What Size Inverter Is Needed to Run a Furnace

The article explains how to determine what size inverter is needed to run a furnace, covering starting and running watts, furnace types, battery and inverter sizing, and practical installation tips for reliable backup power. It gives clear calculations, examples, and safety considerations for homeowners and installers.

Factor	Impact On Inverter Size
Furnace Type	Gas furnace blower motors require moderate starting surge; electric furnaces need very high continuous power
Starting Vs Running Watts	Inverter must handle surge (2–6x running watts) for motor startup
Inverter Waveform	Pure sine wave recommended for motor controllers and controls
Battery Capacity	Determines runtime; expressed in Ah or kWh
Additional Loads	Lights, refrigerator, thermostat and controls increase required inverter size

How Furnaces Use Power

Furnaces fall into two broad categories for power planning: gas furnaces that use electricity mainly for the blower, control board and ignition, and electric furnaces that use electric resistance heat for the heating elements. Gas furnaces have modest continuous loads but can have motor starting surges. Electric furnaces draw very high continuous power and require large inverter and battery systems to operate.

Key Electrical Terms And Why They Matter

Understanding basic electrical terms helps size an inverter properly.

• Running Watts: Continuous power draw during normal operation.
• Starting (Surge) Watts: Short-duration higher power required to start motors or compressors.
• Inverter Continuous Rating: The steady power the inverter can supply indefinitely.
• Inverter Surge Rating: The short burst power the inverter can deliver (often 5–10 seconds).
• Battery Capacity: Stored energy measured in amp-hours (Ah) or kilowatt-hours (kWh), which determines runtime.

Typical Power Requirements For Common Furnaces

A typical modern gas furnace electrical load is small compared with its heat output but varies by blower motor type and features. Electric furnaces are large consumers of electricity and are treated like space heaters for sizing.

Furnace Type	Typical Running Watts	Typical Starting Watts
Gas Furnace With PSC Blower	200–700 W	400–2000 W
Gas Furnace With ECM/Variable-Speed Blower	100–400 W	300–1200 W
Electric Furnace (Whole House)	6,000–25,000 W	Same As Running (no large transient)

Note: ECM (electronically commutated) motors are more efficient and often have lower starting current than older PSC motors, but their variable-speed controls may be sensitive to inverter waveform quality.

Step-By-Step Sizing Method

1. Identify Furnace Electrical Ratings

Locate the furnace nameplate and owner’s manual for running watts, voltage (120V or 240V), and motor type. If values are not listed, check blower motor amp rating or measure with an electrician.

2. Determine Running Watts

Convert amps to watts using W = A × V. Add continuous loads that will run during backup (thermostat, inducer motor, humidifier, controls).

3. Determine Starting Watts

For motor-driven blowers, multiply running watts by a surge factor. Use 2–4× for ECM motors and 3–6× for PSC motors unless manufacturer data shows otherwise.

4. Include Other Household Loads

Add simultaneous loads expected during outage (lights, refrigerator, sump pump). Size the inverter to handle combined running and surge requirements.

5. Choose Inverter Continuous And Surge Ratings

Select an inverter with continuous output above the total running watts and a surge rating above the highest combined starting surge. Prefer a margin of 20–30% for reliability.

6. Size Battery Capacity For Desired Runtime

Calculate energy required: Energy (Wh) = Running Watts × Hours. Add 20–30% extra for inverter inefficiency and avoid depleting batteries fully—use 50% depth-of-discharge for lead-acid or 80–90% for lithium depending on chemistry.

Practical Examples

Example A: Gas Furnace With ECM Blower

Running: 300 W for blower and controls. Starting surge: 900 W (3×). If refrigerator (600 W running, 1,200 W surge) is also on, total running = 900 W and peak surge = 2,100 W. Recommended inverter: at least 1,200–1,500 W continuous with a surge capability around 2,500–3,000 W. For 4 hours runtime at 900 W, battery energy needed = 3,600 Wh. Allowing 90% usable (lithium) => ~4,000 Wh battery capacity.

Example B: Electric Furnace

An electric furnace rated 12,000 W cannot reasonably be powered by a typical residential inverter and battery backup. For continuous 12 kW load, a grid-interactive generator or whole-home backup system sized for 12 kW continuous and substantial battery bank would be required. In most cases, electric furnaces are better served by emergency heat plans rather than inverter-only backup.

Inverter Types And Waveform Considerations

Inverter choice affects compatibility and reliability:

• Pure Sine Wave: Recommended for HVAC systems, motor controls, and electronic thermostats due to clean power and reduced electrical noise.
• Modified Sine Wave: May run simple motors but can cause overheating, noise, or erratic behavior in modern ECM motors and electronics.
• Hybrid Inverters / Inverter-Chargers: Combine inverter, charger, and transfer switch functions and are ideal for home backup systems with battery storage.

Battery Sizing And Chemistry

Battery chemistry affects usable capacity, lifespan, and installation constraints.

• Lead-Acid (AGM, Flooded): Lower upfront cost but recommended maximum 50% depth-of-discharge to prolong life, larger physical footprint.
• Lithium-Ion (LiFePO4): Higher upfront cost but allows deeper discharge (80–90%), longer cycle life, and higher usable energy density.

Examples: For a 4,000 Wh energy need, lead-acid at 50% DoD requires an 8,000 Wh battery (~200 Ah at 12V). Lithium at 90% usable requires ~4,444 Wh (~370 Ah at 12V) but with smaller weight/size than lead-acid equivalents depending on voltage configuration.

Installation And Safety Considerations

Working with HVAC and power systems involves safety and code compliance. Key points:

• Have a licensed electrician evaluate loads and wiring, especially for 240V equipment.
• Install a transfer switch or interlock to prevent backfeeding to the grid.
• Ensure inverter has required certifications (UL, IEEE) for home use.
• Follow manufacturer guidance for generator/inverter connection to furnace controls; some furnaces require neutral/ground bonding or special wiring.
• Locate batteries in a ventilated, temperature-controlled space per manufacturer recommendations.

Cost Considerations And Return On Investment

System cost depends on inverter size, battery capacity, and installation complexity. Typical price drivers:

• Inverter/charger: $500–$5,000 depending on capacity and features.
• Battery bank: $500–$15,000 depending on chemistry and capacity.
• Installation and transfer switch: $500–$2,500 depending on complexity.

For gas furnace backup where only the blower and controls are needed, modest systems (1–3 kW inverter with 3–5 kWh battery) can provide several hours of operation at relatively low cost. Whole-house backup including electric furnaces will require significantly larger investment or a generator.

Tips For Reliable Operation

• Prioritize loads: Identify essential circuits (furnace blower, thermostat, refrigerator, lights) and design the inverter system for those.
• Test the system: Conduct a simulated outage to verify furnace operation and thermostat behavior on inverter power.
• Use dedicated circuits: Keep furnace on its own inverter-protected circuit to avoid unexpected load interactions.
• Maintain batteries: Follow charge cycles and temperature recommendations to maximize lifespan.

When To Choose A Generator Instead

Portable or standby generators are appropriate when continuous high-power loads are needed or when extended runtime is required without costly battery banks. Generators pair well with inverters for hybrid solutions: the generator recharges batteries while the inverter handles clean power delivery for sensitive electronics.

Checklist For Choosing The Right Inverter Size

1. Confirm furnace voltage and motor type from the nameplate or manual.
2. Calculate running watts and estimate starting surge using motor type multipliers.
3. Add other simultaneous loads to get total running and surge requirements.
4. Select an inverter with continuous rating above total running watts and surge rating above combined startup surge with a safety margin.
5. Size battery storage for desired runtime and account for inverter efficiency and depth-of-discharge limits.
6. Choose pure sine wave inverters for HVAC systems, and plan for proper transfer switching and code-compliant installation.

Resources And Further Reading

Consult manufacturer documentation for furnace electrical specs, inverter datasheets for surge and continuous ratings, and local electrical code or a licensed electrician for installation requirements. Industry guides from inverter manufacturers and HVAC associations provide detailed compatibility charts and best practices.

If a homeowner needs a quick rule of thumb: for a typical gas furnace blower, plan for an inverter of 1,500–3,000 W continuous with a surge capacity of 2,500–5,000 W depending on blower motor type and additional loads. Electric furnaces generally require whole-house backup solutions or generators due to their high continuous demand.