Two Batteries, a Heat Pump, and a Three-Day Whole-Home Backup Request
A homeowner with a 9.6 kWdc PV system and a 200 A main panel wants "whole-home backup" for a 3-day outage. Critical loads include a 1/2 hp well pump (LRA 48 A at 240 V), a variable-speed heat pump (soft start, 3.5 kW running), refrigerator, freezer, lights, router, and a 1.5 kW microwave. The homeowner has two 13.5 kWh batteries proposed, each with 13.5 kWh nominal and 90% usable, and a 7.6 kW backup inverter. Winter production averages 12 kWh/day; summer averages 38 kWh/day. The main panel has no spare breaker spaces; the well pump is on a 30 A double-pole breaker. The utility requires a visible lockable disconnect and does not allow backup loads to exceed the service rating. The homeowner says, "I don't need load management; just back up everything."
1. Calculate whether two batteries can support the critical loads for 3 days without PV. State assumptions and show the reasoning.
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Usable battery capacity is 2 × 13.5 kWh × 0.90 = 24.3 kWh. Critical-load energy must be estimated by appliance, not by the whole-home bill. A reasonable winter critical case without the heat pump might be: well pump 1.5 kWh/day, refrigerator 1.5, freezer 1.2, lights/router 1.0, microwave 0.3-about 5.5 kWh/day. At that load, 24.3 kWh ÷ 5.5 = about 4.4 days of autonomy, so two batteries could cover a 3-day outage if the heat pump is excluded and the batteries are fully charged.
With the heat pump, the answer changes. At 3.5 kW running and a 50% winter duty cycle, the heat pump alone uses about 42 kWh/day. Even at a 30% duty cycle it uses about 25 kWh/day, which exceeds the usable battery capacity in less than a day. Two batteries cannot provide whole-home winter backup for 3 days with the heat pump running normally.
Also check surge. A 48 A LRA at 240 V is about 11.5 kVA; a 7.6 kW inverter may not start the well pump without a soft starter or a load-management scheme. The battery may have surge capability, but the inverter is the likely bottleneck. The honest answer is: critical-load backup yes, whole-home 3-day backup no-unless the heat pump is shed, the well pump is soft-started, or a generator is added.
2. What design and code issues must be resolved before promising whole-home backup?
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First, the main panel has no spare breaker spaces. A backup system will likely require a critical-load subpanel and a transfer switch or backup gateway. Decide whether the transfer switch is service-rated and whether it switches the neutral; that affects grounding and bonding. The backup loads must not exceed the service rating, and the utility’s visible lockable disconnect must be located per its rules.
Second, calculate the actual backup load, including motor starting. The well pump’s 30 A breaker and 48 A LRA need a starting study; a soft starter may be required. The heat pump is variable-speed and may have low starting current, but its running load still dominates energy. The 7.6 kW inverter must be checked against the sum of backup loads and surge, not just the battery kWh.
Third, apply the equipment requirements: NEC Article 706 for energy storage systems, working space and clearances, rapid shutdown, disconnects, overcurrent protection, and manufacturer installation instructions. Check temperature and ventilation for the batteries and clear access for emergency responders. Obtain permits and inspections before energizing. Do not promise "whole-home" until the load calculation, single-line diagram, and utility approval are complete.
3. How would you explain the difference between "whole-home backup" and "critical-load backup" to the customer without overselling?
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Use the customer’s own numbers. Two batteries provide 24.3 kWh usable. In winter, the heat pump at 3.5 kW can use that in about seven hours of continuous running, and a normal winter duty cycle can use 25–40 kWh/day. So a 3-day whole-home outage in winter is not realistic without a generator or major load shedding. By contrast, a critical-load panel with refrigerator, freezer, lights, router, well pump, and limited microwave use might need 5–6 kWh/day, which the batteries can support for several days if they start full.
Explain that PV is not a guaranteed refill during an outage. Winter production averages 12 kWh/day, but snow, clouds, and short days can reduce that to near zero for a day or more. Summer is much better at 38 kWh/day, but the customer should not plan around the average.
Offer a written load-management plan: heat pump locked out or cycled, well pump operated in batches, microwave limited, and a generator input if the customer wants longer coverage. Provide a table of outage durations under winter and summer assumptions. This gives the customer a real choice between a smaller critical-load system and a larger whole-home system, rather than a promise that the equipment cannot keep.