It is designed for website visitors who want practical battery planning without needing advanced electrical knowledge. The calculations below are estimates, but they help explain the most important relationships between volts, amps, watts, amp-hours and watt-hours.

This page is useful for small electronics, camping power stations, home backup systems, UPS planning, RV batteries, marine batteries, solar storage and off-grid projects. It focuses on safe planning and clear formulas. It does not replace the manufacturer’s documentation, local electrical codes or advice from a qualified electrician for high-power systems.

Important safety note: batteries can store a large amount of energy. Do not build, modify, open or rewire battery packs unless you are trained and following the manufacturer’s instructions. For lithium, high-current, vehicle, solar or home backup systems, use certified equipment and professional installation where required.

Quick Battery Calculator Formulas

The most useful battery calculations are based on a few simple formulas. These formulas work for many battery types, including Li-ion, LiFePO4, lead-acid, AGM and gel batteries, as long as the correct voltage, capacity, efficiency and usable depth of discharge are used.

What you want to calculate	Formula	Example
Watt-hours	Wh = Volts × Ah	12 V × 100 Ah = 1,200 Wh
Amp-hours	Ah = Wh ÷ Volts	1,200 Wh ÷ 12 V = 100 Ah
Runtime in hours	Runtime = Usable Wh ÷ Load W	960 Wh ÷ 100 W = 9.6 hours
Load in watts	W = Volts × Amps	12 V × 5 A = 60 W
Charging time	Hours = Battery Wh ÷ Charger W	1,200 Wh ÷ 300 W = 4 hours before losses
Solar charge estimate	Daily Wh = Panel W × sun hours × efficiency	400 W × 5 × 0.75 = 1,500 Wh/day

For real-world planning, always include losses. Inverters, chargers, cables, battery management systems and temperature can reduce usable energy. A simple estimate is to use 85% to 95% efficiency for direct DC use, 75% to 90% for AC inverter use and a lower value for harsh conditions.

Battery Runtime Calculator

Runtime tells you how long a battery can power a device. The basic method is to calculate total battery energy in watt-hours, reduce it to usable energy, then divide by the load in watts.

1. Find the battery voltage.
2. Find the battery capacity in amp-hours.
3. Multiply volts by amp-hours to get watt-hours.
4. Apply usable depth of discharge.
5. Apply inverter or system efficiency.
6. Divide usable watt-hours by the device load in watts.

Runtime formula: runtime hours = battery volts × battery amp-hours × usable depth of discharge × efficiency ÷ load watts.

Example: a 12 V 100 Ah LiFePO4 battery has about 1,200 Wh of nominal energy. If you use 90% depth of discharge and 90% inverter efficiency, usable energy is 1,200 × 0.90 × 0.90 = 972 Wh. A 120 W load would run for about 972 ÷ 120 = 8.1 hours.

Battery Capacity Calculator

Capacity planning works in the opposite direction. Start with the load, decide how many hours of runtime you need, then calculate the required battery size. This is helpful for backup power, emergency lighting, routers, laptops, fridges, CPAP machines, pumps, cameras and monitoring equipment.

Required battery Wh: required Wh = load watts × required hours ÷ usable depth of discharge ÷ efficiency.

Required battery Ah: required Ah = required Wh ÷ battery voltage.

Example: if you need to run a 75 W device for 10 hours from a 12 V battery through an inverter, and you assume 85% inverter efficiency and 80% usable depth of discharge, the battery should provide 75 × 10 ÷ 0.85 ÷ 0.80 = about 1,103 Wh. At 12 V, that is about 92 Ah. In practice, a 12 V 100 Ah battery would be a reasonable planning size, subject to the battery type and manufacturer limits.

Battery Charging Time Calculator

Charging time depends on battery size, charger power, battery chemistry, charge limits and the final slow charging phase. A rough estimate can be made from watt-hours and charger watts, but real charging is rarely perfectly linear.

Basic charging estimate: charging hours = battery Wh ÷ charger watts ÷ charging efficiency.

For a 12 V 100 Ah battery, nominal energy is 1,200 Wh. With a 300 W charger and 90% charging efficiency, estimated time is 1,200 ÷ 300 ÷ 0.90 = about 4.4 hours. Some batteries may take longer near full charge because the charging current tapers.

Always use a charger that matches the battery chemistry and voltage. A charger for one battery type may not be appropriate for another. Follow the battery manufacturer’s charging limits, temperature limits and safety instructions.

Common Battery Types and Usable Capacity

Two batteries with the same printed amp-hour rating may not provide the same usable energy. Battery chemistry, age, temperature, discharge rate and safe depth of discharge can make a large difference.

Battery type	Common use	Typical usable planning range	Notes
LiFePO4	Solar storage, RV, marine, backup power	80% to 95%	High usable capacity, long cycle life, requires proper BMS and compatible charger
Li-ion	Laptops, power banks, tools, portable stations	70% to 90%	High energy density, sensitive to heat and charging limits
Flooded lead-acid	Automotive, older backup systems	30% to 50%	Lower usable depth for long life, needs correct maintenance and ventilation
AGM lead-acid	UPS, mobility, marine, backup	40% to 60%	Sealed design, still affected by deep discharge and high temperature
Gel lead-acid	Standby, mobility, specialized uses	40% to 60%	Needs correct charging profile to avoid damage
NiMH	AA/AAA rechargeable cells, small electronics	60% to 80%	Useful for low-power devices, not usually used for large backup systems

For conservative planning, use the lower end of the range. This gives more margin for battery age, cold weather, high load, wiring losses and real-world inefficiency.

Common Battery Voltage Models

Battery systems are often described by nominal voltage. The same watt-hour requirement can be met with different voltage systems, but higher-voltage systems may reduce current for the same power level. The right choice depends on equipment compatibility, inverter requirements and safety rules.

Nominal model	Typical use	Example capacity	Nominal energy
3.7 V cell	Small electronics and power banks	3 Ah	11.1 Wh
5 V USB output	Phones, tablets, USB devices	20 Ah at 5 V	100 Wh before conversion losses
12 V battery	RV, marine, small solar, backup	100 Ah	1,200 Wh
24 V battery bank	Medium solar and inverter systems	100 Ah	2,400 Wh
36 V battery	E-bikes and mobility equipment	15 Ah	540 Wh
48 V battery bank	Larger solar, telecom, backup systems	100 Ah	4,800 Wh
51.2 V LiFePO4	Rack batteries and home energy storage	100 Ah	5,120 Wh

When comparing batteries, watt-hours are usually more helpful than amp-hours. A 12 V 100 Ah battery and a 24 V 50 Ah battery both contain about 1,200 Wh nominal energy, even though the amp-hour rating is different.

Inverter Battery Calculator

Many appliances use AC power through an inverter. Inverters are useful, but they introduce losses. They also have maximum continuous and surge power ratings. The battery must support both the total energy needed and the current required by the inverter.

AC runtime estimate: runtime = battery Wh × usable depth of discharge × inverter efficiency ÷ AC load watts.

Example: a 24 V 100 Ah battery bank has 2,400 Wh nominal energy. With 80% usable depth and 88% inverter efficiency, usable AC energy is 2,400 × 0.80 × 0.88 = 1,690 Wh. A 300 W appliance would run for about 5.6 hours.

For refrigerators, pumps, compressors and power tools, check surge requirements. A device may need much more power for a short time when starting than it uses during normal operation. Use manufacturer data instead of guessing when sizing an inverter or battery system.

Solar Battery Calculator

A solar battery estimate starts with daily energy use. Add the wattage of each device, multiply by hours used per day and total the result in watt-hours. Then compare the daily energy need with expected solar generation and battery storage.

Daily load: daily Wh = device watts × hours per day.

Solar production estimate: daily solar Wh = panel watts × peak sun hours × system efficiency.

Battery storage target: battery Wh = daily Wh × days of autonomy ÷ usable depth of discharge ÷ efficiency.

Example: if a small cabin uses 1,500 Wh per day and needs two days of backup, with 80% usable depth and 90% system efficiency, storage target is 1,500 × 2 ÷ 0.80 ÷ 0.90 = about 4,167 Wh. That is close to a 12 V 350 Ah system, a 24 V 175 Ah system or a 48 V 87 Ah system, before adding extra margin.

UPS and Backup Battery Calculator

UPS and backup systems are usually designed for a specific goal: keep equipment running long enough to save work, shut down safely or cover a short outage. For routers, modems, network equipment, security systems and small office devices, even a modest battery can make a large difference.

Backup runtime: runtime = usable battery Wh ÷ total protected load watts.

For example, if a modem and router use 25 W combined, a usable 200 Wh backup battery could power them for about 8 hours. If a desktop computer and monitor use 250 W combined, the same usable battery would last less than one hour. The load matters as much as the battery size.

For business, medical, security or critical systems, do not rely only on rough estimates. Use manufacturer runtime charts, professional assessment and tested backup procedures.

Example Device Runtime Table

The table below shows rough runtime examples using 500 Wh of usable energy. Real results vary by device, temperature, battery age, inverter losses and duty cycle.

Device or load	Approximate power	Estimated runtime from 500 Wh usable
Phone charging	10 W	About 50 hours of charging output
LED light	12 W	About 41 hours
Wi-Fi router	15 W	About 33 hours
Laptop	60 W	About 8 hours
Small fan	40 W	About 12.5 hours
Mini fridge average load	50 W	About 10 hours
TV	100 W	About 5 hours
Desktop PC and monitor	250 W	About 2 hours

Battery Calculator Checklist

Before choosing a battery, collect the key details. This makes the calculation more accurate and reduces the chance of buying a battery that is too small, too large or incompatible with the equipment.

• Total load in watts
• Required runtime in hours
• Battery voltage
• Battery chemistry
• Nominal amp-hours or watt-hours
• Usable depth of discharge
• Inverter or charger efficiency
• Peak or surge load
• Charging source and charger rating
• Operating temperature
• Manufacturer discharge and charge limits
• Safety approvals and installation requirements

Battery Calculator FAQ

Is watt-hours better than amp-hours?

Watt-hours are usually better for comparing total battery energy because they include voltage. Amp-hours are useful only when comparing batteries at the same voltage.

Why does my battery run for less time than the formula says?

Common reasons include inverter losses, lower usable depth of discharge, cold temperature, old batteries, high discharge rate, inaccurate load estimates and battery protection limits.

Can I use the full capacity of a battery?

Not always. Some batteries can be deeply discharged, while others last much longer when only partially discharged. Use the battery manufacturer’s recommended depth of discharge for planning.

How much extra battery capacity should I add?

A practical margin is often 20% to 30% above the calculated minimum. More margin may be needed for cold weather, critical loads, battery aging, solar uncertainty or high surge loads.

Can this calculator size a home battery system?

It can help with early estimates, but home battery systems should be designed with professional guidance, certified equipment and local code requirements. Whole-home backup can involve high power, grid interconnection and safety rules.

Final Planning Advice

A good battery calculator does not only produce a number. It helps you understand the assumptions behind that number. The safest approach is to calculate energy in watt-hours, apply realistic efficiency and usable capacity values, check the manufacturer’s limits, then add a sensible margin for real-world conditions.

For small devices, the estimates on this page can be enough to compare battery options. For larger systems, use these calculations as a starting point before getting professional advice. Batteries are most useful when they are sized carefully, charged correctly and used within safe operating limits.