When shopping for a solar generator or portable power station, you will see battery capacity listed in two different units: watt-hours (Wh) and amp-hours (Ah). These two numbers describe related but distinct aspects of a battery, and understanding the relationship between them is essential for choosing the right size and estimating runtime. This article explains the difference between power and energy, shows you how to convert amp-hours to watt-hours (and vice versa), and covers real-world factors like depth of discharge and inverter losses that affect usable capacity.
Power vs Energy: Watts and Watt-Hours
To understand battery capacity, start with the difference between power and energy. Power (measured in watts, W) is the rate at which electricity flows at any given moment. Energy (measured in watt-hours, Wh) is the total amount of electricity used or stored over time.
A simple analogy: power is like the speed of a car (miles per hour), and energy is like the distance traveled (miles). A battery rated in watt-hours tells you how much total energy it can deliver, much like a fuel tank’s capacity in gallons. The watt rating of an inverter or appliance tells you how fast that energy can be drawn.
For example, a device that draws 100 watts running for 10 hours consumes 1,000 watt-hours (100 W × 10 h = 1,000 Wh). A battery with 1,000 Wh of capacity could theoretically power that device for 10 hours, ignoring losses.
Amp-Hours and Why Voltage Matters
Amp-hours (Ah) measure the battery’s charge capacity. One amp-hour means the battery can deliver 1 amp of current for 1 hour. However, amp-hours alone do not tell you how much energy is stored, because energy depends on voltage.
Energy (Wh) = Charge (Ah) × Voltage (V). This is the fundamental relationship. Two batteries with the same amp-hour rating can store very different amounts of energy if they operate at different voltages. A 100 Ah battery at 12 volts stores 1,200 Wh (100 × 12). The same 100 Ah at 48 volts stores 4,800 Wh.
When comparing batteries, always look at watt-hours rather than amp-hours, or at least note the voltage. Portable power stations often list capacity in watt-hours because their internal battery voltage varies (for example, a 12.8V LiFePO4 pack or a 51.2V high-voltage pack). Amp-hours are more common on individual deep-cycle batteries used in RV or off-grid systems.
Converting Amp-Hours to Watt-Hours
The conversion is straightforward: Watt-hours = Amp-hours × Voltage. To convert watt-hours to amp-hours, divide by voltage: Amp-hours = Watt-hours ÷ Voltage.
Below are step-by-step examples using common battery voltages found in solar generators and portable power stations.
Example 1: 12V System (Lead-Acid or LiFePO4)
A common deep-cycle battery rated at 100 Ah at 12 volts. Multiply: 100 Ah × 12 V = 1,200 Wh. If you have a device that draws 150 watts, the estimated runtime (ignoring losses) is 1,200 Wh ÷ 150 W = 8 hours.
Example 2: 12.8V LiFePO4 Battery
Many lithium iron phosphate (LiFePO4) batteries have a nominal voltage of 12.8 volts. A 100 Ah battery at 12.8 V stores 100 × 12.8 = 1,280 Wh. The slightly higher voltage gives more energy than a standard 12V lead-acid battery of the same amp-hour rating.
Example 3: 51.2V High-Voltage Battery Pack
Some larger portable power stations use a 51.2V internal battery. A 100 Ah pack at 51.2 V stores 100 × 51.2 = 5,120 Wh. This is why high-voltage systems can pack more energy into a smaller amp-hour number. Always check the voltage when reading amp-hour specs.
Usable Capacity: Depth of Discharge and Inverter Losses
The rated capacity (in Wh or Ah) is the total energy the battery can store when new. However, you cannot always use 100% of that energy without damaging the battery or causing voltage drops. Two key factors reduce usable capacity: depth of discharge (DoD) and inverter efficiency.
Depth of Discharge (DoD)
Battery chemistry determines how deeply you can safely discharge it. Lead-acid batteries (flooded, AGM, gel) typically have a recommended maximum DoD of 50% for good cycle life. Discharging them deeper shortens their lifespan. Lithium batteries, especially LiFePO4, can often be discharged to 80% or even 100% DoD without significant damage, though many manufacturers recommend 80% for longevity. For more on battery types, see our article on LiFePO4 vs lithium-ion.
To calculate usable energy, multiply the rated Wh by the maximum DoD fraction. For a 1,200 Wh lead-acid battery at 50% DoD: usable = 1,200 × 0.5 = 600 Wh. For a 1,280 Wh LiFePO4 battery at 80% DoD: usable = 1,280 × 0.8 = 1,024 Wh.
Inverter Losses
Portable power stations and solar generators include an inverter to convert DC battery power to AC household power. Inverters are not 100% efficient; a common planning assumption is 85% to 90% efficiency. That means 10% to 15% of the energy is lost as heat during conversion. To account for this, multiply the usable battery energy by the inverter efficiency.
Example: A LiFePO4 battery with 1,024 Wh usable (after DoD) and an inverter at 85% efficiency gives 1,024 × 0.85 = 870 Wh of usable AC energy. This is the number you should use when estimating how long an AC appliance will run.
Comparing Labels Across Products
When you look at different portable power stations, you may see capacity listed in watt-hours, amp-hours, or both. Some products list amp-hours at the battery voltage, while others list amp-hours at the AC output voltage (120V). Always check which voltage is being used. If a power station says “100 Ah at 120V,” that would be 12,000 Wh (100 × 120), which is enormous and unlikely. Usually, amp-hour ratings refer to the internal battery voltage.
To compare fairly, convert everything to watt-hours. If a product only lists amp-hours, find the nominal battery voltage (often in the specifications or manual) and multiply. This is especially important when comparing a 12V system to a 48V system. A 50 Ah battery at 48V stores 2,400 Wh, while a 100 Ah battery at 12V stores only 1,200 Wh — the higher-voltage system has more energy despite half the amp-hours.
When sizing a solar generator for your needs, the usable watt-hour capacity is the most important number. Start with the total watt-hours of the devices you plan to run and how long you need them. Our guide on how to size a solar generator walks through that process step by step.
Example Conversion Table
The table below shows common battery sizes at different voltages, along with estimated usable watt-hours after accounting for depth of discharge and inverter losses. These are planning estimates; actual numbers depend on the specific battery and inverter.
| Battery Rating (Ah) | Nominal Voltage (V) | Rated Energy (Wh) | Chemistry & DoD Assumption | Usable Energy (Wh) | Usable AC Energy (Wh) at 85% Inverter Efficiency |
|---|---|---|---|---|---|
| 100 | 12.0 | 1,200 | Lead-acid, 50% DoD | 600 | 510 |
| 100 | 12.8 | 1,280 | LiFePO4, 80% DoD | 1,024 | 870 |
| 50 | 51.2 | 2,560 | LiFePO4, 80% DoD | 2,048 | 1,741 |
| 200 | 12.0 | 2,400 | Lead-acid, 50% DoD | 1,200 | 1,020 |
| 200 | 12.8 | 2,560 | LiFePO4, 80% DoD | 2,048 | 1,741 |
| 100 | 51.2 | 5,120 | LiFePO4, 80% DoD | 4,096 | 3,482 |
Note: Inverter efficiency varies. Some high-quality inverters reach 90% or more. Check your unit’s specifications for a more accurate number.
FAQ
How do I convert watt-hours to amp-hours?
Divide the watt-hours by the battery voltage. For example, a 2,000 Wh battery at 12.8 V gives 2,000 ÷ 12.8 ≈ 156 Ah. If you need to size a battery for a known energy requirement, use this formula to find the amp-hour rating at your system voltage.
Why is my actual runtime shorter than the calculated watt-hour capacity suggests?
Several factors reduce real-world runtime: depth of discharge limits, inverter losses, battery age, temperature, and the fact that some appliances draw surge currents at startup. Always plan for at least 15–20% less than the theoretical maximum. Also, if you are running DC devices directly from the battery, you avoid inverter losses.
Should I use the battery’s nominal voltage or the inverter’s output voltage for conversion?
Always use the battery’s nominal voltage (the internal DC voltage of the battery pack). The inverter’s output voltage (120V AC) is not relevant for calculating stored energy. The amp-hour rating on a portable power station is almost always at the battery voltage, not the AC output.
Can I use amp-hours to compare batteries of different chemistries?
Only if the voltages are the same. If one battery is 12V and another is 24V, the amp-hour numbers are not directly comparable. Convert both to watt-hours first. Also consider depth of discharge: a 100 Ah LiFePO4 battery (12.8V) provides more usable energy than a 100 Ah lead-acid battery (12V) because of higher DoD and slightly higher voltage.
Putting It All Together
Understanding watt-hours vs amp-hours helps you make informed decisions when buying a portable power station or building a solar generator system. Always convert to watt-hours for a true comparison, and remember to factor in depth of discharge and inverter efficiency to estimate usable energy. When shopping for a portable power station, look for the watt-hour rating in the specifications, and use that number to match your energy needs. For more on charging times and system sizing, see our guides on solar generator charging time and how to size a solar generator.
