Generator sizing is the process of calculating the maximum continuous and starting wattage required by your electrical loads and battery chargers to select a machine that won't stall, overheat, or get rejected by sensitive inverter electronics. What this changes in a real installation is whether your battery bank actually charges during a multi-day grid outage, or if your inverter's AC input relay simply refuses to close because the generator's voltage and frequency are too unstable under load. Most DIYers commonly confuse running watts with starting (surge) watts, but the more critical and frequently missed factor is ignoring the power factor and charger efficiency when calculating the true AC draw of a high-amperage battery charger.
The Core Math: Running, Surge, and Charger Efficiency
Sizing a generator for a standard off-grid cabin is as simple as adding up appliance nameplates. Sizing a generator to feed a modern hybrid inverter-charger (like those from Victron, Sol-Ark, or SMA) requires calculating the DC charge demand and working backward through the charger's efficiency curve. Battery chargers are not 100% efficient, and they draw reactive power, meaning the AC watts pulled from the generator are always higher than the DC watts pushed into the batteries.
Worked Numeric Example: 48V 5000VA System
Let’s size a generator for a popular setup: a Victron MultiPlus-II 48/5000/70 charging a 48V 200Ah LiFePO4 bank, while simultaneously running essential house loads.
- Target DC Charge Power: 70A × 52V (absorption voltage) = 3,640W DC.
- Account for Efficiency (88%): 3,640W / 0.88 = 4,136W AC.
- Account for Power Factor (0.95): 4,136W / 0.95 = 4,353 VA (Apparent Power drawn from generator).
- Base House Loads: Fridge, router, LED lights, and well pump controller draw a continuous 1,200W.
- Total Continuous Running Watts: 4,353W + 1,200W = 5,553W.
Now, factor in the surge. If your 1/2 HP well pump kicks on while the batteries are bulk charging, the pump demands roughly 3,000W of starting surge for a few seconds. Your generator must handle a peak load of 8,553W without the voltage sagging below the inverter’s low-voltage disconnect threshold. A standard 7,000-watt portable generator will instantly choke and stall in this scenario.
Where You Meet This in Practice: Inverter AC Input Relays
Where you meet this in practice is at the AC input relay of your hybrid inverter. Think of the inverter's AC input relay like a strict club bouncer: it constantly monitors the generator's voltage, frequency, and Total Harmonic Distortion (THD) before allowing power to pass through to the charger and house loads.
If you undersize your generator, the moment the inverter commands a 70A bulk charge, the generator's engine bogs down. The frequency drops from 60Hz to 57Hz, and the voltage sags from 120V to 108V. The inverter's bouncer sees this "dirty" power, assumes the grid has failed, and opens the AC input relay to protect your electronics. The house seamlessly switches to battery power, the generator spins uselessly in the yard, and your batteries never recharge. To prevent this, modern inverters allow you to program an AC Input Current Limit. If you have an 8,000-watt generator (roughly 66A at 120V), you must set the inverter's AC input limit to 60A to reserve headroom for motor surges.
Decision Tree: Picking the Right Generator Class
Use this decision matrix to match your inverter/battery topology to the correct generator class, terminating in a specific, proven part number.
| System Topology | Max Charge + Base Load | Required Generator Class | Concrete Pick (Part Number) |
|---|---|---|---|
| 12V / 2000VA Inverter | ~800W Continuous | 2kW Portable Inverter Gen | Honda EU2200i (06220) |
| 24V / 3000VA Inverter | ~1,800W Continuous | 3.5kW - 4.5kW Portable | Westinghouse iGen4500 (WGen4500) |
| 48V / 5000VA Inverter | ~5,500W Continuous | 9kW - 10kW Open Frame | Champion 100554 (100554) |
| 48V / 8000VA+ Inverter | ~8,500W+ Continuous | 14kW+ Air-Cooled Standby | Generac Guardian 70432 (70432) |
Common Sizing Mistakes That Trip Inverter Relays
Beyond raw wattage, three specific electrical characteristics will cause a correctly sized generator to be rejected by a hybrid inverter:
1. Ignoring Total Harmonic Distortion (THD)
Standard open-frame contractor generators produce a modified or "dirty" sine wave with a THD of 10% to 20%. High-end hybrid inverters (like the SMA Sunny Island or Schneider Conext) require AC input with a THD of less than 5%. If the THD is too high, the inverter will reject the generator, regardless of available wattage. Always select an "inverter generator" or a premium standby unit that explicitly guarantees <5% THD.
2. Floating vs. Bonded Neutrals
Most portable generators under 8,000 watts have a "floating neutral" (the neutral wire is not bonded to the chassis ground). When you plug a floating neutral generator into an inverter's AC input or a home transfer switch equipped with GFCI or AFCI breakers, the breakers will immediately trip because they detect an imbalance. For home backup integration, your generator must have a bonded neutral, or you must install a neutral bonding plug at the generator receptacle.
3. Undersizing the Receptacle and Wiring
If your generator outputs 9,200 watts at 240V, it is pulling roughly 38 amps. You cannot use a standard 30A L14-30R twist-lock receptacle and 10 AWG wire; the plug will melt. You must ensure the generator is equipped with a 50A L14-50R receptacle and wire your transfer switch feed with 6 AWG copper (THHN in conduit or 6/3 NM-B for short indoor runs) to handle the continuous load safely without voltage drop.
FAQ: Generator Sizing Edge Cases
Can I use a 2000W tailgater generator to charge a 48V battery bank?
Only if you strictly limit the inverter's AC input current to 15A (roughly 1,800W at 120V) via the inverter's software settings. The 48V charger will pull maximum power, the generator will bog down, and the inverter will drop the connection. By software-limiting the AC input, you force the charger to draw only what the 2kW generator can supply, resulting in a slower but stable bulk charge.
Does a dual-fuel (gas/propane) generator lose power on propane?
Yes. Propane has a lower energy density than gasoline. Expect a 10% to 15% drop in maximum continuous wattage when running on LPG. If your math requires exactly 8,000 running watts, a "9,000-watt" dual-fuel generator will fall short on propane. Always size the generator based on its propane rating if you plan to use it as your primary emergency fuel.
What is "Generator Support" or "Grid Assist"?
Found in premium inverters (like the Sol-Ark 15K or Victron Quattro), this feature allows the inverter to seamlessly blend generator power with battery power. If the generator is rated for 5,000W, but the house suddenly demands 8,000W for a well pump surge, the inverter will pull the 5,000W from the generator and instantly inject the remaining 3,000W from the battery bank. This allows you to buy a smaller, cheaper generator than you would otherwise need.
The Default Recommendation
If you are building a standard 48V whole-home backup system and want a single default recommendation without running complex load calcs: buy a 10,000-watt running / 12,500-watt surge dual-fuel inverter generator with a factory-bonded neutral and an L14-50R receptacle, such as the Champion 100554. Priced around $1,600, it covers 95% of hybrid inverter charging profiles and standard home motor surges, keeps THD under 5% to satisfy strict inverter relays, and provides the 50A headroom required for safe 6 AWG wiring to your transfer switch.
