How many RPMs does an alternator require

Basic math problem: you want to power 5 to 10 inverters, each with a capacity of 8-10 kW. This means you presumably want a total output power of from 40kW to 100kW.

But you want to use a 5-10 HP motor to drive alternators producing 40-100kW. Ten HP is about 7.5kW. Not possible. You can't get out more than you put in.

And you want to run the 7.5kW (7500 Watt) motor driving the alternators charging the batteries powering the inverters with a 1.5kW (1500 Watt) inverter. Not possible. Again, you can't get more out than you put in.

Basic electrical problem: you can't just connect 5-10 cheap DC to AC inverters in parallel (through breakers into a power panel). Their waveforms won't sync up. The most likely result will be blown inverters. You could have an explosion and/or fire.

You might be able to do this with grid-tie inverters -- if you can solve the above problem. But they're not going to be cheap.

Even if you can solve those problems, I'm sorry to say that this idea still isn't practical. You'll lose big time on (in)efficiency.

If your inverters are top quality (expensive) ones, each will lose only 5-15% of the energy in conversion. But my guess is that you're looking for a low cost fix, so you'll probably use Chinese Cheapies. These simple "modified sine" (really modified square wave) inverters are designed for the lowest possible price, not for efficiency. Typical efficiency is poor, 65-75%.

Large, expensive motors can get well into the 90s for efficiency, but a typical mass produced induction motor (think circular saw or washing machine) will be 75-85% efficient (losing 15-25% of its input as heat) at full load. When powered from the crummy waveform a cheap inverter produces, it'll be even less efficient.

I assume you want to use vehicle alternators -- again, they're cheap. For the automakers, low cost is everything. They don't care about efficiency. Typical auto and light truck factory alternator losses run 40% (60% efficiency).

The charging efficiency of a lead battery can be 95% when it's charged with a really good charger. But your alternators won't come close to that. Most likely they'll overcharge your batteries, wasting at least 20-40% of the energy they produce.

Then you lose another 25-35% with another cheap inverter downstream of the batteries.

So let's look at the bottom line, with cheap inverters and automotive alternators. As an example, we'll start with 100 Watt hours of energy produced by your PV or wind machine.

100 Wh * 70% efficient inverter == 70 Wh to motor
70 Wh * 75% efficient motor == 52.5 Wh to alternator
52.5 Wh * 60% efficient alternator == 31.5 Wh to battery
31.5 Wh * 80% efficient battery charging efficiency == 25.2 Wh to inverter
25.2 Wh * 70% efficient inverter == 17.6 Wh available for use

So, overall, your system -- even if it worked without blowing up -- would be only 17.6% efficient. It would lose over 80% of the energy it produced before you could do anything with that energy!

The problem is all those conversions -- from DC to AC in the first inverter, from electrical to mechanical energy in the motor, back to electrical energy in the alternators, from AC to DC (also in the alternators), then from DC to AC (in the inverters). You lose energy at every conversion.

Ideally, you'd use the energy your PV and wind machine produce directly. That's often impractical, but there's plenty of good quality commercial renewable energy gear available. A typical brand name PV inverter runs at 90-95% efficiency.

Of course, it might cost more than your idea would -- or it might not, especially if you buy used gear. In any case, you'll save money on the PV panels and wind machines by not needing as many for a given amount of usable energy.

Sometimes the standard solution really is the most practical and most affordable one.

The efficiency of the alternator is terrible under most circumstances.

The Alternator itself is only usually about 50-55% efficient. For example a 100A alternator will provide an output of about 1.5kW at 15V, but will dissipate about 750W internally. That's why they have such a big fan on the front of the alternator.

If you then consider the mechanical power required to spin and cool the alternator your overall efficiency drops to about 20-25% including the engine losses.

If you like to deal in HP, then 1.5kW is about 2HP. At an overall efficiency of 20% this means you will need about 10HP from an engine to provide this.

You could read this report from Remy which lays out some of the losses. You will notice that the losses are dominated by the engine and mechanical drive system. From the report this graph may address the alternator losses alone:

The amount of power at idle, normal and maximum is alternator dependent. Most alternators need to spin at about 6000 rpm or more to provide maximum output, and in most vehicles you will see max alternator rpm around 8-10000 rpm.

The efficiency (and power output) of an alternator can be improved markedly by moving to synchronous rectification and raising the output voltage using load dump configurations. For example a 48V 100A output may improve to almost 75% or better.

At what RPM does a alternator charge?

Will an alternator work at low RPM?

How much torque is needed to turn an alternator?

How do you calculate alternator RPM?