Watts (Power), Watt-hours (Energy), Amps (Current), Amp-hours (Charge), and Volts (Potential) are the units used to measure electricity, and they are often confused and ununderstood.
After reading this quick article, you’ll learn about what these units represent in a battery, and you’ll also learn about concepts like charge capacity, discharge rate, power, energy, energy capacity, depth of discharge, and usable battery capacity.
What does 100ah mean on a battery?
Ah or Amp-hours, is an electrical measurement unit that represents the “Charge Capacity” of a battery. A 100Ah rating on a battery means that the battery can supply 1 Amp of current for 100 hours, 10 Amps of current for 10 hours, or – ideally – 100 Amps of current for 1 hour.
For example, the runtime provided by a fully charged 100Ah battery that is supplying 40 Amps of current could – ideally – be calculated as such:
Runtime (hours) = Charge Capacity of the battery (Amp-hours) ÷ Amp draw (Amps)
Runtime (hours) = 100 Amp-hours ÷ 40 Amps
Runtime (hours) = 2.5 hours
Ideally, the charge capacity (Amp-hours) of a battery would be constant no matter how fast you discharge it, or in other words, no matter how high the discharge rate is. For example, a 100Ah battery should – ideally – be able to supply 200 Amps for 30 minutes.
However, in practice, the rate at which you discharge a battery will have an effect on its capacity. This effect is referred to as the Peukert Effect or Peukert’s law.
The severity of this effect will depend on the chemistry of the battery. Lead-Acid batteries, in particular, are more prone to this effect than other battery chemistries such as lithium-ion.
For example, a fully charged 100Ah Lead-Acid battery supplying 150 Amps of current (1.5C discharge rate) might only last 15 to 20 minutes, when it should – ideally – last 40 minutes.
Related: How long will a 100Ah battery last?
This is why when most manufacturers specify a charge capacity for a battery (in Amp-hours), they also specify the discharge rate (also referred to as C-rate) at which that charge capacity is actually true:
So, if you buy a Lead-Acid battery that says 100Ah at 20 hours, it means that the battery will last 20 hours if you discharge it at 5 amps (0.05C discharge rate). However, if you discharge it at more than 5 amps, it will likely provide less energy than the rated 100 amp-hours.
On the other hand, if you buy a 100Ah Lithium battery, you can discharge it at up to 100 Amps (1C discharge rate) and it will still supply 90-98% of its rated charge capacity.
How many watts is a 100Ah battery?
Watts, or W for short, is an electrical measurement unit that represents “Electrical Power”. And Electrical Power (in Watts or kiloWatts) is the rate at which Electrical Energy (Watt-hours or kiloWatt-hours) is being transferred (produced or consumed).
Also, Electrical Power (Watts) is the product of Current (Amps) and Voltage (Volts):
Power (Watts) = Current (Amps) x Voltage (Volts)
The amount of Power (in Watts) that a 100Ah battery produces depends on what the load requires. A 12V-100Ah battery running a DC (Direct Current) load that demands 50 Amps of current (@ 12 Volts), will produce 600 Watts of power.
Power (Watts) = Current (Amps) x Voltage (Volts)
Power (Watts) = 50 Amps x 12 Volts
Power (Watts) = 600 Watts
If the DC load demands 100 Amps of current (@ 12 Volts), the battery will supply 1200 Watts, or 1.2 kiloWatts, of power (100 Amps x 12 Volts).
If it’s an AC (Alternating Current) load, you will need an inverter to convert to low DC voltage (12V) of the battery to a higher AC voltage (120V) which the AC load requires.
Related: What size inverter for a 100Ah battery?
If the AC load requires 10 amps at 120 Volts, the inverter will pull 100 Amps at 12 Volts, and the 100Ah battery will produce 1200 Watt of power:
Power (Watts) = Current (Amps) x Voltage (Volts)
Power (Watts) = 10 Amps x 120 Volts
Power (Watts) = 600 Watts
However, it is important not to confuse Electrical Power (W or kW) with Electrical Energy (Wh or kWh). While Watt-hours (or kiloWatt-hours) represent the amount of energy that’s stored, consumed, or generated, as mentioned above, Watts or kiloWatts represent the rate at which energy is transferred.
Power (Watts) = Energy (Watt-hours) ÷ Time (hours)
or
Energy (Watt-hours) = Power (Watts) x Time (hours)
For example, if a 100Ah battery supplies 100 watts of power for 2 hours, by the end of those 2 hours, it could be said that the battery has supplied 200 Watt-hours of energy (100 Watts x 2 hours).
How many kWh (or Wh) is in a 100ah battery?
kWh (kiloWatt-hour) or Wh (Watt-hour) is the unit that measures Electrical Energy. A 12V-100Ah battery can store 1200Wh or 1.2kWh of energy, and therefore could be said to have 1.2kWh of energy capacity.
If the 100Ah battery is rated at 24 Volts, such as this Ampere-Time battery, it can store 2400Wh or 2.4kWh of energy.
Energy Capacity of the battery (Watt-hours) = Charge Capacity of the battery (Amp-hours) x Voltage of the battery (Volts)
To put this amount of energy in perspective, a 32″ LED TV uses about 40 Watt-hours of energy per hour of runtime. This means that a 12V-100Ah battery has enough energy (1200Wh or 1.2kWh) to run this TV for up to 30 hours.
Related: How long will a 100Ah battery run a TV?
Still, it is important to note that, for some batteries, this energy capacity is not 100% usable. Depending on the type of battery you’re using, you might not be able to utilize its full capacity.
Batteries lose percentages of their rated capacity over time. For example, a battery that could initially store 1.2 kWh of energy might only be able to store 0.6 kWh of energy after 200 or 300 charge/discharge cycles.
However, there is a correlation between how fast a battery loses its ability to store energy, and how deep that battery is usually discharged. The deeper you usually discharge a battery, the fewer charge/discharge cycles you’ll have left on that battery before its capacity starts decreasing dramatically.
On the bright side, this means that you can optimize the lifespan of the battery by only using a certain percentage of its capacity when you discharge it. This optimal percentage of capacity that you can use is referred to as the Depth Of Discharge (or DOD for short).
For example, a Lead-Acid battery that’s usually fully discharged will only last about 150-200 charge/discharge cycles before its rated capacity drops by about 30-40%. If the same battery is only discharged to 50% of its rated capacity (50% DOD), it would last about 500 to 600 cycles before any significant losses in capacity.
Yes, this means that you’ll need twice the batteries for the same amount of energy, but, your batteries will last 3 times longer. This is more cost-efficient in the long term.
This is also true for Lithium batteries, but these batteries inherently offer more charge/discharge cycles than their cheaper Lead-Acid counterpart.
For example, at a DOD of 100%, a Lithium battery will last about 1500 to 2500 cycles. At a DOD of 80%, which is generally the recommended DOD for these batteries, the same battery would last 3000 to 5000 cycles.
In any case, the recommended Depth Of Discharge of the battery should be taken into consideration when determining how much energy the battery can offer. The usable capacity of your battery will not only depend on its rated capacity, but also on its recommended DOD:
Usable Capacity (Watt-hours) = Rated Capacity (Watt-hours) x recommended Depth Of Discharge (%)
