What it costs to run, and what a quote is worth
A season of cooling or heating at a given efficiency, what an upgrade actually saves, and the outdoor temperature where a heat pump hands over to the expensive heat.
The load and the prices
Run hours are the rough part of this. A rule of thumb is 1,000 to 1,500 cooling hours in a hot climate and 500 or fewer in a mild one — your own bills are better evidence than any number here.
Cooling, and what an upgrade saves
Heating: gas against a heat pump
Which one is cheaper is a question about your two utility prices, not about the equipment. Both answers are common.
Heat pump balance point
Two straight lines crossing: your heat loss rises as it gets colder, the heat pump's capacity falls. Below where they meet, the back-up heat runs.
Reading a quote with these numbers
Efficiency alone rarely justifies replacing a working system
Run the upgrade saving above against the price of the job and the payback is usually measured in decades. That is not an argument against replacing equipment — it is an argument against efficiency being the reason. Replace things because they have failed, because the repair is a large fraction of the replacement, or because the system was never right. Take the efficiency as a bonus.
Where the maths does change is on the cheap end: a system running at half its rated airflow because of a filter and a crushed duct is losing more than the difference between two SEER2 ratings, and fixing it costs a fraction of a new unit.
SEER2 is a rating, not a promise
SEER2 and HSPF2 are measured on a test stand under a defined sequence of conditions. A real installation delivers less, sometimes much less, and the gap is almost always airflow, duct leakage into unconditioned space, and a charge nobody verified.
Use these figures to compare two options against each other, which is what they are good at. Do not expect the absolute number to match a utility bill.
What the back-up heat does to a bill
A heat pump running below its balance point makes up the shortfall with auxiliary heat, which on resistance strips has a COP of 1 — roughly a third as efficient as the heat pump was managing above the balance point. A single cold snap with the strips running can produce a bill that looks like a fault.
It is usually not a fault. It is a balance point in the wrong place, a thermostat calling auxiliary heat earlier than it needs to, or a defrost cycle that is not clearing.
Related
The annual service checklist · Airflow, split and static pressure · All HVAC tools