Size

There is a lot of confusion over battery sizes. This is due to the way the industry will document things.

Definitions

These definitions are simplified and only a guilde. They approimate how things are converted

How are things connected together

We have this thing called a metre. It is a length was (untile recently) defined by a bit of metal. Originally defined as 1, tenmillionth of the distance between the equator and north poll. More recently it is defined speed of light in some amount of time.

Then we have seconds based on a day.

Finally we have celsius - based on freezing and boiling pure water at sea level.

Combine these together, we can then get Joules and Watts - its all connected.

Amps vs Volts vs Watts

What really matters is watts. This is a measurement of energy. Volts and Amps will be moved between at each point in your system. Main transmission lines are often 20,000 volts, while single phase in my house is 240 volts to ground, and 3 phase is 400 between each phase.

Watts = Amps * Volts. So your Kettle is 10Amps, 240Volts it is therefore 2400 Watts.

Amp Hours and Watt Hours

Joules are energy and defineds as a watt in 1 second. But in energy terms we use Amp Hours and Watt Hours.

When you pay your electricity, it is measured in Watt Hours, or more specifically KWH - 1000 Watt Hours.

Batteries

Since our batteries are DC and a specific volts, we often see mentioned Amp Hours. This is especially true for 12 Volt batteries.

However, to compare everything equally, we must convert to Watt Hours.

So a 12V 100Ah battery = 1200 Watt Hours. And a 24V 50Ah battery = 1200 Watt Hours.

Battery Type

The battery type is very significant. And this is where we have a bit of a painful situation.

Lead Acid 100Ah batteries may contain 100Ah of power at 12 volts, but you generally can not access more than 50Ah without damaging the battery. 80Ah depth of discharge will generally permenantly damage the battery.

Where as Lithium Batteries can be discharged 80-95%.

So when you purchase a 100Ah lithium battery, you can get almost twice the power of a 100Ah lead acid battery.

Some Sizes Compared

My Electric Bike

Lithium Ion battery, 72Volts and 80Ah = 5760 Watt Hours.

My Caravan Battery

A 12 volt lithium Ion battery, 100Ah = 1200 Watt Hours

Tesla Power Wall

These are variable, but the internet suggests 13000 Watt Hours.

My 22 Year Old House Solar

48 Volt Lead Acid, XXX Amp Hours = ???

But note these are Lead Acid, so I can only use about XXX

Your Phone

Approximately 3000mAh, and 3.7Ah (aka, a single sell lithium) = 11 Watt Hours

Car Starter Battery

All different sizes but approximately often about 40Ah = 480 Watt Hours. This is lead acid, and you can’t even really use all that power, these are high current batteries to start your car.

Prius 2010 Battery

These are 1.2V nickel-metal-hydride cells. Totally = 1600 Watt Hours