Volts, Watts, Amps. What does it all mean anyway? Perhaps a useful analogy is thinking about electricity like water. I mean, both are currents, right?
Voltage can be like water pressure. A higher pressure can "do" more than a lower one.
Amps can be like Gallons Per Hour/Minute. Whether it's at a very low pressure (such as water flowing down a wide stream) or a very high pressure (such as from a water main), it's still measured in gallons per hour/minute.
Watts is like total water used/required. A change in Voltage/Water Pressure is directly proportional to Amps/Gallons Per Minute needed/supplied.
A battery is like a water tank. It can only hold so much juice. The voltage is similar to putting some air-pressure in that tank, and as you use it, it will drop.
Now, carrying on with this example, you'd need a minimum (and in many cases, maximum) pressure to do some tasks. Too much pressure can burst the pipes, blows the seals and innards of your gadgets, just like too much voltage can fry fuses, electronics, bulbs, and even wires. All your electrical gadgets are designed to work with a specific voltage.
Knowing that, however, is only half the battle. We know a starter needs 12 volts to spin, but a 12 V watch battery clearly isn't going to do the job. Now we're talking amps. Back to our water example, even under no pressure, we can still harness water using a water-wheel to power entire mills. We just need a wide trough to get a large volume of water onto that wheel. On your bus you have a nice wide wire from the battery to the starter to serve the same purpose.
Using water, and knowing the weight/density of water, we can actually calculate how much we would need to pass over that water wheel to power a given load, and it's no different with electricity. Volts x Amps = Watts. The math works the same in reverse, both directions. Watts / Amps = Volts. We probably won't need this often since we generally know the voltage we need to be working with, so we use ... Watts / Volts = Amps. We can obtain the wattage of many things we need to power and usually have established the voltage, so we use these two bits of math to calculate wattage or amperage, as needed. A margin of error and safety is essential, so figure adding a 10% (minimum) margin for this at the end of your estimations.
Also take into consideration that changing from one voltage to another incurs a loss, such as inverting (from DC to AC) as well as rectifying (from AC to DC).