When a hydronic boiler fails, the first question is how long the repair will take. An ACHR News piece on repairing or renting during a boiler failure starts from the same priority: the summary says "getting it back online is the priority." It also notes that temporary heating can keep a building operational when repairs run long. In a house, that usually means a portable electric heater running in one or two rooms while the boiler waits on parts or a technician.
That heater is a new appliance on an existing branch circuit, and it deserves the same arithmetic you would give any other load. Most homeowners check the heater's comfort settings and never check the breaker it shares.
Work the Numbers Before the Plug
Start with the nameplate watt rating. Divide watts by volts to get amps. A common 1500 W heater on a 120 V circuit draws about 12.5 A (1500 divided by 120). A standard 15 A household branch circuit is usually limited to 12 A for continuous loads, which is 80 percent of its rating. A 1500 W heater on that circuit is already at or above the continuous guideline before anything else plugs in. A 1200 W unit draws 10 A, which leaves roughly 2 A of headroom for everything else on the same breaker.
On a 20 A circuit, the continuous limit is 16 A, or about 1920 W at 120 V. That is why the breaker label matters as much as the heater label. Open the panel, find the breaker that feeds the room, and write down what else is on it. Refrigerators, lighting, and bathroom fans are common shared loads, and they all add to the total.
If you have a clamp meter, you can check the real draw instead of relying on the label. With the heater on high and the other loads on that circuit running, clamp the hot conductor at the panel and read the amps. A multimeter alone will not do this job without a clamp accessory. A reading that sits near the breaker rating means the circuit is working too hard, and the fix is a different outlet or a lower-wattage unit, not a bigger breaker.
Cords, Outlets, and Clearances
Plug the heater directly into a wall outlet. Extension cords and power strips are the usual problem in a temporary setup. A thin cord carrying 12 A or more can run warm at the plug, and a cheap power strip can overheat at its connections. If a cord is unavoidable, use one rated for the load and keep it short and uncoiled.
Check the outlet and plug by hand after the heater has run for a while. Warmth at the faceplate, a discolored receptacle, or a plug that feels soft is a reason to stop and move the heater. Breakers that trip repeatedly are also a signal to reduce the load, not to reset the breaker over and over. If you see scorching or smell burning insulation, shut the heater off and call a licensed electrician.
Clearance matters as much as amperage. Keep the heater away from bedding, curtains, couches, and anything that can be knocked into it. Follow the clearance printed on the unit, which is often three feet or more. Set it on a flat, stable surface where it will not be tipped, and look for a model with an overheat cutoff and a tip-over switch before you buy or rent one.
A Short Outage Checklist
Read the wattage on the heater and convert it to amps. Find the breaker that feeds the room and list what else shares it. Compare the total against the 80 percent continuous limit for that breaker size. Plug straight into the wall, then check the outlet and plug for warmth after the unit has been running. Keep the printed clearance, and keep the heater away from doorways and walkways where someone could trip over the cord.
Write the circuit and the heater rating on the same sheet you use to track the outage. When the boiler is back, the same sheet tells you exactly what the temporary setup was asking of the house, and whether the circuit needs a different plan for the next cold snap.