Your Circuit Fortress is designed to handle 95%+ of outages on battery and solar alone. The generator is insurance you'll rarely use. But when you need it, you want it sized right and ready to go. Here's everything you need to know about your generator options.
Every Circuit Fortress system follows one rule for generators: size for the maximum your inverter can accept. A bigger generator charges your batteries faster, runs for less time, burns less fuel per kWh stored, and is better prepared for future load growth (EV charger, heat pump, etc.).
A 13kW generator costs nearly the same as a 6kW. There's no reason to go small. The Sol-Ark manages everything — it pulls what it needs from the generator, charges the battery bank at max rate, then shuts the generator off. Bigger gen = shorter runtime = less fuel = less noise.
Sol-Ark 8K: 40A GEN port — up to 9.6kW generator
Sol-Ark 12K: 50A GEN port — up to 12kW generator
Sol-Ark 15K: 100A GEN port — up to 19.2kW generator (future-proof choice)
Generators larger than the GEN port connect to the GRID port instead. Your installer handles the wiring.
This is what most Circuit Fortress systems ship with. A portable dual-fuel or tri-fuel generator stored in your garage, rolled out and connected when needed for extended outages. The Sol-Ark manages the charging — you just plug it in and start it.
Best value in the 13kW class. Fits the Sol-Ark 12K gen port perfectly. Runs at ~80% load while charging, which is the most fuel-efficient operating point. Recharges a full 28.6 kWh battery bank in ~3-4 hours, then you shut it off.
If you have the Sol-Ark 15K with its massive 100A gen port, this is your match. Inverter-type output means ultra-clean power (<3% THD). Charges faster than anything else in the portable class.
Pros: Low cost ($1,500-3,500). No permits for the generator itself. No concrete pad or gas line. Store inside when not needed. Cons: Manual setup during outage — you roll it out, connect it, start it. Must be outside while running.
A permanent standby generator sits outside your home on a concrete pad, connected to natural gas or propane, with an automatic transfer switch (ATS). When the Sol-Ark signals that battery SOC has dropped below your threshold, the generator starts itself, charges the batteries, then shuts off. Zero human intervention. You can be asleep or away from home.
Great match for Sol-Ark 12K systems. Fits the GEN port. Auto-starts via Sol-Ark's 2-wire AGS signal or its own ATS. Built-in weekly exercise keeps it ready. Weatherproof enclosure.
The big one. Future-proof for fully electrified homes (EV + heat pump + induction). Connects to Sol-Ark 15K's 100A GEN port or GRID port for the 12K. Runs the entire house plus charges batteries simultaneously.
Pros: Fully autonomous — zero intervention. Runs on plumbed natural gas (unlimited fuel) or a large propane tank. Weatherproof. Weekly auto-exercise. Cons: $8,000-14,000 all-in. Requires permits (electrical, gas, possibly zoning). Concrete pad. Permanent fixture.
This is the breakthrough option. What if you could get standby-like auto-start behavior at portable generator prices? With the right generator and a simple outdoor setup, you can.
Sol-Ark inverters have a built-in AGS (Automatic Generator Start) feature — a dry contact relay that closes when your battery drops below a configurable SOC threshold (you set it: 0-25%). This sends a low-voltage 2-wire signal. Any generator that accepts a 2-wire start/stop input will auto-start, charge your batteries to 100%, then auto-stop when Sol-Ark opens the relay. No human needed.
This is the first and only portable generator with factory 2-wire auto-start. It ships with a 2-wire start cable with terminal block. Connect two wires to Sol-Ark's AGS terminals and you have fully autonomous backup. Inverter-type output means clean sine wave safe for all electronics. Tri-fuel means you can run it on propane (recommended for outdoor semi-permanent setup — fuel never goes stale).
Note: At 7.8kW rated, this is undersized vs the Sol-Ark 12K's full 12kW gen port. It will charge batteries at a slower rate than a 13kW DuroMax. Your system design uses the maximum generator for the port — this option trades some charging speed for full auto-start capability.
The GM10500XiT at 7.8kW rated is the only portable with native auto-start, but it's smaller than the optimal generator for most Sol-Ark systems. Your estimator recommends the maximum generator for your inverter port — don't downsize just for auto-start. If auto-start is important AND you want maximum charging speed, consider a permanent standby generator or the Atkinson GSCM kit on a larger portable.
If you want to keep a larger portable (like the DuroMax XP13000HXT) but add auto-start capability, aftermarket solutions exist:
An aftermarket auto-start/stop controller that handles choke management, preheat, crank, retry on failed start, and cooldown. You specify your generator model and inverter — they configure the kit for your exact setup. Proven in the off-grid community. This is the way to get auto-start on a full-size portable.
OEM conversion kit that adds dry contact 2-wire start to Generac standby models that don't have it natively. Only works with specific Generac Guardian and Protector models.
If you're going the auto-start portable route, the generator needs to live outside, ready to start on signal. Here's how to do it right.
Think carport-style lean-to, not a shed. A simple three-sided shelter with a roof is all you need. Cost: $300-800 for a basic structure.
Once you permanently wire a portable generator (hardwired to Sol-Ark's GEN port through an inlet box), an inspector may classify it as a permanent installation regardless of the generator being "portable." Plan for an electrical permit. If connecting propane, a gas permit may also be required. Massachusetts 527 CMR 12.00 applies. Check your town's requirements.
If your generator sits outside for months waiting for an outage, fuel matters. Gasoline degrades in 3-6 months — the carburetor gums up, the gen won't start when you need it most. Propane never goes stale. The generator is always ready, even after a year of sitting idle.
The Genmax GM10500XiT is tri-fuel: gasoline, propane, and natural gas. For a semi-permanent outdoor setup, propane is the clear choice. If your home already has propane (heating, cooking), you can tap into the existing supply.
| Propane Tank | Capacity | Runtime @ 75% | Cost | Notes |
|---|---|---|---|---|
| 20 lb (BBQ tank) | 4.7 gal | ~7 hours | $20-25 swap | Keep 2 on hand. Easy to swap. |
| 100 lb tank | 23.6 gal | ~36 hours | $150-200 | Dedicated tank, refill by truck. |
| 250+ gal buried | 250 gal | ~385 hrs (16 days) | $500-1,500 | Permanent. Auto-delivery available. |
Natural gas is the ultimate setup if you have a gas line — unlimited fuel, zero maintenance, zero refueling. The GM10500XiT supports it natively.
The portable auto-start setup saves $5,000-11,000 compared to a standby install for the same autonomous functionality. The trade-off: you'll need to swap propane tanks during extended multi-day events, and the generator is slightly undersized vs a full standby. For 95%+ of outages where the generator never even fires (battery + solar handles it), the portable auto-start is the clear winner on value.
Regardless of which option you choose, run your generator for 10-15 minutes every month. This circulates oil, charges the onboard starter battery, prevents carburetor buildup (gasoline models), and confirms it starts reliably. The Sol-Ark can be configured to auto-exercise weekly if your generator supports 2-wire start.
Run your free estimate — we'll recommend the optimal generator for your specific system, load, and inverter. Then use the interactive configurator to see how different setups affect your autonomy.
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