My truck supposedly needs a new alternator, so I am searching websites that sell them. THere is a 95amp and 130amp model, and I don’t know which one my truck takes. Since I have a 4 cyl engine and no power anything (windows, locks, seats, etc) I’m assuming the 95amp.
I assume if I need the 130 but put the 95 in it will burn up. What happens if the 130 is installed and I only need a 95?
It will last longer.
I’m no EE, but wouldn’t your engine use more fuel also?
No. Alternators do not run at full output all the time, they are demand driven units.
If (and I do mean IF) you ran two identical engines with both alternators full fielded, the engine with the 130 A alternator would use more fuel. But as I said alternators don’t run that way as a rule.
You do need to check that the mounting and harness are the same for both alternators. I have seen cases where one model car had two different alternators available, but they were NOT interchangeable.
This is not a common occurrence, but it can happen.
what Rick said. an alternator can be considered as a current source, in that it will only supply the current demanded by the rest of the car/truck’s electrical system. if your vehicle only needs, say, 50 amps, then either alternator will be adequate.
You can put rechargeable batteries under your truck to take advantage of the extra amps that spill out. 
if you’re driving down the road using 70 amps for the stereo then your mileage will suffer with the 130 amp unit. If you’re driving down the road using 70 amps for the stereo with the 95 amp unit you will burn it up.
FYI, if your engine is leaking oil and it’s getting on the alternator it will shorten it’s life.
The effect an loaded alternator will have on fuel consumption is very small compaired to the fuel consumption of just running the engine. 95 amps is around 1300 watts and 135 amps is around 1850 watts. Or less than 3 hp. And that is fully loaded. As stated an alternator is only part loaded.
The 135 amp alternator is likely to be a bigger, heavier unit. Thus more weight to be moved, thereby also increasing fuel consumption and reducing mileage some. But this is likely to be even less of a factor.
Manufacturers put the minimum output alternator on the vehicle to begin with. There are upgrades for people who have more needs. An alternator that can put out more amps does no damage to your system. The extra amps are not put out unless you need them. You will be able to run more electrical items at a lower speed and recharge the battery at a faster rate, but not too fast. By all rights, the higher output alternator should cost more. I’d sure go for the higher output if I had the choice.
Just to reiterate the above. Alternators do not deliver their rated current all the time. The rating is a maximum safe working rating. If your vehicle needs 10A, the alternator will deliver 10A, if it needs 100A, so long as it is able, the alternator will deliver that. When it is delivering smaller currents, the alternator presents a proportionately lower load to the vehicle’s engine.
There is some reason to believe that an alternator with a higher rating will last longer. It will probably run a little cooler, and the recitifier diodes be a bit beefier than a smaller unit. But it may weight a tiny bit more.
If you install an alternator that is too small it should not “burn up”. It would only do that if it’s a poor design. It will just not deliver enough amps to feed the car. The battery will make up the difference. Eventually you will find that the battery is drained and the car won’t start.
One potential problem with installing a larger alternator is if the wiring is not sized to the higher amps. If it is sized for 95A and you pump 130A through it, you could potentially melt the insulation, short it and have a fire. But if an optional 130A alternator was factory installed on that model vehicle, it is likely the wiring is already sized for the larger alternator.