Big loads that need care: pumps, heat pumps, ovens, EV chargers

Big loads are the ones that decide how large a backup system needs to be. They're also the ones electricians most often suggest leaving off. That isn't a rule. It's a trade-off, and you're the one making it.

Why size and starting surges matter

Two things make a load hard to back up. It draws a lot of power while it runs. Or it draws a burst of power at the moment it starts.

Motors can draw several times their running current as they start, as covered under starting surges on the appliance labels page. A backup system has a maximum output. A motor's startup burst can be above it, even if its running load is well below.

Resistance heating is a different case. A cold heating element "will draw a high current" at first.¹ It keeps doing so "until the metallic element reaches operating temperature."¹ The source's examples are a kiln and filament lamps, so ask about your own oven in its manual.

Well pumps

A well pump moves water with a motor, often deep in the well. If you're on well water, no power can mean no water. That's a good reason to consider it.

It's also a motor, so the starting burst applies. Find the pump's label, or the label on the control box. Note both the running figure and any starting figure the maker gives. Your electrician and the pump's manual are the place to settle it.

Also ask your electrician what kind of circuit the pump is on.

Heat pumps and air conditioners

A heat pump "uses electric power to transfer heat by compression."² That compression is done by a motor-driven compressor, so the same starting question applies.

Some systems also have backup electric heating elements, and those can draw a lot on their own. Ask your installer how the unit is wired and whether the elements can be kept off the backup circuit.

A transfer switch can help here. Some switches let you prioritize "optional circuits, such as heating and cooling equipment."³ Whether your product can do that is a question for its manual.

Electric ovens and cooktops

These have heating elements and are usually large loads. Cooking is a real need for many families. But the load is large for how short a time you use it.

Some households leave the oven off and keep a microwave or a single plug-in cooking appliance on the backup side. Others keep gas cooking and keep the electric oven off. Put the choice on your list and explain it to your electrician.

EV chargers

Charging a car is a big, long load. Wikipedia says Level 2 AC charging can "supply between 6 and 80" amps.⁴ It also gives a "significant charging speed increase over AC Level 1 charging."⁴ A charger at the high end is a lot to carry. Many people leave it off the backup panel entirely.

If you do want it on, ask whether the charger can be set to a lower current. Your charger's manual says what's possible.

How to decide

• Look up the running and starting figures. Start with reading appliance labels.
• Decide what happens if the load can't run: no water, no heat, a slower charge.
• Ask whether a lower setting, a soft starter or a smaller replacement unit is an option. Only your installer should add parts.

This is design work for a qualified installer, (see why this is licensed work). Don't attach anything to a motor or its wiring yourself.