Battery Capacity Explained in Amp Hours, Watt Hours, and Runtime

The battery capacity indicates the amount of energy that a battery can store. It sounds simple. However, most consumers make the wrong comparison. They compare mAh to mAh or Ah to Ah without taking a look at the voltage.

This is the straightforward answer. The best way to compare battery capacity is in watt-hours (Wh). Wh already accounts for voltage. Ah & mAh are only part of the story when you know the voltage as well.

We will go through the units, the formulas, and some examples of running time. You'll also see where rated capacity and usable capacity split apart.

Key Takeaways

• Watt-hours (Wh) are a good way to compare batteries because they already take into account the voltage.

• The comparison of mAh and Ah is only fair when they are at the same voltage. More mAh does not necessarily mean better.

• Usable capacity is less than rated capacity. This is because of loss, draw, and discharge limits.

• The runtime estimates are based on some assumptions. These are device wattage, temperature, and efficiency.

• The BLUETTI Elite 30 V2 is compact enough to fit phones and laptops. The bigger Elite 300 suits RVs and home backup.

What is the Meaning of Battery Capacity?

The capacity of a battery is a measure of the energy or charge stored in the battery. There are three units in that sentence. mAh is commonly used for small electronics. Ah is commonly used in car and marine batteries. Power stations generally employ Wh.

Amp Hours, Milliamp Hours, & Watt Hours Defined

An amp-hour (Ah) is a unit of electric charge. It indicates the number of amps that a battery can deliver in 1 hour. A milliamp hour (mAh) is the same concept, but on a smaller scale. 1 Ah equals 1,000 mAh.

A watt-hour (Wh) is a measure of energy, not charge. It is a combination of voltage and current. That makes it the most convenient unit to compare the capacity of products.

Why Voltage Makes Ah and mAh Matter

Two batteries can have the same Ah rating. They can still contain significantly different energy. A 10Ah battery at 3.7V has much less capacity than a 10Ah battery at 48V. The missing component is always voltage. To convert Ah or mAh to watt-hours, multiply by the voltage.

What Does 5,000mAh Battery Capacity Mean?

Phone power banks typically have a 5,000mAh rating. It is not much of a story by itself. The typical power bank cell voltage is 3.7V. Using Wh = V × mAh ÷ 1,000, a 5,000mAh battery holds about 18.5Wh. That's two phone batteries. It's not enough to run a laptop.

How to Measure Battery Capacity

Various products rely on various units. There is a practical reason for each choice. mAh is for small devices. Ah refers to automotive and deep-cycle batteries. Wh is used for all comparisons between brands and voltages. Higher systems will typically provide kWh, which is 1,000 Wh.

When to Use mAh

For small, low-voltage electronics, mAh is a good unit to use. Capacity is almost always expressed in phones, earbuds, and gadgets this way. Because phones typically use similar lithium battery architectures, mAh is commonly used for comparing phones, although Wh remains the more accurate energy comparison.

A power bank and a power station have a unique feature. They do not always have the same voltage. After crossing categories, mAh is no longer a good comparison.

When to Use Ah

Ah is most prevalent on car, marine, and RV deep-cycle batteries. These are typically operated at a constant voltage, typically 12V. Thus, Ah is a convenient shorthand in one category.

Trouble starts when you compare a 12V battery to a 48V system by Ah alone. The same Ah rating can be double the energy stored at double the voltage.

Why is Wh Better for Comparing Stored Energy?

Watt-hours eliminate the voltage problem altogether. The number already contains the voltage. This is the most equitable method of comparing battery types. This is a fair comparison of power banks, power stations, and home backup batteries. Keep this one thing in mind. When the voltage differs, compare in Wh.

Your Battery Capacity Calculator and Conversion Formulas

You don't need an engineering degree for this. Most situations are covered by 3 formulas.

Battery capacity formula box

Convert Ah to Wh

Multiply the voltage by the amp-hour rating. A 12V, 100Ah battery stores 12 × 100, or 1,200Wh. This is the most common calculation used by RV and marine shoppers.

Convert mAh to Wh

Multiply the voltage by the mAh and then divide by 1,000. A 20,000mAh power bank at 3.7V stores about 74Wh. That's about 4 or 5 smartphone charges.

Convert Wh to Ah

Divide watt-hours by voltage. A 288Wh battery at 16V works out to 18Ah. This is a simplified calculation using a 16V equivalent voltage and does not represent the published Ah rating of the BLUETTI Elite 30 V2.

How Do You Estimate Battery Runtime

If you know the Wh capacity of a battery, you can estimate the runtime. There is no guarantee of runtime. A lab test is not always a true reflection of real conditions. Assume all numbers in this table are approximate.

The Runtime Formula for AC Loads

The estimated runtime in hours is (Rated Wh × usable efficiency) ÷ device watts. For example, Runtime = (RatedWh × Efficiency) / LoadW. Usable Wh accounts for depth of discharge and inverter loss, which are discussed below. Most power stations produce 80-90% of their rated Wh as usable AC energy.

Worked Examples for a Laptop, Router, and Small Appliance

Let's take a 288Wh battery, with 85% usable energy, which is approximately 245Wh. Look on the nameplate of your device for the exact wattage. Another good choice is BC Hydro's free online calculators.

Runtime worksheet

The Difference Between Rated Capacity and Usable Capacity

The number on a spec sheet is the rated, or nominal, capacity. It is measured by manufacturers in a lab setting. Usable capacity is nearly always less in the real world.

That rated number is affected by several factors. None of them means the battery is faulty. They are simply a part of the battery's operation.

Depth of Discharge and Battery Management Limits

Most lithium batteries will not discharge to 0%. A battery management system, or BMS, holds a small reserve. This helps to maintain long-term health. However, this reduces available volume slightly.

Inverter Loss, Self-Consumption, and Temperature

Some energy is lost as heat when converting DC energy to AC energy. This is approximately 10-15%, depending on inverter efficiency, load conditions, and operating conditions. The usable capacity is further diminished by standby draw and cold temperatures. A battery in a cold garage may run shorter than one indoors.

State of Charge Compared to State of Health

These two terms often get mixed up. State of charge is the current level of charge in the battery, similar to a fuel gauge. The state of health reflects the capacity remaining compared to a new battery. The 85% usable-capacity assumption is a runtime calculation assumption that accounts for available energy after considering system losses and usable limits. It is not the same as state of charge or state of health.

What Battery Size is Suitable for Each Application?

Not all households require the same capacity, and that's a good thing. If you match capacity to actual use, costs are kept to a minimum. In our experience, it's easy to underbuy for a camping trip. You may regret it by day two.

Phones, Laptops, and Everyday Communication

A smaller battery is good for phones, tablets, and a laptop or two. Typically, 200- 300 Wh is sufficient. It remains light enough to throw in a bag.

Camping, RV Life, and Portable Refrigeration

Camping and RV living require more. Running a fridge, fans, or lights for days adds up. A medium to large battery, typically 1,000Wh to 3,000Wh, provides a greater margin for error.

Emergency Prep and Home Backup Loads

The Government of Canada's power-outage preparedness guidance has one simple suggestion for power outages. Have a battery pack to recharge critical devices several times. Whole-home backup is another. This includes a fridge, lighting, & communications. Larger units, as well as home battery backup systems, are perfect here. They work better there.

BLUETTI Cases for Compact and Higher Capacity Needs

The first is ideal for phones, laptops, and small essentials on the move. That's the BLUETTI Elite 30 V2. It has a capacity of 288Wh. This includes a weight of close to 4.3kg. The BLUETTI Elite 300 is another excellent option. This has a capacity of 3,014Wh and an output of 2,400W. It is more suitable for RV appliances or short-term home backup. Neither unit is designed to power your entire house during an extended power outage.

How Can You Check Your Battery Capacity and Health

There is no special training required to check capacity. A few simple habits give you a good read over time.

App Readings, System Readings, and Battery Monitors

Most phones, laptops, and power stations display a percentage of charge. Many, such as BLUETTI units, also display real-time wattage via an app. A special battery monitor provides more accurate voltage and battery life readings.

Controlled Load Testing and When to Call in the Professionals

A controlled load test is a battery discharge test conducted at a known wattage and timed. Compare that result to the rated Wh. Stop using a battery if it drains quickly, gets hot, or swells. Call a qualified technician.

How Can You Protect Battery Capacity Over Time

A few habits go a long way toward preserving capacity. Health Canada's lithium-ion battery safety guidance has a simple rule for storage.

Maintain a battery level of about 50% charge. Monitor it every 3 months. A simple reminder system, such as recording the last inspection date, can help prevent long-term storage issues.

Avoid constant full discharges. Do the same for unnecessary fast charging. Health Canada recommends chargers with recognised Canadian certification marks. Check for CSA, cUL, or cETL. Uncertified chargers might not comply with the fundamental safety requirements.

Compare Batteries in Watt Hours and Plan for Real World Losses

Battery capacity isn't one single number. It's a small family of related units. Amp-hours and milliamp-hours are units of charge. Watt-hours are a measure of actual energy and allow for a fair comparison between voltages. Realistic runtime estimates are based on usable capacity, not rated capacity.

Match the size of a power bank or a larger station to actual requirements. That's more important than trying to get the highest number on the shelf.

FAQ

• Is a higher mAh rating always better?

Not necessarily. Battery capacity mAh only compares fairly at the same voltage. Check watt-hours when comparing across different products.

• What is the difference between battery capacity and battery power?

Capacity means the amount of energy the pack can store. The rate at which it delivers that energy is measured in power output. That is measured in watts.

• How many watt-hours are in a 12V, 100Ah battery?

That cell has a capacity of 1,200Wh. That is equal to V × Ah. Usable energy is slightly lower.

• Why is the usable capacity of a battery system less than the rated capacity?

The usable capacity is reduced by depth of discharge, inverter loss, standby draw, and cold temperatures.

• How much battery capacity do I need for a power outage?

This depends on what you are going to run and how long you are going to run it. Know the wattage of your devices, estimate the hours of use, and then size your battery accordingly.