What you'll see

The complaint isn't a breakdown — the system "works," but the bill jumped. This is a runtime-and-efficiency call, and the worst thing you can do is shrug and say "summer's hot." Energy use comes down to how long the equipment runs times how much it draws while running, plus any electric resistance heat quietly piling on. Your job is to find what's making the system run longer than it should, draw more than it should, or run expensive backup heat it shouldn't need.

Start by separating the three drivers: excessive runtime, degraded efficiency, or auxiliary/strip heat running when it shouldn't.

Walk it in order

  1. Get the story and the season. Heat pump or AC? Gas or electric heat? Did the bill spike in a cold snap (aux heat) or a heat wave (cooling runtime), or for no weather reason (a fault)? A heat pump leaning on electric strips in winter is the single biggest "high bill" driver there is.
  1. Heat pump: check whether aux/strip heat is running too much. Clamp the strip-heat circuit and watch. Strips should only run on a real aux call (cold day, defrost, recovery from setback). If they're energizing during normal heat-pump operation — stuck sequencer/relay, miswired stat, wrong O/B logic putting the unit in cooling so aux carries everything, or a thermostat with too-aggressive aux staging — the meter spins. Emergency-heat mode left on by the homeowner does it too.
  1. Check for excessive runtime causes. A system that runs much longer than needed burns more power:
  • Low charge / leak: lost capacity means long run times to barely keep up. Gauge it.
  • Dirty condenser or evaporator: poor heat transfer, high head, long runs, high amps. Clean coils are free efficiency.
  • Dirty filter / weak airflow: the system runs longer to move the same heat; can also freeze the coil.
  • Short-cycling: lots of starts burn inrush energy and never reach efficient steady-state (see the short-cycling article).
  • Thermostat/setback fighting: big setbacks on a heat pump trigger aux on recovery — sometimes deep setbacks cost more on a heat pump.
  1. Measure the draw. Clamp the compressor, fan, and blower amps and compare to the nameplate. A compressor pulling well above RLA, or a motor overamping, costs money every minute it runs — and points at high head, a weak capacitor, a failing motor, or a mechanical problem.
  1. Check duct leakage and insulation. Conditioned air leaking into an attic or crawlspace is pure waste — the equipment runs fine but you're paying to heat/cool the void. Disconnected branches, leaky plenums, and uninsulated attic duct all show up as a high bill with "normal" equipment readings. A big supply/return imbalance pulls in unconditioned air too.
  1. Consider sizing and condition. A grossly oversized, short-cycling system, or an old low-efficiency unit, simply costs more to run. That's a replace/upgrade conversation, not a repair — but confirm it's not a fixable fault first.

What "normal" looks like

  • Compressor amps: at or below nameplate RLA when running; well above is a problem.
  • Strip heat: off during normal heat-pump operation; on only for aux/defrost/recovery. Each strip stage draws a lot — that's why it dominates a bill.
  • Coils: clean; head pressure and amps in normal range.
  • Runtime: long and steady in design conditions is efficient for a right-sized system; constant running in mild weather is a flag.
  • Duct leakage: minimal; supply air reaching registers near coil temperature, not warmed/cooled by the attic.

Common faults & what they mean

  • Stuck/over-eager aux or emergency heat: the #1 heat-pump bill killer. Strips running with the compressor or left in emergency mode. Clamp and confirm.
  • Wrong O/B logic / miswired stat: heat pump effectively in cooling, aux carrying the whole load. Brutal on the bill.
  • Low charge / dirty coils / weak airflow: lost capacity, long runs, high amps. The efficiency-loss bucket.
  • Overamping compressor or motor: high draw every minute. High head, weak cap, failing component.
  • Duct leakage / poor insulation: paying to condition the attic. Normal equipment readings, high bill.
  • Oversizing / old low-efficiency equipment: inherently costly. Upgrade conversation after ruling out faults.

Tech tips & gotchas

  • On a heat pump, check the strips first. Electric resistance heat dwarfs everything else on the meter. A stuck sequencer or emergency-heat switch left on explains most winter bill spikes in one clamp-meter reading.
  • Deep thermostat setbacks can backfire on heat pumps. Recovering from a big setback kicks on aux heat — sometimes a setback raises the bill. Set expectations.
  • Dirty coils are silent money. No breakdown, just high head and long runs. Cleaning is the cheapest efficiency you can sell.
  • Duct leakage hides behind "normal" gauge readings. If the equipment checks out but the bill's high, get in the attic and look for leaks and disconnects.
  • Clamp the amps — don't assume. An overamping compressor costs real money and points at a real fault. The nameplate is your benchmark.
  • Separate "fault" from "physics." A hot week or a cold snap genuinely costs more. Confirm there's an actual fault before you sell a repair, and be honest when it's just the weather plus an oversized unit.

Safety / code notes

  • De-energize before clamping inside panels where you must open them; many amp readings can be taken at the disconnect or whip without exposing yourself.
  • Electric strip heat draws heavy current — verify the circuit, breaker, and wiring are sized and intact per NEC; a stuck contactor on strips is a fire/energy risk.
  • On gas systems, a "high bill" plus poor performance can mean a combustion problem — verify combustion and CO, and that venting/combustion air meet IMC §701.