Choosing the right inverter to run a furnace requires understanding the furnace’s startup surge, continuous running watts, and the supporting battery and charging system. This guide explains how to size an inverter for various furnace types and scenarios, offering practical calculations, example setups, and installation tips to ensure reliable operation during grid outages or off-grid living.
| Furnace Type | Typical Starting Watts | Typical Running Watts |
|---|---|---|
| Electric Forced-Air Furnace | 5,000–15,000 W (depends on elements) | 3,000–12,000 W |
| Gas Furnace (Blower & Controls) | 500–1,500 W | 200–800 W |
| Small Millivolt/Wall Furnace | 100–400 W | 50–200 W |
How Inverters Work And Why Size Matters
An inverter converts DC from batteries to AC for household appliances. Inverter capacity is expressed in continuous watts and surge (peak) watts. Continuous watts determine how long an inverter can sustain the load; surge watts handle the brief startup current required by motors and heating elements. Undersizing an inverter can cause nuisance shutdowns or damage, while oversized units increase cost and battery demand.
Furnace Types And Their Power Requirements
Different furnace types have very different power needs. Electric furnaces with resistance heating elements draw large continuous current and huge startup power. Gas furnaces primarily use electricity for the blower, controls, and ignition; these are far less demanding. Ductless mini-splits or heat pump furnaces have variable compressors and may require moderate surge capacity.
Electric Resistance Furnaces
Electric forced-air furnaces are essentially banked heating elements and can draw several kilowatts continuously. A typical 3-ton electric furnace or heat-only system may need 9,000–12,000 watts continuously. These usually require a whole-house inverter or generator rather than a small inverter.
Gas Furnaces
Gas furnaces rely on natural gas or propane for heat; electricity operates fans, control boards, and ignition. Typical running loads are 200–800 watts, with startup surges up to 1,500 watts depending on blower size and ECM motor type. Gas furnaces are often the easiest to run on an inverter-based backup system.
Heat Pumps And Mini-Splits
Heat pump systems include a compressor and outdoor unit that create higher startup draws. Running watts vary by capacity, typically 1,000–4,000 watts, with surge multipliers of 2–4x for compressor start. Variable-speed inverters on modern heat pumps reduce surges, but older units may need larger inverter surge capacity.
Determining Required Inverter Size: Step-By-Step
Step 1: Identify continuous running wattage of the furnace (check the nameplate or manual). Step 2: Identify startup or locked-rotor watts for motors or the inrush current for heating elements. Step 3: Add other simultaneous loads (lights, fridge, well pump). Step 4: Choose an inverter whose continuous rating exceeds the total running watts and whose surge rating exceeds the largest startup surge.
Typical Calculation Example
For a gas furnace with a 500 W blower and a 1,200 W startup surge: running watts = 500 W; surge = 1,200 W. Add refrigerator (700W start, 150W run) starting together: combined running = 650 W; combined surge = 1,900 W. A 2,500 W continuous inverter with 4,000 W surge would be a safe choice.
Battery Capacity And Inverter Efficiency
Inverter efficiency (usually 85–95%) and battery capacity determine runtime. Convert AC loads to DC required battery amp-hours using formula: Battery Ah = (AC Watts × Hours) / (Battery Voltage × Inverter Efficiency). For example, a 1,000 W furnace running 4 hours on a 12 V battery at 90% efficiency requires about 370 Ah. Always size battery bank with depth-of-discharge and charging method in mind.
Startup Surge Versus Running Load — Why Surge Matters
Motors, compressors, and some electronic blowers produce a transient inrush current several times higher than running current. Inverter surge rating must cover these peaks for short durations (typically a few seconds). Pure sine wave inverters handle inductive loads better and are recommended for sensitive furnace controls and ECM motors.
Examples Of Inverter Sizing For Common Furnace Scenarios
| Scenario | Estimated Running Watts | Estimated Surge Watts | Recommended Inverter |
|---|---|---|---|
| Gas Furnace Only (ECM Blower) | 300–600 W | 1,000–1,500 W | 1,500–2,000 W Pure Sine Inverter |
| Small Electric Furnace/Space Heater | 1,500–3,000 W | 1,500–3,000 W | 3,500–5,000 W Inverter (or generator) |
| Whole House Electric Furnace | 6,000–12,000 W | 6,000–15,000 W | Whole-house inverter or backup generator with appropriate panel transfer |
Additional Loads To Consider
While sizing an inverter to run a furnace, include simultaneous essential loads such as the refrigerator, well pump, lighting, medical devices, and charging equipment. Two or three medium loads starting at once can surpass surge capacity quickly. Prioritize loads and choose an inverter with headroom or a load management system.
Inverter Types And Features To Prefer
Pure sine wave inverters are recommended for furnaces, especially those with sensitive electronics or ECM motors. Look for inverters with high surge capacity (2–3x continuous rating), built-in transfer switches or external ATS compatibility, low THD (<5%), and robust cooling. Hybrid inverters with built-in chargers and solar integration simplify renewable backup setups.
Practical Installation Tips
Use a dedicated transfer switch or subpanel for furnace circuits to isolate inverter-supplied circuits from the grid. Ensure inverter input voltage matches battery bank voltage (12V, 24V, 48V commonly). Install proper fusing, battery disconnects, and ventilation for batteries and inverter. Verify that furnace controls are compatible with inverter output waveform.
Safety, Codes, And Professional Help
Local electrical codes may require licensed electrician installation for inverter tie-ins, transfer switches, and permanent battery systems. Permits and inspections often apply to backup power installations. Engaging a licensed electrician ensures compliance with National Electrical Code (NEC) and local requirements and reduces risk.
Cost Considerations And Alternatives
Large inverters and battery banks are a significant investment. For electric furnaces, a backup generator may be more cost-effective than scaling batteries for high continuous loads. For gas furnaces, smaller inverter and battery systems are economical. Consider hybrid systems combining solar, battery, and generator for resilience and cost management.
Maintenance And Performance Optimization
Maintain batteries (especially flooded lead-acid) according to manufacturer guidance and keep inverter firmware updated. Test the transfer switch periodically and perform load tests to confirm surge handling. Monitor inverter temperature and ventilation to prevent derating due to heat.
Frequently Asked Questions
Can A Small Inverter Run My Furnace?
Small inverters may run the blower and controls of a gas furnace, but not electric resistance furnaces. The key is matching both the continuous load and startup surge. Check furnace nameplate data for accurate sizing.
Is A Pure Sine Wave Inverter Necessary?
Yes, for most furnaces and modern ECM motors a pure sine wave inverter is recommended to ensure reliable operation and prevent control board issues. Modified sine inverters can cause overheating or erratic behavior in sensitive electronics.
How Long Will A Battery Run A Furnace?
Runtime depends on inverter efficiency, battery capacity, and furnace load. Example: a 1,000 W furnace with a 48 V 200 Ah battery bank (9.6 kWh usable at 50% DoD) could run approximately 7–8 hours excluding inverter losses. Real-world runtime will be lower when accounting for inefficiencies and other loads.
What About Inrush Current For Old Furnaces?
Older furnaces with PSC motors or large compressors may have higher inrush currents. When in doubt, measure startup current with a clamp meter or consult the manufacturer to determine correct surge capacity.
Key Takeaways For Sizing An Inverter To Run A Furnace
- Determine actual running and startup watts from furnace specifications.
- Choose a pure sine inverter with continuous and surge ratings above combined loads.
- Include additional household loads and add headroom for safety.
- Size battery bank based on desired runtime, inverter efficiency, and depth-of-discharge.
- Consider professional installation to meet code and safety requirements.
For complex systems or whole-house backup, consulting a licensed electrician or a certified solar/energy systems designer will ensure the inverter, battery, and transfer equipment are properly sized and safely installed.