Battery Capacity: A Guide to Understanding

Knowing the size of your battery (or battery capacity), is critical if you want to accurately predict what appliances it will be able to power, and for how long.

Indeed, this fact is so important that most solar generator manufacturers actually include the capacity rating in the name of their product. 

For example, you can see that in one of our own products, the  Bluetti AC200P/2000Wh.

In this article we will discuss what battery capacity is, how to calculate the right capacity for your energy needs, and more. 

What is battery capacity?

Battery capacity is defined as the total amount of electricity generated due to electrochemical reactions in the battery and is expressed in ampere hours (Ah), watt hours (Wh) or kilowatt hours (kWh).

Generally, car batteries or "vanlife" batteries are sold under their charge capacity (Ah) rating while solar generators are sold under their energy capacity rating (Wh). 

In summary: Watt-hours (Wh) = energy capacity, while ampere-hours (Ah) = charge capacity.

Why do some batteries have a higher capacity than others? 

Batteries come with varying levels of capacity. Generally, the capacity of a battery is determined by the following factors: 

Number and size of plates in a cell

The amount of plates, or their size indicates the total amount of active substance that allows energy to be stored. 

This means that the battery in question, will have an increased ability to store more or deliver more energy should their be more active ingredient in the battery plates.

Density of the electrolyte

If manufacturers opt for higher density electrolyte inside a battery, the overall capacity will increase (to some degree). 

However, with added density, this also equates to shorter battery life. therefore, if higher capacity is what you are after, it is not as simple as increasing the density of the electrolyte. 

Lastly, the overall capacity of a battery also depends on its age. The more a battery is used, the more you can expect its overall capacity to decrease.  

How do you calculate the capacity of a battery?

Contrary to popular belief, calculating your battery capacity is really not that hard. Let us explain. 

So, depending on the type/ size battery you buy, you may notice it comes listed in either, mAh, Ah, Wh, or kWh. 

Generally small powerbanks come listed in mAh (milliampere hour), car batteries in Ah (ampere hours), solar generators in Wh (watt hours) and residential energy storage systems in kWh (kilowatt hour).

But how do we convert one unit into another. 

It's pretty easy, as long as you have one unit of measurement plus the batteries voltage you can always convert one into the other. 

Here's an example: 

Ah To Wh/kWh

You are able to calculate a batteries energy capacity by multiplying its voltage (V) by its nominal capacity (Ah).

V x Ah = Wh/kWh

Let's say you have a 100Ah 12V battery and you want to learn how many Wh hours it has. 

• 100Ah x 12V = 1200Wh/ 1.2kWh

Keep in mind though, that just because two batteries have the same charge capacity (Ah) it does not mean they will necessarily have the same energy capacity. 

For example: 

• 100Ah 12V battery = 1200Wh
• 100Ah 24V battery = 2400Wh

Generally, most household appliances are rated on how much power they require to function. i am sure you have noticed the sticker on the back indicating their power rating in Watts (W = voltage x current).

This is why knowing the energy capacity can be much more useful than knowing the charge capacity, assuming you aim to power household appliances with your battery system.

Battery capacity vs battery life

It is actually quite common for people to confuse battery capacity with battery life at first. 

We recommend knowing the difference before buying any battery as there is actually an incredibly big difference between the two. 

Battery capacity

As we have already mentioned, battery capacity is defined as the total amount of electricity generated due to electrochemical reactions in the battery and is expressed in ampere hours (Ah), watt hours (Wh) or kilowatt hours (kWh). This is the measurement which indicates what your battery will be able to power and for how long. 

Battery life

Battery life on the other hand indicates how many life cycles it has before it starts degrading. The biggest influencing factor here is the battery type/ chemistry. 

As you can see cycle life varies greatly. This is why working out your levelized cost of storage (LCOS) is so important when buying a battery. 

For example, let's say you are looking to buy a 100Ah battery. 

You are trying to decide between a lead-acid or LiFePO4 battery

To calculate LCOS you need to know the total energy output of the battery and the total upfront cost, along with the batteries DOP. 

As you can see, overtime the LiFePO4 battery is roughly 2 times cheaper and lasts 5X as long. 

This is why Bluetti only uses LiFePO4 batteries in their latest solar generators. 

Final thoughts

We hope you found this article informative. Remember, when working out your batteries capacity, it is also important to work out its LCOS. By doing this, you will ensure you are getting the best value for money overtime. 

Additionally, a batteries charge capacity (Ah) can be converted to energy capacity by simply multiplying its voltage (V) by its nominal capacity (Ah).