Watt Hours Vs Amp Hours

Watt-Hours vs. Amp-Hours: What Every Van Builder Needs to Know

If you’re building your own van’s electrical system, you’ve probably seen batteries advertised in amp-hours (Ah), while solar panels and appliances often list power in watts or watt-hours (Wh).

Here’s the problem: mixing these up can lead to a seriously undersized system — and that’s how you end up with a dead fridge, no lights, and warm beer halfway through your road trip.

By the end of this post, you’ll know exactly what these terms mean, how they’re connected, and how to use them to plan your off-grid electrical setup with confidence.

1. Amp-Hours: The Current Over Time

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

Think of amps like the width of a water hose — the bigger the hose, the more water (current) can flow. Amp-hours tell you how much total flow you get over a certain period.

Example:

• A 100Ah battery could, in theory, supply:
  • 1 amp for 100 hours
  • 5 amps for 20 hours
  • 10 amps for 10 hours

Key point: Amp-hours alone don’t tell the full story — you also need the voltage.

2. Watt-Hours: The Real Energy Number

A watt-hour (Wh) measures total energy — this is the true “fuel tank size” for your battery.

Here’s the connection:

Watts = Volts × Amps
Watt-hours = Volts × Amp-hours

So if you know a battery’s amp-hours and voltage, you can find watt-hours.

Example:

• 100Ah at 12 volts = 1,200Wh
• 100Ah at 24 volts = 2,400Wh

Why it matters:
Two batteries can have the same amp-hour rating but store very different amounts of energy if their voltages are different. That’s why watt-hours are better for comparing systems.

Example of our Victron LFP-12.8/300 packs nominal energy at 3840Wh

Lithium Batteries can be found here: https://www.vanlifeoutfitters.com/category/camper-van-electrical-system-parts/batteries/

The industry is headed towards rating battery power by the nominal energy voltages. Instead of seeing 12, 24 or 48 volts the industry uses 12.8, 25.6 and 51.2 volts as the nominal values for watt hour calculations.

3. Why Watt-Hours Matter More for Off-Grid Systems

Amp-hours are useful when you’re staying in one voltage (most van systems are 12V), but watt-hours let you:

• Compare batteries of different voltages (12V, 24V, 48V).
• Calculate your daily power needs accurately.
• Match your battery capacity to your solar panel output and appliance usage.

If you only look at amp-hours, you might undersize your system and run out of battery faster than you expect.

4. How to Calculate Your Daily Power Needs

Before buying batteries, figure out how much energy you use in a typical day.
Here’s how:

Step 1: List your appliances.
Write down every device you’ll use: fridge, lights, fan, water pump, laptop, etc.

Step 2: Find the wattage.
Look for a sticker on the device, the manual, or search online. If it only lists amps, use:

Watts = Volts × Amps

Step 3: Multiply by hours used per day.
This gives you watt-hours for that device.

Step 4: Add everything up.
That’s your daily total watt-hours.

Example Daily Usage Table:

Step 5: Add a safety margin.
Cloudy days, inverter inefficiencies, and battery aging happen. Add 20–30% to be safe:

3650Wh × 1.25 ≈ 4,562.5 Wh/day needed.

5. Converting Between Ah and Wh

If you already know one measurement, here’s the cheat sheet:

• Ah = Wh ÷ Volts
• Wh = Ah × Volts

Example:

• You need 4,562.5Wh per day.
• With a 12V system: Ah = 4,562.5 ÷ 12 ≈ 381Ah
• So you’d want at least a 460Ah lithium battery bank (lithium batteries can use most of their capacity, however manufactures only recommend 80% depth of discharge or less; lead-acid can only use about 50%).
• With your daily Wh usage calculated, remember you must have a charging source that is capable of replenishing the used energy each day too. This can be from solar, dcdc chargers, 2^nd alternator, shore power or generator.

6. Common Mistakes to Avoid

• Comparing Ah without checking voltage.
  A 100Ah 12V battery is not the same as a 100Ah 24V battery.
• Forgetting inverter losses.
  Converting 12V DC to 120V AC loses ~10–15% of energy.
• Only planning for sunny days.
  Always size for your worst-case scenario.
• Not tracking usage.
  A battery monitor like a Victron SmartShunt helps you see real-world consumption.

7. Final Tips for DIY Van Builders

• Always start with watt-hours when planning your system.
• Use efficient appliances — LED lights, efficient DC fridges, and laptops instead of power-hungry desktops.
• If you can, test your system before hitting the road.
• Remember: you can always add more solar panels later, but battery capacity is harder to upgrade in a cramped van.

Conclusion:
Understanding watt-hours vs. amp-hours is one of the most important steps in building a reliable off-grid van electrical system. Once you get the hang of converting between them, you can size your battery bank, solar array, and charging system with confidence — and keep your lights on and fridge cold, no matter where the road takes you.

If you need assistance putting together your electrical system, please call 754-444-8704 an agent at Vanlife Outfitters will provide their expertise in the perfect electrical system for your application.