Understanding Amp-Hours and Watt-Hours: 12V Power Explained

“My battery is 100Ah but my fridge drained it in 8 hours.” Sound familiar? Most RV and off-grid power problems come back to one thing: not understanding how to measure and budget energy. Amp-hours and watt-hours are the units that make this manageable — once you understand what they mean, sizing a battery bank and estimating run times becomes straightforward math.

What Is an Amp-Hour?

An amp-hour (Ah) is a measure of electrical charge — specifically, how much current a battery can deliver over time. A 100Ah battery can theoretically supply:

• 100 amps for 1 hour, or
• 10 amps for 10 hours, or
• 1 amp for 100 hours

In practice, discharge rate affects capacity. Lead-acid batteries lose usable capacity as discharge rate increases (the Peukert effect). A 100Ah AGM battery discharged in 5 hours may only deliver 80Ah. LiFePO4 batteries are largely immune to this effect and deliver close to rated capacity regardless of discharge rate — one of their underappreciated advantages.

The key practical point: amp-hours are measured at the battery’s voltage (12V for most RV systems). To convert to usable energy, you need to account for voltage.

What Is a Watt-Hour?

A watt-hour (Wh) is a measure of energy — it combines voltage, current, and time into a single number that’s easier to work with across different voltages. The conversion is simple:

Wh = Ah × Volts

A 100Ah battery at 12V contains 1,200Wh (1.2 kWh) of stored energy.

Watt-hours are more useful than amp-hours for planning purposes because the wattage ratings on appliances are already in watts. You don’t need to know the operating voltage to do the math.

Calculating Your Daily Energy Consumption

List every device in your RV, its wattage, and average daily run time:

This example system needs about 1,500Wh per day. Now you can size your battery bank and solar array to match.

Converting Wh to Required Battery Capacity

Once you know your daily consumption in watt-hours, convert back to amp-hours at 12V. This is the same calculation at the heart of how to size your 12V battery bank:

Required Ah = Wh ÷ 12V

1,500Wh ÷ 12V = 125Ah of usable capacity needed per day

But usable capacity depends on battery chemistry:

• AGM at 50% DoD: need 250Ah of rated capacity to get 125Ah usable
• LiFePO4 at 80% DoD: need about 156Ah of rated capacity to get 125Ah usable

For a 2-day autonomy buffer (common for cloudy-day solar resilience), double those numbers. A LiFePO4 bank of 200–300Ah handles the example load comfortably with solar supplementing daily.

Why You Need a Battery Monitor

Knowing your battery is “100Ah” is theoretical. Knowing that you’ve consumed 72Ah since your last full charge, your current draw is 8.3A, and you have an estimated 3 hours of runtime left — that’s actionable information. That’s what a battery monitor gives you. See our roundup of the best battery monitors for RVs and boats for top picks at every budget.

A shunt-based monitor like the Victron SmartShunt measures every amp flowing in and out of your battery bank. It tracks cumulative amp-hours consumed, calculates state of charge, and with Bluetooth, sends all of that data to your phone in real time. The Renogy 500A monitor covers the same bases at a lower price point if budget is a concern.

Without a monitor, you’re relying on voltage to estimate state of charge — which is unreliable (LiFePO4 voltage is especially flat across most of its charge range) and dangerously inaccurate under load.

Common Misconceptions Cleared Up

“My 100Ah battery should run my fridge for 100 hours.” No — that assumes 1 amp of draw. A 45W fridge draws about 3.75 amps at 12V. Divide 100Ah by 3.75A = roughly 26 hours for a full lithium battery or 13 hours for an AGM at 50% usable.

“I have 400W of solar so I can run anything.” Solar output depends on sun hours. 400W of panels in 5 peak sun hours = 2,000Wh generated per day — but that’s peak conditions. Factor in partial shading, cloud cover, and angle losses; realistic output may be 1,200–1,500Wh. Cross-check that against your daily consumption.

“The battery is at 12.4V so it’s half full.” Voltage as a state-of-charge indicator is imprecise, especially under load and immediately after charging. Use a battery monitor instead of relying on voltage alone.

Putting It Into Practice

The numbers aren’t complicated — they just need to be done. Add up your loads in watt-hours, divide by 12 for amp-hours, apply your battery chemistry’s usable percentage, and multiply by your desired autonomy days. That’s your minimum battery bank size. The 12V RV electrical basics guide covers the system context, and choosing the right battery chemistry helps you pick the right DoD multiplier for your situation.

Once your system is running, a quality battery monitor removes all the guesswork. You’ll know exactly how much energy you have, how fast you’re using it, and whether your solar is keeping up. Managing a 12V system intelligently starts with measuring it accurately.