A few basic terms can help to understand and visualize electrical energy storage devices.
Specific Energy is the kwHrs/Kg.
Specific Energy is important, because you want as much energy stored with as little weight as possible. However, compared to an automobile a boat can much better tolerate weight.
Volumetric Energy Density is the kwHrs/unit volume.
Volumetric Energy Density is important because whatever the weight, the smaller the package size, the easier it is to integrate into the space you have available.
Power (watt or kw) is the rate at which a device can accept or discharge its energy.
Power is important for the rate of charge and how much you can draw in a short time for a rapid acceleration for example. High power rates can allow the instantaneous drawing of huge amounts of energy when required. Low power rates cause inefficiency because when you try to draw it, the energy is converted to thermal energy which can actual damage the device and reduces the amount of energy available.
Energy (kwHr) is the actual amount of energy stored.
Energy is the amount of power you have over time, which relates to the time and distance that you can run. But you have to calculate the total efficiency of the system in order to determine how long your storage device will actually run your electric motor or what have you.
Deep Discharge Cycles are the number of times you can fill and empty the storage device before it fails or falls below a certain percentage of its original capacity.
Deep discharge cycles relate to the life of the storage device. This is complicated somewhat by the fact that in some storage devices (such as deep cycle lead acid batteries) a partial discharge/charge of the device can be more efficient than full discharge. In a situation in which one deep discharge is anticipated on a daily basis, you can calculate the life of the storage device.
Efficiency is the ratio of the energy put into the device compared to what comes out.
Efficiency is very important, especially throughput efficiency or in/out efficiency. This tells you how much of the energy you put into a device will be retrievable. The amount that is not retrievable is generally lost as thermal energy which also generally shortens the life of the device.
Shelf Life is how long the device can endure when not in use or with a minimal maintenance charge.
Shelf life relates to how long the device can sit unattended without self-destructing. This is very important in uses which are occasional as in recreational boats.
Maintenance is the amount of effort, expense, and risk that is required to utilize the device over its life. Obviously the less maintenance required, the more useful the device.
The Self-discharge Rate is what percentage of the energy is lost over time when the device is charged but unused. The only type of energy storage that does not appreciably self-discharge is the potential difference created by gravity. If you lift a weight away from the earths center, you have stored energy in that mass which can be released when it falls back to its original position. This is why pumped hydro storage is very attractive on a large scale, although impractical for vehicles because of the scale of the device. Lead acid batteries discharge at about 3%/month when fully charged. Super capacitors can self-discharge that much in a day, depending on the technology. The Lithium titanate technology from Altairnano self-discharges at about 10% per month. The self discharge rate of a device is sometimes gradually reduced as the voltage goes down. So for example a fully charged 5 volt supercapacitor may self-discharge more rapidly until 75% of its energy remains at which time the rate is rapidly reduced.
These issues are what make the Altairnano technology a real breakthrough in batteries. But right now the company seems to be going through some difficulties as the CEO has stepped down and they have had to have a recall of their earlier battery packs from Phoenix Motorcar Company because of a heating problem. If these people can work out their problems, their technology represents a quantum leap in electrical energy storage technology.