Electric Heat Strips: How Many kW Do You Need?

When your heat pump can't keep up on the coldest days, electric heat strips provide supplemental heating. They're reliable, inexpensive, and simple — but they're also extremely expensive to run. One kilowatt of electric heat costs the same as one kilowatt of electricity from your utility, making strip heat the most expensive way to generate warmth.

What Are Heat Strips?

Electric heat strips are resistive heating coils (like a toaster, but larger) installed inside your air handler or furnace. When activated by a supplemental heat relay, they heat the air passing over them.

Each heat strip module is typically rated at 5 kW (17,065 BTU/hr). Multiple modules can be staged in sequence. Common configurations include:

• 5 kW: Small air handlers (1.5-2 ton)
• 10 kW: Medium air handlers (2-3 ton)
• 15 kW: Large air handlers (3-5 ton)
• 20 kW: Commercial or oversized residential

The Cost of Electric Strip Heat

One kW of electricity converts directly to 3,413 BTU/hr of heat. Unlike a heat pump (which can deliver 2-3x more BTU per kW), electric resistance heat is 100% efficient at converting electricity to heat — but that's actually worse, because you're paying for every single BTU.

How to Size Heat Strips Properly

Heat strips should be sized to cover the difference between your heat pump's heating capacity at its lowest operating temperature and your home's heating load at that same temperature.

Here's the step-by-step:

1. Find your heat pump's heating capacity at 17°F (or lowest operating temp) — this is usually 60-70% of its 47°F rating
2. Find your home's heating load at 17°F — from your Manual J heating calculation using your winter design temp
3. The difference is the supplemental heat required
4. Convert BTU to kW — divide by 3,413 to get the strip heating capacity needed

Why Oversized Heat Strips Are a Problem

Many contractors install heat strips based on air handler size, not actual supplemental heating need. A 3-ton air handler might come with 15 kW strips, when only 5 kW is ever needed. This creates problems:

• High electric bills: When strips activate, you're spending $1.95/hour instead of $0.65/hour
• Short cycling: Oversized strips satisfy demand too quickly, then shut off — wasting energy
• Reduced heat pump run time: Strips "overwrite" the heat pump's more efficient operation
• Uncomfortable temperature swings: Heat pump provides gradual, even heating; strips blast on and off

Staging Heat Strips

Quality heat pump systems stage their heat strips in 5 kW increments. This allows the system to activate only the strips needed:

• Stage 1: First 5 kW kicks in at, say, 30°F outdoor temp
• Stage 2: Additional 5 kW at 20°F
• Stage 3: Full capacity at 10°F or below

This staged approach minimizes electric costs while ensuring comfort at design conditions.

The Alternative: Dual-Fuel

If you're in a climate where heat strips would run frequently, consider a dual-fuel system (heat pump + gas furnace). The gas furnace handles cold-weather heating more economically than electric strips, and the heat pump covers mild weather efficiently.

For more details, see our article on Heat Pumps vs. Furnaces: Calculating the Switchover Point.