Capacity is about how much, not how fast
Battery capacity tells you how much energy a battery can store. It is the size of the tank, not the speed of the tap. A separate figure, the power rating, tells you how quickly the battery can deliver or accept energy. Mixing these two up is the most common source of confusion when reading a specification.
Kilowatt-hours and kilowatts
Two units do most of the work.
- Kilowatt-hour (kWh) measures energy. One kilowatt-hour is the energy used by a 1-kilowatt appliance running for one hour. This is the unit on most electricity bills, and the same quantity is described in the article on energy units.
- Kilowatt (kW) measures power, the rate at which energy is used or delivered at a moment.
An everyday comparison: a water tank’s size in liters corresponds to kilowatt-hours, and the rate at which the water can flow out through the pipe corresponds to kilowatts.
Small batteries use the same units scaled down. Watt-hours (Wh) are one-thousandth of a kilowatt-hour, and milliamp-hours (mAh) are common for phones and small gadgets.
Worked examples
Duration from capacity and power. A battery holds 10 kWh, and a device draws 2 kW. Time equals energy divided by power, so 10 divided by 2 is 5 hours. If the draw doubles to 4 kW, the time halves to 2.5 hours.
Energy from power and time. A 3 kW heater running for 2 hours uses 3 times 2, or 6 kWh. A 10 kWh battery could supply that, with some left over, at least on paper.
Power limit. A home battery might have 10 kWh capacity but a 5 kW maximum output. It cannot run a combination of appliances that adds up to 8 kW at the same time, even though there is plenty of energy stored. This is why both numbers matter.
Amp-hours and watt-hours
Many small batteries list capacity in amp-hours (Ah) or milliamp-hours (mAh), which measure charge rather than energy. To convert to energy, multiply by the voltage. For example, a 3.7-volt battery rated at 3,000 mAh (3 Ah) holds about 3.7 times 3, or 11.1 Wh. Comparing batteries with different voltages by mAh alone can be misleading, which is why watt-hours give a fairer comparison.
Nominal versus usable capacity
Specifications often distinguish between several figures.
Nominal (or gross) capacity is the total energy the battery can theoretically hold. Usable capacity is the amount you can actually draw. Manufacturers often reserve a portion at the top and bottom of the range to protect the cells and extend their life. A battery sold as 13.5 kWh nominal might offer a little less in practice. The difference depends on the design, so check which figure is quoted.
This distinction matters in comparing products. Two batteries with the same nominal figure could deliver different amounts of usable energy.
Why real-world results differ
Several factors mean you will not always get the labeled number.
- Age. Capacity declines gradually, as described in the article on battery degradation. A battery that is several years old will hold less than when new.
- Temperature. Cold weather temporarily reduces the energy you can extract, and extreme heat can speed up wear.
- Discharge rate. Drawing power very quickly can lower the effective capacity somewhat.
- Conversion losses. Inverters, chargers and cables lose some energy as heat. For a home battery, the energy that ends up powering your appliances is usually less than the energy that went in, and the ratio is the round-trip efficiency.
- Test conditions. Laboratory tests are carried out under controlled conditions that may differ from daily use.
C-rate
You may see a figure called the C-rate, which links power to capacity. A 1C rate means a battery would be fully discharged in one hour; 0.5C means two hours; 2C means thirty minutes. For a 10 kWh battery, 0.5C corresponds to 5 kW, since 10 times 0.5 equals 5. The C-rate shows quickly whether a battery is designed for long, slow delivery or short, intense bursts.
Capacity in vehicles
In an electric vehicle, capacity is quoted in kilowatt-hours, and the range depends on how efficiently the vehicle uses that energy. A car using 0.2 kWh per kilometer with a 60 kWh battery would travel about 300 kilometers under those conditions, since 60 divided by 0.2 is 300. Real range varies with speed, weather, load and driving style, and the usable capacity may be lower than the nominal. Charging rate, in kilowatts, determines how long a refill takes, which is discussed in the article on electric vehicles and the grid.
Capacity in grid and home storage
Larger systems are described using megawatts and megawatt-hours, where one megawatt-hour equals 1,000 kilowatt-hours. A system described as 100 MW / 400 MWh can deliver up to 100 MW and has enough energy for four hours at full output, since 400 divided by 100 is 4. Describing a project by only one number would hide that distinction. The comparison of grid batteries and home batteries shows how these sizes differ in practice.
Questions to ask when reading a spec
- Is the capacity figure nominal or usable?
- What is the maximum continuous power output, and the peak?
- What is the warranty, and does it guarantee a minimum capacity over time?
- Under what temperature and conditions were the figures measured?
- What is the round-trip efficiency?
For purchases or installations that affect your home wiring, the sizing and installation should be done by a qualified, authorized electrician. This article is for general understanding and is not a recommendation for any product.