Understanding Amp Hours vs Watt Hours

In this lesson, we’re tackling a common point of confusion:

👉 If I have a 100 amp hour battery… why doesn’t it last 100 hours?

The short answer is: it all depends on how you use it. But to really understand, we need to look at the relationship between amp hours (Ah) and watt hours (Wh).

Understanding Amp Hours vs Watt Hours – VIDEO

COMING SOON

Amp Hours Explained

An amp hour (Ah) is a measure of how much current a battery can provide over time.

• 1 amp for 1 hour = 1 amp hour
• 100 amp hours could mean:
  • 1 amp for 100 hours
  • 100 amps for 1 hour
  • Or any mix in between

⚡ Example loads:

• A 1A light bulb could run for 100 hours on a 100Ah battery.
• A microwave pulling 100A might run for only 1 hour.

Watt Hours Explained

A watt hour (Wh) measures energy over time, combining both voltage and current.

Formula:

Watt Hours = Amp Hours × Battery Voltage

So:

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

💡 Key takeaway: Even if two batteries are both rated at 100Ah, the one with a higher voltage has a much higher capacity in watt hours.

Battery Size Analogy: Fuel Tanks in Cars

Think of watt hours like fuel mileage.

• A Prius with a 10-gallon tank might go 400 miles.
• An F-150 with the same 10-gallon tank might only go 200 miles.

Both tanks hold the same amount of fuel, but the efficiency is different. Similarly, two batteries with the same Ah rating can give very different Wh output depending on voltage.

Why Do Some 100Ah Batteries Look Bigger?

Take this comparison:

• A 100Ah, 12V Battle Born battery = 1,200 Wh
• A 100Ah, 48V rack battery = 4,800 Wh

Even though they’re both 100Ah, the 48V battery contains four times the usable energy, which explains its larger size.

Real-World Example: Running a Laptop

Let’s say you have a 100W laptop you want to run for 5 hours.

1. 100W × 5 hours = 500Wh
2. 500Wh ÷ 12V = 41.67Ah

So running your laptop will use about 42Ah. On a 100Ah battery, that’s almost half its capacity.

Planning Your Battery Bank

The important thing isn’t just the battery label, but the total watt hours available.

• Add up your daily power usage in Wh (we cover this in our Power Audit lessons).
• Size your battery bank so it can handle those watt hours comfortably.
• Then, size your charging system (like solar) so you can recharge your batteries.

Party-Saving Example: The Blender Test 🥤

Because what’s more important than making margaritas off-grid?

• A 1,200W blender running for 1 hour = 1,200Wh.
• That’s your whole 12V, 100Ah battery drained in one go!
• But on a 48V, 100Ah battery (4,800Wh), you’d barely scratch the surface.

Key Takeaways

• Amp hours alone don’t tell the full story. Always convert to watt hours to compare batteries.
• Voltage matters. Higher-voltage batteries with the same Ah rating have more total capacity.
• Plan with watt hours. Base your system design on your actual energy needs, not just Ah labels.