FLA to Watts: How to Convert Battery Amp-Hours

To convert FLA (flooded lead-acid) battery capacity in amp-hours (Ah) to watts, multiply the amp-hours by the battery voltage: Watts = Amp-hours × Volts. This gives the total energy stored in watt-hours (Wh), which is the same as watts delivered for one hour. For example, a 100 Ah 12V FLA battery holds 1,200 Wh (1.2 kWh).

Formula

The conversion uses a single multiplication:

E (Wh) = C (Ah) × V (V)

• E = energy in watt-hours (Wh) or watts for one hour
• C = battery capacity in amp-hours (Ah)
• V = battery nominal voltage (V)

To get kilowatt-hours (kWh), divide by 1,000. This formula assumes the battery is discharged at its rated C-rate (usually 20 hours for FLA) and that voltage stays constant at the nominal value, which is an approximation.

Worked Example

Using the default values from the calculator: a 100 Ah battery at 12V.

1. Write down the formula: E = C × V
2. Plug in the numbers: E = 100 Ah × 12 V
3. Multiply: E = 1,200 Wh
4. Convert to kWh: 1,200 ÷ 1,000 = 1.2 kWh

The calculator returns 1,200 Wh (and 1.20 kWh), matching the hand calculation.

What Is a Flooded Lead-Acid Battery?

A flooded lead-acid (FLA) battery is the traditional type of lead-acid battery with liquid electrolyte (sulfuric acid) that can be topped up with distilled water. It is common in cars, golf carts, forklifts, and off-grid solar systems. Unlike AGM or gel batteries, FLA batteries require regular maintenance, vent gases during charging, and must be kept upright.

The capacity of a battery is usually printed on the label in amp-hours (Ah), often at a 20-hour discharge rate. That means a 100 Ah battery can deliver 5 amps for 20 hours. The voltage is nominal: 12V for most automotive and small solar batteries, 6V for golf cart batteries (often wired in series), and 2V per cell for large stationary banks.

Why Convert Ah to Watts?

You need watts (or watt-hours) to compare batteries with different voltages, to size solar panels or chargers, or to estimate how long a load will run. For example, a 12V 100Ah battery stores more energy than a 6V 200Ah battery? Let’s check:

• 12V × 100Ah = 1,200 Wh
• 6V × 200Ah = 1,200 Wh

They store the same energy. Amp-hours alone don’t tell you the energy; voltage matters. Watts also help you understand how fast a battery can be drained. A 100W light bulb running on a 12V battery draws about 8.3 amps (100W ÷ 12V). A 100Ah battery could theoretically run it for 12 hours (100Ah ÷ 8.3A), but in practice less due to losses.

How to Choose the Right Battery Size

When planning a battery bank, you usually start with your daily energy needs in watt-hours. Then you divide by the system voltage to get the required amp-hours. For example, if you need 2,400 Wh per day and you use a 12V system, you need at least 200 Ah of battery capacity. But FLA batteries should not be discharged below 50% depth of discharge (DoD) to avoid damage, so you’d double that to 400 Ah.

Steps to size a battery bank:

1. List all loads and their wattage.
2. Estimate hours of use per day for each.
3. Multiply watts × hours to get watt-hours per day.
4. Sum all watt-hours.
5. Divide by system voltage (12V, 24V, 48V) to get amp-hours.
6. Apply a safety factor: for FLA, multiply by 2 (50% DoD) or more for cold temperatures.

Common Mistakes and Misconceptions

One common mistake is confusing watts with watt-hours. Watts is a rate (energy per time), while watt-hours is a total amount. A battery “holds” watt-hours, not watts. Saying a battery is “1,200 watts” is imprecise; it’s 1,200 watt-hours.

Another mistake is using the CCA (cold cranking amps) rating for capacity. CCA is a measure of short-term power delivery, not energy storage. A car battery might have 700 CCA but only 50 Ah. You cannot convert CCA to watts meaningfully.

Also, the nominal voltage is not constant. A 12V FLA battery actually sits around 12.7V when fully charged and drops to about 10.5V when empty. Using 12V is a good average, but for precise work you might use the actual voltage over time (Peukert’s law accounts for this).

Limits of the Simple Formula

The Ah × V formula assumes ideal conditions. Real FLA batteries lose capacity when discharged quickly (Peukert effect). A 100Ah battery rated at 20-hour discharge may only deliver 70Ah if drained in 1 hour. Also, you cannot use 100% of the rated capacity without damaging the battery; for FLA, the usable capacity is typically 50% for deep-cycle use, less for starting batteries.

Temperature also affects capacity: cold batteries deliver less energy. The formula gives a theoretical maximum, not a practical one. For real-world planning, multiply the result by the depth of discharge (0.5 for FLA) and an efficiency factor (around 0.85 for charging/discharging).

Practical Examples

Let’s apply the conversion to real scenarios:

• RV refrigerator: A 12V fridge draws 60W. How many hours can a 100Ah battery run it? First, 100Ah × 12V = 1,200Wh. Then 1,200Wh ÷ 60W = 20 hours. But with 50% DoD, only 10 hours.
• Solar system: You have a 48V battery bank with 200Ah. Energy = 200 × 48 = 9,600Wh (9.6kWh). That’s enough to run a 1,000W appliance for 9.6 hours (theoretically).
• Golf cart: Six 6V batteries wired in series give 36V. If each is 200Ah, the bank is 200Ah at 36V = 7,200Wh. A 2,000W motor could run for 3.6 hours at full power.

Frequently Asked Questions

Can I convert FLA to watts directly? What about other battery types?

Yes, the conversion is the same for any battery: watts = amp-hours × volts. AGM, gel, and lithium batteries all use the same formula. The difference is in the usable capacity (lithium can go to 80-100% DoD, FLA only 50%).

What is the difference between watts and watt-hours?

Watts measure power (how fast energy is used), while watt-hours measure energy (how much total). A 100W bulb uses 100 watts of power; in 2 hours it consumes 200 watt-hours. A battery rated in watt-hours tells you how much energy it can supply.

How many watts is a 12V 100Ah battery?

It is 1,200 watt-hours (1.2 kWh). That means it can deliver 1,200 watts for 1 hour, or 100 watts for 12 hours, theoretically. In practice, with a 50% depth of discharge, you get about 600 usable watt-hours.

Why is my battery’s amp-hour rating not matching my calculations?

Because the rating is based on a 20-hour discharge rate. If you draw more current, the battery delivers fewer amp-hours due to internal resistance and chemical limits. Also, temperature and age reduce capacity. Check the battery’s datasheet for ratings at different discharge rates.

Can I use this formula to size an inverter?

Yes. To find the inverter size, add up the wattage of all appliances you want to run simultaneously. Then ensure your battery bank can supply that wattage and enough watt-hours for the duration. For example, a 1,000W inverter on a 12V battery draws about 83 amps (1,000W ÷ 12V), so you need a battery that can handle that current and provide the needed energy.

How do I convert watts back to amp-hours?

Divide watts by volts: Ah = Wh ÷ V. For example, 1,200Wh ÷ 12V = 100Ah. This is useful when you know your energy needs in watt-hours and want to select a battery in amp-hours.

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in ah | Battery capacity | Ah | 100 | 0
in v | Battery voltage | V | 12 | 0
out wh | Energy in watt-hours | Wh | ah * v | 0
out kwh | Energy in kilowatt-hours | kWh | ah * v / 1000 | 2