Many portable power stations show Ah and Wh ratings. However, these ratings tell only part of a battery's story. What rating actually matters?
Typically, Wh is the more useful number when considering portable power stations. Amp-hours measure a battery's charge over time, while watt-hours use voltage to determine stored energy. The relationship is: Wh = Ah × V
The voltage is crucial, batteries with the same Ah rating can possess varied amounts of storage energy capacity.
When comparing portable power stations’ capacity, use Wh. However, to understand the battery or DC system, you need the battery voltage and then the Ah. Lastly, to see if the power station can handle a certain appliance see the output watts (W).
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What does Amp-Hour (Ah) Mean?
An amp-hour, essentially, is a capacity of charge.
In simplified terms, with a battery that has a 20Ah rating, one could theoretically expect it to provide a constant current draw of 20 amps over one hour, 2 amps over 10 hours, or another equivalent combination. Actual battery performance can vary with discharge rate, temperature, battery condition, and system limits.
There is also a very important limitation. That is Ah does not tell us the energy of the battery.
Why? Because energy is a product of charge and voltage.
A 20Ah battery which is operating at 12.8V is not comparable with a 20Ah battery operating at 51.2V, both have the same rating and charge capacity, but the latter has much higher energy potential.
For that reason, comparing Ah can be a disastrous purchasing decision.
Watt-hours (Wh) Definition
A watt-hour (Wh) is used to measure energy.
A watt-hour is the amount of energy used by a one-watt load operating for one hour. Wh is generally more useful than Ah alone when comparing stored energy because Wh accounts for both charge capacity and voltage.
One of the reasons why many portable power stations list their ratings in watt-hours is because: How long can this power station run my refrigerator? How many times can I recharge my laptop? Which power station has more stored energy? Do I have enough stored energy with 1,024Wh for my backup power needs?
Wh gives you a more relevant starting point.
To better understand how battery capacity translates into real-world use, explore what 1,024Wh means for portable power station capacity and runtime.
Why Is Voltage So Important When Assessing Ah?
When interpreting an Ah rating, voltage is essential.
Generally,
Wh = Ah × V
And, with Wh and V known,
Ah = Wh ÷ V
Let’s take two batteries that both have a 20Ah rating.
Battery A = 12 V
12V × 20Ah = 240Wh
Battery B = 24 V
24V × 20Ah = 480Wh
Even though both batteries share a 20Ah rating, battery B stores considerably more energy.
This is why a battery with a lower Ah rating can actually have as much or more energy than another battery if it operates at a sufficiently higher nominal voltage.
A Case Study Comparing the GEYOTO N300 and the GEYOTO N1000
Let's take a look at two of GEYOTO's current portable power stations.
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The GEYOTO N300 is rated at 256Wh, 12.8V, and 20Ah. The GEYOTO N1000 is rated at 1024Wh, 51.2V, and 20Ah.
Take a look at something interesting: they both have a 20Ah rating!
Looking only at Ah makes one believe that their battery capacities are similar, but they are not.
The formula for the N300:
12.8V × 20Ah = 256Wh
The formula for the N1000:
51.2V × 20Ah = 1024Wh
The N1000 stores four times as much energy as the N300 despite both having the same 20Ah charge rating.
An Ah comparison is not adequate when analyzing the energy capacity of a battery in comparison to other units like watt-hours. If battery voltage is not known, then Wh is the better quantity to use.
Ah to Wh conversion:
The formula uses the battery's nominal voltage.
Wh = Ah × V
Example:
20 Ah × 12.8V = 256 Wh
If we have a 20 Ah battery at 24V:
20 Ah × 24 V = 480 Wh
The nominal battery voltage is essential to this calculation, and you can not assume that a portable power station uses a 12V battery. Portable power stations can use a range of internal nominal voltages.
GEYOTO's N300 and N1000 have nominal voltages of 12.8V and 51.2V, respectively.
To convert Wh to Ah
To convert Wh to Ah, you also need the battery's nominal voltage:
Ah = Wh ÷ V
For instance, let’s take a 51.2V battery with a 1024Wh capacity:
1,024 Wh ÷ 51.2 V = 20 Ah
Voltage is critical to this calculation. If we use 12 volts, the Ah would be completely different.
With a “Wh to Ah” converter that does not consider voltage, you can end up with misleading values.
Which Matters More for a Portable Power Station: Ah or Wh?
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When comparing portable power station capacity, for most users Wh is the better metric.
We would use Wh when we want to know:
- The total energy stored
- The expected runtime of appliances
- The capacity of various batteries and power stations
- The energy available for use during a power outage
- If a larger capacity power station is worth the cost and space
While Ah does serve a purpose in some areas, it should not be used on its own to compare battery capacities when different voltages are used.
In general, when shopping for a portable power station, comparing Wh gives a better, fairer comparison.
Does More Wh Mean a Power Station Can Run Any Appliance?
No, in this situation ‘Wh’ and ‘W’ need to be differentiated.
- Wh is for measured energy capacity.
- W is for measured power output.
A power station may have stored energy, but may still not be able to run an appliance when the power demand for the appliance is greater than the station’s output capability.
The GEYOTO N300, for instance, is rated at 300W continuous AC output, and has a peak output at 500W; whereas the N1000 is rated at 1800W output with a peak of 3000W.
Therefore, we should ask two different questions when choosing a power station.
- Does it have sufficient Wh for the required runtime?
- Does it have sufficient W output to run the appliance?
To see how these output limits apply in practice, compare the rated and peak wattage of the GEYOTO N300 and N1000.
Why Theoretical Runtime is Not Guaranteed
Watt-hours and appliance wattage can provide a useful starting point for estimating runtime, but a simple theoretical calculation does not represent the full amount of usable runtime you should expect from a portable power station.
There are many environmental and operating factors that will affect the actual runtime.
Conversion efficiency, device consumption, temperature, battery health, startup loads, and other factors will affect the actual runtime of the system.
Finally, converting stored energy to AC power using an inverter consumes some of the stored energy.
Any runtime figures shown for GEYOTO products are reference estimates, and actual runtime can differ depending on the appliance and operating conditions.
Tips on its Comparison: Ah vs Wh
In discussing portable power stations, a comparison of Amp hours vs Watt hours is the relevant measure.
Four things to remember:
★ Ah is the unit of electric charge.
★ Wh measures stored energy and reflects both charge capacity and battery voltage.
★ Wh = Ah × V.
★ W is the measure of output power and has to be checked individually from the battery's energy capacity.
Bottom Line
Ah should not be considered as a totally irrelevant measure. It is a meaningful measure once the battery's voltage is known and used in conjunction with the Ah measure.
However, the measures of Ah alone can be misleading when discussing portable power stations with batteries of different voltages. As an example, both the GEYOTO N300 and N1000 are rated at 20Ah, but their respective 12.8V and 51.2V battery systems yield 256Wh and 1,024Wh, respectively.
For most buyers, the easiest way to compare is to look at Wh to determine stored energy, look at W to see if the power output supports the appliance, and only in this context look at Ah relative to battery voltage.
For a more detailed view of other specs, see the N300 and N1000 specs.