Do you know how to do simple arithmatic?
1350W/12V= 112.5 Amps. A car battery might boil or explode with such a high continuous current. Maybe you need 2 car batteries in parallel to handle the high current. How will you charge them?

The inverter project you posted is a piece of junk and doesn't work. It blows up its capacitors because they are backwards and has a very low power output when it works. Larger transistors won't make any difference to its low power.

Originally Posted by kukla

I have pretty much no idea what I'm doing.

No kidding!
It is a good thing that you asked.

The coffee machine that operates from the 12V lighter plug in a car needs a car battery to power it, not an inverter. How are you going to charge a car battery in school? It might last a week then you could carry it home for charging each weekend.

But no 12v coffeemaker is going to make coffee with that speed. As a sanity check, look at the size of cable alone needed to pass over 1000W @12v. That would require 2 ga cable or better, and that's just to prevent the cable from overheating. The voltage drop is another issue. It is likely a 12v coffeemaker is made to comply with a more or less standard limit of 10 amps from the lighter socket, so it's 120W. This will mostly work, but it's going to be slow. There's a question of just how hot that water's going to be able to get even if you wait forever.

Actually it's not impossible. I have a big alternator that puts out like 130 amps. If the engine is running, and you have excellent connectors and HUGE cables running to a sizable inverter, this is entirely possible to run your badass coffeemaker. I've run a small microwave with the engine running. I've run a 6000 BTU air conditioner. Not at the same time of course.

He could install a gasoline powered electric generator in his school locker to power the coffee maker.

Hasn't the school got a powerpoint in the hall or locker room to use ?

Somehow the cleaners need access to 120 Vac for the vacuum.

Just my 2 cents worth

Good luck with your coffeemaker.

Good coffee is essential.

Originally Posted by RODALCO

Hasn't the school got a powerpoint in the hall or locker room to use ?

Somehow the cleaners need access to 120 Vac for the vacuum.

Just my 2 cents worth

I was thinking of that as well... If it only takes 30 seconds to spit out the coffee, then just find an outlet near by and plug it.

Why not just make the coffee at home and take it in a flask?.

Originally Posted by HarveyH42

There must be some method of boiling water with reasonable sized battery power.

Nope.
Heating something takes power. Power is volts times amps. A battery must be pretty big to supply amps of current for a time long enough to heat a cup of water.

Originally Posted by Nigel Goodwin

Why not just make the coffee at home and take it in a flask?.

I do admire the logic in this suggestion

I expect you want the machine in the locker for a coolness factor, so you can open your locker and turn on this big coffee maker in there?

An invertor for that power level is unrealistic (if it is to be supplied from 12v at least). Supplied from a bank of 4 car batteries in series supplying 48vdc, possibly, but it is unusual and requires a bespoke design. Also bear in mind you would be generating mains voltages at high power levels which, although may not be referenced to earth (?) could still be fatal if the wiring wasn't suitably insulated.

Stick with Nigels suggestion , or use the machine from a wall outlet.

I calculated how many watts would be required to raise the temperature of 12oz (340g) of water by 75C in 5 minutes. I got 366 watts. This assumes that ALL the power goes into heating the water, and none is lost.
So, if you wanted to do it with a 12V battery, you would need about 30 amps (again, with no losses). To do it in 10 minutes would require 15 amps, etc.
Of course, the actual current would be more, or the time would be longer, due to heat loss.