Understanding Amp Hours, Watt Hours & Battery Sizing for Your Setup

Choosing the right battery for your solar setup doesn’t have to be confusing. Understanding Amp Hours (Ah), Watt Hours (Wh), and how much power you actually need is key to avoiding over- or under-sizing your system. This guide breaks it down simply so you can plan confidently—whether you’re powering a fridge, lights, or your entire campsite.

? What Are Amp Hours (Ah)?

Amp Hours (Ah) tell you how much current (in amps) a battery can deliver over time.

• Example: A 100Ah battery can deliver 1 amp for 100 hours, or 10 amps for 10 hours.

• It’s a measure of capacity, not power.

But Ah only gives part of the picture—you also need voltage to understand total energy.

⚡ What Are Watt Hours (Wh)?

Watt Hours (Wh) = Total energy stored. It's the full picture of how much work a battery can do.

? Formula:
Watt Hours (Wh) = Amp Hours (Ah) × Voltage (V)

Example:
A 12V 100Ah battery = 1,200Wh of energy
(100Ah × 12V = 1,200Wh)

? How to Estimate Your Daily Power Needs

Use this simple process:

1. List your devices

2. Check their power rating (in watts)

3. Estimate how many hours per day they’ll run

4. Multiply watts × hours = Wh per day

? Choosing the Right Battery Size

Now that you know your daily energy use, choose a battery that:

• Can store that much energy (in Wh)

• Won’t discharge below safe limits (especially for AGM/lead acid)

✅ Recommended Battery Sizing Rule:

• Lithium batteries: Use 80–90% of rated capacity

• AGM/Lead acid batteries: Use 50% of rated capacity (to prevent damage)

Example:
You need 730Wh/day.

• For lithium (usable ~90%):
  → Battery should be ~900Wh = 75Ah at 12V

• For AGM (usable ~50%):
  → Battery should be ~1,460Wh = 122Ah at 12V

? Allow for Efficiency Losses & Backup

Add 20–30% margin for:

• Inverter losses

• Cloudy days

• Future expansion

So, for 730Wh/day, a good safe minimum:

• Lithium: 100Ah 12V battery

• AGM: 150Ah 12V battery

? Fast Reference Table

⚠️ Important Tips

• Match voltage: A 24V system needs fewer amps than a 12V system for the same power.

• Consider your charge rate: Your solar input must replace your daily usage.

• Use a battery monitor or voltmeter to track real-world consumption.

• Don’t mix battery types or sizes in the same system unless they're designed to work together.