Determining the electric backup heat capacity for a fully electric heat pump system
How much electric backup heat does an all-electric heat pump system need? Enough to cover the gap between what your heat pump can deliver on the coldest night and what your home loses, and ideally enough to keep the house safe if the heat pump ever stops. Here is how we work it out for Toronto and GTA homes.
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🔌 Why a Fully Electric System Needs Backup
In a fully electric system, a cold-climate heat pump does the heating and an electric heat strip inside the air handler fills in when it cannot keep up. Even the best cold-climate heat pumps lose output as it gets colder, while your home loses more heat. Where those two lines cross is the balance point. Below it, backup heat makes up the difference. Homes that keep a gas furnace use it as backup instead; that is a hybrid system.
🧮 The Three Numbers We Use
| Number | What it is | Where it comes from |
|---|---|---|
| Design heat loss | Heat your home loses per hour at the local design temperature (about -20°C for Toronto) | A room-by-room heat-loss calculation |
| Heat pump output at design temperature | What the outdoor unit actually delivers at that temperature, not its nominal size | Manufacturer extended performance data |
| Deficit | Design heat loss minus heat pump output | The minimum backup needed |
⚖️ Minimum Backup or Full Backup?
Minimum: cover the deficit
The strip only covers what the heat pump cannot. Smallest heater and electrical load, but if the heat pump fails in January the house will slowly cool.
Full: cover the whole heat loss
The strip alone can heat the house at design temperature. Bigger heater and breaker, but the home stays warm even if the outdoor unit is down.
What we usually recommend
Size close to full backup where the electrical panel allows it, especially for homes with no other heat source. We check the panel before quoting.
📐 Worked Example
| Step | Value |
|---|---|
| Design heat loss at -20°C | 40,000 BTU/h |
| Heat pump output at -20°C | 26,000 BTU/h |
| Deficit | 14,000 BTU/h = about 4.1 kW |
| Minimum strip | 5 kW |
| Full backup (40,000 ÷ 3,412) | about 11.7 kW, so a 10 to 15 kW strip |
1 kW of electric heat = 3,412 BTU/h. Example only; every home needs its own heat-loss calculation.
For the kW-to-breaker side of the job, see how to size an electric heater for a heat pump.
⚡ Check the Electrical Panel
A 10 kW heat strip on 240 V draws about 42 amps on its own. Add the outdoor unit and the rest of the house, and many older 100 A services are not enough. We check your panel during the site visit and tell you if an upgrade is needed before anything is ordered.
💡 Prefer to keep gas? A hybrid system uses your furnace as backup and needs no large heat strip. Compare both in hybrid vs fully electric.
🔗 Related Guides & Services
- ✓ Sizing an Electric Heater
- ✓ Fully Electric Heating
- ✓ Hybrid vs Fully Electric
- ✓ What Is a Cold-Climate Heat Pump?
- ✓ Heat Pump Installation
- ✓ Central Heat Pumps
- ✓ Hybrid Heat Pumps
- ✓ Heat Pump Rebates
⚡ Get Your Backup Heat Sized
Send us a few photos and tell us about your home. We will recommend the right system and give you a clear written price.
or fill out our form for a free consultation · Serving Toronto & the GTA