A panel label is a starting point, not the answer
A home with 100-amp service needs an assessment, not an automatic yes or no. The available electrical capacity depends on the service, panel and wiring condition, calculated household loads, proposed equipment and applicable requirements. Empty breaker spaces alone do not prove that there is enough capacity.
A panel replacement, a utility service upgrade and a new equipment circuit are also different jobs. One does not always require the others. The proposal should identify the actual change needed and the trade responsible for it.
What an electrical assessment should establish
- The existing service rating, main panel condition and usable circuit arrangements.
- The calculated loads for the home and other planned electrical appliances.
- The heat pump's specified circuit and overcurrent-protection requirements.
- Any indoor electric backup heat, which must be included in the assessment.
- The wiring route, disconnect requirements, installation access and permit scope.
A qualified electrical professional should establish the electrical scope using the selected equipment's ratings and the applicable calculation method. You do not need to remove a panel cover or inspect internal wiring yourself. If the HVAC proposal excludes electrical work, identify who will price and complete it before scheduling depends on it.
Existing service, load management or an upgrade?
The review may support a suitable system on the existing service, identify circuit work, or show that a larger change is required. Equipment selection and permitted load-management approaches can affect the result, but a smaller electrical demand must still satisfy the home's heating and cooling needs.
PG&E's home-electrification planning guide describes ways to plan around existing capacity. Its PanelBoost information also explains an eligibility-based load-management option. These are planning resources, not confirmation that a particular property qualifies or that the program is included in an HVAC installation.
Keep future plans in the calculation
Tell the assessor about an EV charger, electric water heater, kitchen change, ADU or other planned loads. A decision based only on today's equipment can miss a project already on the drawing board. For an ADU, include the full proposed use and electrical design rather than treating the mini split as the only added appliance.
Retaining a serviceable gas furnace as backup is another system-design question, not a shortcut around electrical assessment. A dual-fuel arrangement needs compatible equipment and controls, an appropriate operating strategy and its own complete installation scope.
What should appear in the written estimate?
Look for the proposed equipment models, whether backup electric heat is included, the electrical work included or excluded, the responsible contractor, permit responsibilities and dependencies involving the utility. If an upgrade is necessary, compare its cost and timing as part of the whole project rather than treating it as an unexplained allowance.
Known electrical work should be coordinated before the old heating or cooling equipment is disconnected. Equipment supply, utility coordination, permits and access can affect the sequence. Our replacement planning checklist explains the difference between preparation time and the shutdown window.
Capacity is only one part of the choice
Also compare the home's load, duct condition, sound and placement requirements, realistic heat pump running costs and any rebate application requirements. The heat pump service page explains the equipment options; the electrical assessment establishes what the property can support.
Common questions
Can a 100-amp service support a heat pump?
It can in some circumstances, but the service rating alone is not enough to decide. The existing loads, electrical condition, proposed equipment and any electric backup heat need a qualified assessment. Available breaker spaces do not by themselves establish capacity.
Does an empty breaker space mean no panel upgrade is needed?
No. Physical space and electrical capacity are different. The panel and service condition, load calculation, equipment ratings and circuit requirements still need to be checked.
Who should confirm the electrical work before a heat pump installation?
A qualified electrical professional should assess the property's electrical requirements. The installation proposal should identify who is responsible for that work, what is included, and how electrical permits or utility coordination affect the schedule.