FAQ
TL;DR: A 12V 100A alternator makes ~1.2 kW DC; a 90% inverter yields ~1.08 kW AC. “12V x100A = 1200W.” [Elektroda, vodiczka, post #17340167]
Why it matters: This helps DIY power builders and off‑grid tinkerers size alternator–inverter systems without overloading, voltage sag, or false expectations.
Quick Facts
- Automotive alternators regulate near ~14.5 V; the regulator boosts rotor current to hold setpoint. [Elektroda, sanfran, post #17339989]
- 12V × 100A ≈ 1.2 kW DC; with ~90% inverter, ~1.08 kW usable AC. [Elektroda, vodiczka, post #17340167]
- Typical alternator efficiency ≈ 50%; 1 kW electrical needs ~2 kW mechanical input. [Elektroda, kkknc, post #17340305]
- Car alternators aren’t built for continuous full-load duty; honor continuous vs peak ratings. [Elektroda, sanfran, post #17340110]
- Parallel alternators can multiply output until drive/excitation limits cause voltage drop. [Elektroda, sanfran, post #17339989]
How much AC power can a 12V 100A alternator deliver through a 12/230V inverter?
Multiply voltage by current for DC watts: 12V × 100A ≈ 1200W. With a 90% efficient inverter, expect about 1080W continuous AC. Keep surge margins for startup loads. “12V x100A = 1200W.” [Elektroda, vodiczka, post #17340167]
What does “real power” mean in this setup?
Real power is what the load actually draws at the regulated output. The alternator’s regulator targets about 14.5 V and increases rotor current to maintain it. At the limit, voltage sags below nominal as excitation maxes out. [Elektroda, sanfran, post #17339989]
Can I run a car alternator at full load continuously?
Not safely. Alternators have instantaneous, hourly, and continuous ratings, and full load is not a continuous-duty condition. Heat and wear accelerate at max output. “Car alternators are not designed for continuous full load operation.” [Elektroda, sanfran, post #17340110]
How much mechanical power do I need to get 1 kW electrical?
Budget about double due to alternator losses. With ~50% alternator efficiency, 1 kW electrical needs roughly 2 kW mechanical at the shaft. Add inverter losses on top when sizing the prime mover and belts. [Elektroda, kkknc, post #17340305]
Will connecting five alternators give five times the power?
In principle, yes, outputs add. In practice, limits appear when the drive lacks torque or alternators hit excitation ceilings, causing voltage drop. Ensure correct RPM, adequate torque, and proper regulation for each unit. [Elektroda, sanfran, post #17339989]
Do magnetic wheels or clutches provide extra (free) power?
No. A magnetic coupling only transmits torque and adds its own losses. It cannot increase net energy. “You will always take out less than you put in.” Use them for isolation, not gain. [Elektroda, vodiczka, post #17340584]
How do I safely find the maximum load my alternator-inverter can handle?
Try this: 1) Assemble the system and connect meters for voltage and current. 2) Add loads gradually (e.g., tools, lamps), tracking voltage. 3) Stop when voltage sags or temperatures rise. That’s your practical limit. [Elektroda, Anonymous, post #17340069]
What happens if I overload the alternator?
The regulator drives maximum rotor current. If demand keeps rising, output voltage drops below the setpoint and the drive may slow. That indicates you have exceeded the alternator or drive capability. Reduce load or increase torque. [Elektroda, sanfran, post #17339989]
What inverter size matches a 100A 12V alternator?
Base it on ~1200W DC available. With a ~90% inverter and real-world losses, select around 800–1000W continuous AC to leave margin for heat and surges. Avoid sizing the inverter above sustained alternator output. [Elektroda, vodiczka, post #17340167]
Is a 100A alternator rated at 12V or 14V?
Automotive alternators are rated around 14V at the terminals. “100A is the rated current efficiency, but at 14V.” Compute watts using the regulated voltage, not the nominal 12V label. [Elektroda, kkknc, post #17340305]
Can I get more energy by stacking generators or stages?
No. To deliver 6 kW you must supply more than 6 kW input due to losses. Electric motors can be 0.97–0.98 efficient, but alternators are lower, so stacking increases losses, not output. [Elektroda, iron64, post #17341181]
How do I know if my alternator speed is sufficient under load?
Watch voltage. Regulators hold near 14.5 V until excitation max is reached. If voltage remains stable under load, RPM and torque are adequate. Droop indicates insufficient speed or torque. Increase drive RPM or reduce load. [Elektroda, sanfran, post #17339989]
What do instantaneous, hourly, and continuous ratings mean for alternators?
Instantaneous is short bursts. Hourly is limited-duration duty with cooldown expectations. Continuous is safe indefinitely within thermal limits. Size your loads to the continuous rating, not the peak or hourly values. [Elektroda, sanfran, post #17340110]
