Ah (amp hours) measures how much current a battery can deliver over time. Wh (watt hours) measures the actual energy stored, and is more useful for comparing batteries. kWh is simply 1,000 Wh and is the unit used for larger home and off-grid battery systems.
Battery specifications can be confusing, especially when different products use different units. Understanding the relationship between amp hours, watt hours and kilowatt hours will help you compare batteries accurately and choose the right capacity for your system.
Amp Hours (Ah): Current Over Time
An amp hour describes how much current a battery can supply over one hour. A 100Ah battery can theoretically deliver 100 amps for one hour, or 10 amps for ten hours. The actual usable capacity depends on the discharge rate (faster discharge reduces effective capacity in lead-acid batteries) and the depth of discharge the battery chemistry allows.
Ah is commonly used for 12V and 24V batteries: lead-acid, AGM, gel and lithium batteries used in vehicles, boats and off-grid systems.
Watt Hours (Wh): The More Useful Figure
Watt hours combine voltage and amp hours into a single energy figure:
Wh = Voltage (V) x Capacity (Ah)
A 12V 100Ah battery stores 1,200 Wh (1.2 kWh). A 24V 100Ah battery stores 2,400 Wh (2.4 kWh). This is why Wh is more useful than Ah alone when comparing batteries that operate at different voltages.
Portable power stations like the EcoFlow DELTA series are rated in Wh: the DELTA 2 is 1,024 Wh, the DELTA Pro is 3,600 Wh. These figures tell you directly how much energy is stored.
Kilowatt Hours (kWh): For Larger Systems
One kWh equals 1,000 Wh. Home battery systems, such as wall-mounted LFP (lithium iron phosphate) batteries, are typically rated in kWh. A 10 kWh home battery can store 10,000 Wh of energy, roughly enough to power a typical UK home overnight without solar generation.
Usable Capacity vs Nominal Capacity
The headline capacity figure is not always fully accessible. Different battery chemistries have different depth of discharge (DoD) limits:
- Lead-acid: typically 50% DoD recommended to protect longevity (a 100Ah battery gives you ~50Ah usable)
- AGM/Gel: similar to lead-acid, around 50 to 60% DoD
- Lithium (LFP): 80 to 100% DoD is safe, meaning a 100Ah LFP battery gives you 80 to 100Ah usable
This is one of the primary reasons lithium batteries offer better practical capacity despite sometimes having a similar or lower headline Ah rating than lead-acid alternatives.
How to Calculate How Long a Battery Will Last
Divide the usable Wh by your average consumption in watts:
Runtime (hours) = Usable Wh / Load (W)
A 1,000 Wh power station running a 100W device will last roughly 10 hours (accounting for inverter efficiency, closer to 8 to 9 hours in practice). Running a 500W appliance halves that to around 4 to 5 hours.
Choosing the Right Capacity
Start by listing the devices you need to power and their wattage, then estimate daily usage in hours. Multiply to get daily Wh demand. Size your battery at 1.5 to 2 times your daily demand so you have reserve capacity and do not regularly discharge to the limits. For solar charging systems, also factor in the number of days of autonomy you want during poor weather.
Comparing Batteries Across Products
Always convert to Wh before comparing. A 200Ah 12V battery and a 100Ah 24V battery both store 2,400 Wh. They are equivalent in energy terms, though the 24V system carries current at half the amperage, allowing thinner cable runs and lower resistive losses, which matters in larger installations.
