AH Calculator
Estimate battery amp-hours, current draw, and runtime with a premium ah calculator built for solar systems, RV power banks, marine batteries, UPS planning, and off-grid energy design. Enter your voltage, power, current, runtime, battery type, and depth of discharge to get practical results instantly.
Battery Capacity Calculator
How to Read the Calculator
This ah calculator supports three practical workflows:
- Find Ah from watts and runtime: Convert appliance power and target hours into a recommended battery capacity.
- Find Ah from amps and runtime: Use a direct current draw for a more exact sizing estimate.
- Find runtime from battery Ah: Estimate how long an existing battery can support your device.
Expert Guide to Using an AH Calculator
An ah calculator helps estimate how much battery capacity you need or how long a battery can run a device. The term AH stands for amp-hour, a standard unit used to describe battery charge capacity. In practical terms, a battery rated at 100 Ah can theoretically deliver 100 amps for 1 hour, 10 amps for 10 hours, or 5 amps for 20 hours. Real world performance depends on voltage, temperature, battery chemistry, discharge rate, usable depth of discharge, and system efficiency. That is why a quality calculator should never stop at the simplest formula. It should also help you translate battery charge into energy and expected runtime.
For most people, the key challenge is that battery planning usually starts with watts, not amps. A CPAP machine, mini fridge, trolling motor charger, router, LED lights, laptop dock, and emergency backup inverter are usually described in watts. A battery bank, however, is commonly labeled in amp-hours and voltage. To bridge that gap, you can use the relationship between power, voltage, and current: watts equal volts multiplied by amps. Once current is known, amp-hours become easier to estimate. If you know the appliance wattage and how many hours it must run, an ah calculator converts that requirement into a battery size recommendation.
What does AH mean on a battery?
AH indicates the amount of electric charge a battery can store and deliver over time. It does not directly tell you total energy unless voltage is included. For that reason, comparing a 100 Ah battery at 12 V with a 100 Ah battery at 24 V can be misleading if you look only at amp-hours. The 24 V battery stores roughly twice the energy because energy in watt-hours is calculated as voltage multiplied by amp-hours. A 12 V 100 Ah battery stores about 1,200 Wh, while a 24 V 100 Ah battery stores about 2,400 Wh before accounting for system losses or battery protection limits.
Core formulas behind an ah calculator
- Amp-hours: Ah = A × h
- Current from power: A = W ÷ V
- Energy: Wh = V × Ah
- Runtime: h = Ah ÷ A
- Adjusted battery sizing: Required Ah = Load Ah ÷ usable fraction ÷ efficiency fraction
The adjusted sizing formula matters because few systems can safely use 100 percent of a battery’s rated capacity every day. Lead acid batteries often last longer when depth of discharge is limited. Lithium batteries such as LiFePO4 usually allow a much greater usable share, often around 80 to 100 percent depending on the battery management system and manufacturer guidance. Efficiency also matters. If your power path includes an inverter, some energy is lost as heat, and that means your battery must be larger than a simple idealized calculation suggests.
How to calculate battery amp-hours from watts
Suppose you want to run a 120 watt load on a 12 volt battery system for 5 hours. First convert watts to amps:
120 ÷ 12 = 10 amps
Then multiply by runtime:
10 × 5 = 50 Ah
That 50 Ah result is the ideal charge requirement with no losses and full battery utilization. In a more realistic design with 90 percent efficiency and 80 percent usable depth of discharge, the recommended battery size becomes:
50 ÷ 0.90 ÷ 0.80 = 69.44 Ah
This is why practical calculators often suggest stepping up to a 75 Ah or 100 Ah battery, especially if startup surges, cold weather, or future load growth are possible.
How to estimate runtime from battery Ah
If you already own a battery, the process reverses. A 12 V 100 Ah battery contains around 1,200 Wh of nominal energy. If your usable capacity is 80 percent and your system efficiency is 90 percent, the practical usable energy becomes:
1,200 × 0.80 × 0.90 = 864 Wh
If your device draws 120 watts, estimated runtime is:
864 ÷ 120 = 7.2 hours
This style of calculation is especially useful for backup planning during outages, solar storage design, recreational vehicle upgrades, and marine electronics sizing.
Battery chemistry and usable capacity
Different chemistries behave differently. Flooded lead acid and AGM batteries are often rated using standardized testing conditions, but actual delivered capacity can change significantly with discharge rate and temperature. Lithium iron phosphate batteries tend to provide flatter voltage curves, greater cycle life, and higher practical depth of discharge. That does not mean every lithium battery should always be used to 100 percent depth of discharge, but it often means the usable share of nameplate capacity can be much higher than for traditional lead acid designs.
| Battery Type | Typical Recommended Daily Depth of Discharge | Typical Practical Efficiency | General Use Notes |
|---|---|---|---|
| Flooded Lead Acid | About 50% | About 80% to 85% | Common in legacy off-grid systems and backup applications, but heavier and more maintenance intensive. |
| AGM | About 50% to 60% | About 85% to 90% | Sealed lead acid option with less maintenance and good vibration resistance. |
| Gel | About 50% to 60% | About 85% to 90% | Useful in some deep cycle contexts, but charging parameters must be controlled carefully. |
| LiFePO4 | About 80% to 100% | About 95% to 98% | Popular in modern RV, marine, and solar storage setups due to long cycle life and strong usable capacity. |
| Lithium Ion | About 80% to 90% | About 90% to 96% | Broad category with varying safety and cycle-life characteristics depending on cell chemistry. |
The values above are broad planning ranges rather than manufacturer specific specifications. Always confirm charge and discharge guidance with the battery brand you intend to use. For authoritative energy system references, see the U.S. Department of Energy at energy.gov, the National Renewable Energy Laboratory at nrel.gov, and battery safety information from institutions such as osha.gov.
Why voltage matters so much
Many users focus only on amp-hours, but voltage strongly affects current and cable sizing. If a 1,200 watt load operates at 12 volts, it draws about 100 amps. The same 1,200 watt load at 24 volts draws about 50 amps, and at 48 volts only about 25 amps. Lower current can reduce conductor size requirements, decrease resistive losses, and improve system efficiency for larger installations. That is one reason larger solar and backup systems often use 24 V or 48 V architectures rather than remaining at 12 V.
| Load Power | Current at 12V | Current at 24V | Current at 48V |
|---|---|---|---|
| 120 W | 10 A | 5 A | 2.5 A |
| 600 W | 50 A | 25 A | 12.5 A |
| 1,200 W | 100 A | 50 A | 25 A |
| 2,400 W | 200 A | 100 A | 50 A |
Common AH calculator use cases
- RV and van life: Size a house battery for lights, fans, refrigerators, laptops, water pumps, and induction cooking.
- Marine systems: Estimate capacity for trolling motors, fish finders, chart plotters, radios, and cabin loads.
- Solar storage: Match battery size to overnight loads and desired backup autonomy.
- Home backup: Understand how long a battery can power internet gear, medical devices, lights, and critical appliances.
- Portable power stations: Compare actual energy needs against nameplate battery capacity and inverter efficiency.
Frequent mistakes people make
- Ignoring inverter losses. AC loads often require extra battery capacity because the inverter is not 100 percent efficient.
- Using only rated battery Ah. Nameplate capacity is not the same as safe daily usable capacity.
- Forgetting surge loads. Motors and compressors can draw much more at startup than during steady operation.
- Confusing Ah and Wh. Amp-hours alone do not fully describe battery energy across different voltages.
- Not allowing a safety margin. Temperature changes, aging, and future load additions can all reduce effective performance.
How much reserve should you add?
In many real installations, adding 10 percent to 25 percent reserve capacity is a wise design choice. If the load is mission critical, used in cold weather, or expected to grow over time, a reserve closer to 25 percent can provide more reliable operation. Batteries also age. Capacity generally decreases over the service life of the pack, so a system that looks perfect on day one may feel undersized after several years of cycling.
Interpreting battery labels and manufacturer data
Battery labels can be confusing because some products emphasize amp-hours while others focus on watt-hours. Deep cycle batteries used in RV and solar markets are usually discussed in Ah at a nominal system voltage, while portable electronics and smaller power stations often emphasize Wh. Both are useful, but Wh creates a more direct energy comparison across voltages. If you are comparing products, convert to watt-hours whenever possible. Then review cycle life, maximum continuous discharge current, temperature range, and warranty details.
Practical example for off-grid planning
Imagine an overnight cabin load consisting of a 70 W refrigerator average draw, 20 W of LED lighting, a 45 W router and communications load, and a 65 W laptop charger used for 3 hours. The continuous baseline is 135 W. Over 10 hours, that baseline consumes 1,350 Wh. The laptop contributes another 195 Wh, for a total of 1,545 Wh. If the battery system is 12 V, the ideal charge requirement is 1,545 ÷ 12 = 128.75 Ah. With 90 percent efficiency and 80 percent usable capacity, the recommended battery bank becomes 128.75 ÷ 0.90 ÷ 0.80 = 178.82 Ah. That is why a 200 Ah class battery bank is often a more realistic solution than a 100 Ah battery for overnight mixed loads.
Final advice for sizing with confidence
An ah calculator is most useful when you treat it as a planning tool rather than a magic answer. Enter realistic load data, use conservative efficiency assumptions, choose a sensible depth of discharge for your battery chemistry, and include reserve capacity. If your system will operate through an inverter, verify continuous and surge ratings. If your loads include motors, compressors, pumps, or heating elements, check startup current separately. Finally, compare your results with battery manufacturer specifications and system design guidance from trusted technical sources.
Done correctly, amp-hour planning can save money, reduce frustration, and prevent undersized power systems. Whether you are building a solar battery bank, selecting an RV house battery, sizing a marine trolling setup, or planning emergency backup for essential devices, this ah calculator gives you a fast and practical starting point.