Battery Wh Calculator

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Battery Wh Calculator

This calculator converts battery voltage and amp-hour (Ah) rating into watt-hours (Wh) — the actual amount of electrical energy a battery can theoretically store. Enter your battery's voltage and Ah rating below and it does the math instantly.

Watt-hours matter because Ah alone only tells you half the story. A 12V 100Ah battery and a 24V 100Ah battery both carry the same amp-hour number, but the 24V battery holds twice the energy. The formula behind this calculator is simple:

Take a 12V 100Ah battery as an example. 12 × 100 = 1,200. So a 12V 100Ah battery has 1,200 Wh of nominal energy — that's the theoretical figure printed on the label, not necessarily what you'll actually get out of the battery in daily use. Real-world usable energy is usually lower once you account for battery chemistry, depth of discharge, temperature, age, and how hard the battery is being worked. The optional fields in the calculator below let you factor those in.

This tool gives a nominal (label-rated) energy figure and, if you fill in the optional fields, a rough usable-energy estimate. It isn't a substitute for your battery manufacturer's datasheet.

Common Battery Wh Examples

If you just want a quick reference instead of typing numbers into the calculator, here's how the most common battery sizes work out:

Notice the pattern: for the same Ah rating, a higher-voltage battery stores more energy. 12V × 100Ah gives 1,200 Wh, 24V × 100Ah gives 2,400 Wh, and 48V × 100Ah gives 4,800 Wh — double the voltage, double the energy, same Ah number.

What Is a Watt-Hour (Wh)?

A watt-hour is a unit of energy — not power. It represents one watt of power sustained for one hour. A battery rated at 1,000 Wh, in theory, could deliver:

• 1 watt for 1,000 hours
• 10 watts for 100 hours
• 100 watts for 10 hours
• 500 watts for 2 hours

In practice, runtime rarely lines up this neatly. Batteries lose some capacity to internal resistance, heat, and voltage sag as they discharge, so the last few watt-hours are usually harder to extract than the first few. Still, this simple relationship is what makes Wh so useful for sizing decisions — it's a common currency you can compare against inverters, solar panels, and appliances.

It helps to keep these four terms straight, since they get mixed up constantly:

What Is Battery Capacity in Ah?

Amp-hours describe how much electrical charge a battery can deliver — 1 amp of current for 1 hour, 2 amps for 30 minutes, and so on. It's the number stamped on nearly every lead-acid and lithium battery label. But Ah alone doesn't tell you how much energy is stored, because it says nothing about voltage.

If you have ever looked at a battery label and wondered what the Ah number actually means on its own, the missing piece is voltage. Take a 12V 100Ah battery:

• 12V — the electrical potential (pressure) the battery operates at
• 100Ah — the charge capacity at that voltage
• 1,200Wh — the nominal stored energy once voltage and Ah are combined

That 100Ah number sounds impressive on its own, but it doesn't tell you the full amount of energy until voltage enters the equation. A 100Ah battery at 12V and a 100Ah battery at 48V are not remotely equivalent — the second one stores four times as much energy.

Battery Wh Formula

Where Wh is watt-hours, V is battery voltage in volts, and Ah is battery capacity in amp-hours. Once voltage is included, the calculation becomes straightforward.

How to Calculate Battery Wh

If you'd rather work it out by hand than use the calculator above, here's the process in four steps.

Step 1: Find battery voltage

Check the battery label or datasheet. Common systems are 12V, 24V, 36V, and 48V.

Step 2: Find battery capacity

Look for the Ah rating, usually printed right next to the voltage — for example, 100Ah.

Step 3: Multiply voltage by Ah

12 × 100 = 1,200 Wh

Step 4: Convert Wh to kWh if needed

1,200 ÷ 1,000 = 1.2 kWh

Or skip the manual math entirely and let the calculator above do it for you, including the optional usable-energy estimate.

12V Battery Wh Calculator

12V is the most common battery voltage for small solar setups, RVs, UPS units, and backup systems. Here's what a range of 12V batteries work out to:

• 12V 7Ah = 84 Wh
• 12V 20Ah = 240 Wh
• 12V 50Ah = 600 Wh
• 12V 100Ah = 1,200 Wh
• 12V 150Ah = 1,800 Wh
• 12V 200Ah = 2,400 Wh

You'll typically see 12V batteries used in UPS systems, small solar setups, emergency lighting, RV house batteries, small DC loads, and portable electronics. It's a convenient voltage because so many inverters, chargers, and accessories are already built around it.

24V Battery Wh Calculator

Move up to 24V and the same Ah rating stores twice the energy. A 24V 100Ah battery:

• 24V 50Ah = 1,200 Wh
• 24V 100Ah = 2,400 Wh
• 24V 200Ah = 4,800 Wh

24V systems are common in mid-sized solar setups, some RV and marine systems, and inverter systems where lower current (compared to 12V for the same power) means thinner cabling is possible.

48V Battery Wh Calculator

A 48V 100Ah battery bank:

48V systems show up frequently in larger solar and home energy-storage setups because delivering the same power at a higher voltage means lower current, which in turn allows thinner wiring and reduces resistive losses. That doesn't make 48V universally "better" — it depends on the equipment you're pairing it with, the size of the load, and how the overall system is designed. A well-matched 12V or 24V system can be exactly right for a smaller setup.

Ah to Wh Calculator

To convert amp-hours to watt-hours, multiply by voltage:

Amp-hours cannot be converted to watt-hours without knowing the voltage. This is the single most common point of confusion when people compare batteries — a "200Ah" battery could be a small 12V unit or a much larger 48V bank, and the Ah number alone won't tell you which.

Wh to Ah Calculator

Going the other direction, divide watt-hours by voltage:

• 1,200 Wh ÷ 12V = 100Ah
• 2,400 Wh ÷ 24V = 100Ah
• 4,800 Wh ÷ 48V = 100Ah

As with the forward calculation, you need to know the battery's voltage to convert Wh back into Ah — there's no way around that requirement.

Battery Wh vs Ah

A 12V 100Ah battery and a 24V 100Ah battery share the same Ah number but store very different amounts of energy — 1,200 Wh versus 2,400 Wh. If you're shopping between batteries of different voltages, Wh is the number that actually tells you how much energy you're getting.

Battery Wh and Runtime

Once you know a battery's watt-hours, you can get a rough runtime estimate against a given load:

Real runtime comes in lower than that ideal number, usually because of inverter efficiency, the battery's own internal losses, how deeply it's allowed to discharge, wiring resistance, temperature, and the battery's age. Here's a more realistic version of the same example:

Treat both of these as estimates, not guarantees — actual runtime depends heavily on the specific battery, inverter, and load involved.

Battery Wh and Inverter Sizing

Wh and W measure different things, and mixing them up leads to undersized systems. Wh describes how much energy is stored; W describes how much power can be delivered at any given instant. A big battery with a lot of stored energy doesn't automatically mean it can supply a large load — that depends on the maximum current the battery and its wiring can handle.

When sizing an inverter, it's worth looking beyond the battery's Wh rating and considering:

• Continuous load the inverter needs to supply
• Surge or startup load, especially for motors and compressors
• Motor starting current, which can spike several times higher than running current
• Battery voltage, since it affects current draw for a given wattage
• Maximum continuous and peak current the battery can safely supply
• Inverter efficiency and its own power draw
• Manufacturer specifications for both the battery and inverter

For example, a 12V 100Ah (1,200 Wh) battery has plenty of stored energy for a 1,500W surge load for a few seconds, but whether it can actually deliver that current safely and continuously depends on the battery's discharge rating, not just its Wh figure.

Battery Wh and Solar Systems

In a solar setup, watt-hours are the bridge between what your panels can produce and what your battery can store for use after dark. Sizing a solar-plus-battery system usually means weighing:

• Solar panel power rating, in watts
• Battery energy capacity, in Wh or kWh
• Daily household or appliance energy consumption
• Charging losses between the panels and the battery
• Inverter losses when converting stored DC energy to AC
• How much of the battery's nominal capacity is actually usable
• Day-to-day variability in solar production (clouds, season, panel angle)

Say a household uses roughly 4 kWh (4,000 Wh) a day. On paper, a single battery rated at 4,000 Wh nominal might look sufficient. In practice, the system usually needs a larger battery than that, because not all of the nominal capacity is available for use once depth-of-discharge limits and conversion losses are factored in.

Usable Battery Capacity vs Nominal Capacity

This distinction matters more than almost anything else on this page. Nominal capacity is the label number — voltage times Ah, with nothing subtracted. Usable capacity is what you can actually draw from the battery in a real system, after accounting for depth of discharge and conversion losses.

The exact usable percentage depends entirely on the battery chemistry and the system it's part of — there's no single universal number. A lithium battery might tolerate a much deeper discharge than a lead-acid battery of the same nominal rating. Always check the manufacturer's recommended depth of discharge rather than assuming a fixed percentage.

Battery Chemistry and Watt-Hours

The Wh figure from the V × Ah formula is based on nominal voltage and the rated capacity printed on the label. What varies by chemistry is how much of that nominal energy can practically be used, and how the voltage behaves as the battery discharges. Common chemistries include:

• Lead-acid — inexpensive, widely available, but typically limited to shallower recommended discharge depths for longer life
• AGM — sealed lead-acid variant, more resistant to vibration and spillage, similar discharge considerations to flooded lead-acid
• Gel — another sealed lead-acid variant, generally more sensitive to charging voltage than AGM
• Lithium-ion — higher energy density, generally tolerates deeper discharge than lead-acid
• LiFePO4 (lithium iron phosphate) — common in modern solar and backup batteries, valued for cycle life and stable discharge voltage

None of this changes the basic Wh formula, but it does change how much of that nominal figure you should plan to actually use. Always defer to the manufacturer's datasheet for real operating limits.

Lead-Acid Battery Wh Calculation

The 1,200 Wh figure is the same regardless of chemistry — it comes straight from voltage times Ah. What differs with lead-acid batteries is that the practically usable portion of that energy is often lower than with lithium, because deep, repeated discharges shorten lead-acid battery life more aggressively. Many lead-acid battery guidelines suggest keeping discharge shallower to preserve cycle life, though the specific number depends on the battery and how it's used.

Lithium Battery Wh Calculation

You'll notice lithium battery labels often list a nominal voltage like 12.8V instead of a flat 12V — that's because LiFePO4 cells have a slightly different resting voltage than lead-acid cells, even though the battery is marketed as a "12V" drop-in replacement. The exact nominal voltage, usable capacity, and recommended depth of discharge vary by manufacturer, so it's worth checking the specific datasheet rather than assuming a round number.

Battery Bank Wh Calculation

Connecting more than one battery changes either the voltage or the Ah rating of the bank, depending on how they're wired.

Batteries in Parallel

Wiring same-voltage batteries in parallel keeps the voltage the same and adds the Ah ratings together.

Batteries in Series

Wiring the same batteries in series keeps the Ah rating the same and adds the voltages together.

With identical batteries, series and parallel wiring can land on the same total nominal energy — the difference is the resulting voltage and current characteristics of the bank, not the total stored energy.

Safety note: combining batteries into series or parallel banks must follow the manufacturer's guidelines for that specific battery model, using matched batteries, appropriate cabling, and correct protection. Mixing battery ages, brands, or states of charge in a bank is a common source of problems.

Battery Bank Wh Formula

Four identical 12V 100Ah batteries store the same total nominal energy — 4,800 Wh — no matter how they're arranged, because the total energy in the bank doesn't change with wiring configuration. What changes is the resulting system voltage and current, which needs to match what your inverter or charge controller is designed for.

How Many Wh Does a 12V 100Ah Battery Have?

That's the label figure. The energy you can actually use day to day will typically be somewhat lower, depending on the battery's chemistry and how deeply you discharge it.

How Many Wh Does a 24V 100Ah Battery Have?

How Many Wh Does a 48V 100Ah Battery Have?

How Many Wh Do I Need?

The battery capacity you actually need comes down to your load and how long you want to run it:

That 2,500 Wh figure is only the starting point — the energy the load actually consumes, before any losses. In a real system, you'd typically want a battery larger than 2,500 Wh nominal, since inverter losses, depth-of-discharge limits, and a safety margin all eat into what's usable. How much larger depends on your battery chemistry and the efficiency of your specific equipment.

Battery Wh Calculator for Backup Power

For general backup-power planning, it helps to add up the wattage of what you actually want to keep running, then multiply by how many hours you need it. A rough set of typical draws:

• LED lights: 5–15W each
• Box fan: 40–100W
• LED TV: 60–150W
• Wi-Fi router: 10–20W
• Laptop: 30–65W
• Refrigerator (running load, with much higher startup surge): 100–800W depending on size

These are general ranges, not guarantees for any specific appliance — always check the wattage on your own device's label before sizing a system around it. Add up the watts of everything you plan to run, multiply by your desired runtime in hours, then compare that figure against a battery's usable — not nominal — Wh rating.

Battery Wh Calculator for UPS

UPS sizing works the same way, with the added consideration of the UPS's own conversion efficiency.

Actual UPS runtime depends on the specific UPS unit, the condition and age of the battery, the connected load, and manufacturer specifications — treat this as a planning estimate, and check your UPS's own runtime charts where available.

Battery Wh Calculator for Solar Inverters

Sizing a battery for a solar inverter setup pulls together everything covered above: battery voltage, Ah, the resulting Wh, inverter efficiency, your load in watts, desired runtime, and depth of discharge.

Run your own numbers through the calculator above using your battery's actual voltage, Ah rating, and your inverter's efficiency spec for a closer estimate.

Common Battery Wh Calculation Mistakes

Mistake 1: Using Ah without voltage

Comparing two batteries by Ah alone ignores voltage entirely, and voltage is half of the energy equation. A "200Ah" battery could be a small 12V unit or a much bigger 48V bank.

Mistake 2: Assuming nominal Wh equals usable Wh

The label number is a ceiling, not a guarantee. Depth of discharge and system losses bring the real, usable number down.

Mistake 3: Ignoring inverter efficiency

Converting stored DC energy to AC power always loses some energy in the process. Skipping this step overestimates runtime.

Mistake 4: Confusing W and Wh

Watts measure the rate of energy use right now; watt-hours measure total energy stored or consumed over time. A 1,000W inverter and a 1,000Wh battery are not the same kind of number at all.

Mistake 5: Assuming battery voltage is always exactly 12V

"12V" is a nominal label. Actual resting voltage might be 12.6V–12.8V for lead-acid, or 13.2V–13.6V for a fully charged LiFePO4 pack marketed as "12V." For quick estimates, the nominal figure is fine; for precise work, check the actual datasheet.

Mistake 6: Ignoring battery age

Capacity fades with cycle count and age. A three-year-old battery rated at 100Ah when new may no longer deliver a full 100Ah, which lowers its real-world Wh as well.

Mistake 7: Assuming every battery can be discharged completely

Discharging most batteries to 0% shortens their lifespan dramatically and, for some chemistries, can damage them outright. Manufacturer-recommended depth-of-discharge limits exist for a reason.

Mistake 8: Comparing batteries using only Ah

Two batteries with the same Ah rating but different voltages store very different amounts of energy. Wh gives you an apples-to-apples comparison; Ah by itself does not.

Watts vs Watt-Hours

A 500W appliance draws power at a rate of 500 watts, continuously, while it's running. A battery holding 2,000Wh stores a fixed quantity of energy that can be drawn down over time. Running that 500W appliance from the 2,000Wh battery would use up its stored energy in about four hours, ignoring losses:

2,000 ÷ 500 = 4 hours

Think of watts as the flow rate and watt-hours as the total volume in the tank — but don't rely on that analogy alone for real calculations; use the actual Wh and W figures from your equipment.

Wh vs kWh

• 500 Wh = 0.5 kWh
• 1,200 Wh = 1.2 kWh
• 2,400 Wh = 2.4 kWh
• 10,000 Wh = 10 kWh

Residential energy storage systems, solar batteries, and utility bills are usually described in kWh simply because the raw Wh numbers would otherwise run into the thousands and become harder to read at a glance.

Battery Energy Formula Examples

Frequently Asked Questions

Battery Wh Calculator Formula Summary

These are simplified planning formulas, not lab-grade predictions. Real battery performance depends on chemistry, temperature, age, discharge rate, and the specific equipment in your system — use manufacturer datasheets for final sizing decisions.

Final Thoughts

Watt-hours describe how much energy a battery actually stores, and the basic formula behind it — voltage multiplied by Ah — is one you can do in your head for most common battery sizes. From there, converting to kWh, estimating usable capacity, and working out rough runtime are all small extensions of the same idea. Just remember that nominal Wh is a ceiling, not a promise: depth of discharge, efficiency, chemistry, and age all determine what you actually get to use.

If you have a battery in front of you right now, plug its voltage and Ah rating into the Battery Wh Calculator above and see exactly how much energy you're working with.

Related Calculators

For sizing a full backup or solar power system, these related tools on Inverter110.com build on the numbers from this page: