Residential · Watts Hillandale, Durham

Storage first, solar second

A household that had lost power three times in one winter and wanted runtime before generation.

Storage27 kWh usable (2 modules)
Inverter7.6 kW continuous
Backed up loadCritical loads subpanel
Array added7.9 kW DC, six months later

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Record

Full specification

Storage: 27 kWh usable (2 modules)
Inverter: 7.6 kW continuous
Backed up load: Critical loads subpanel
Array added: 7.9 kW DC, six months later
Measured outage: 31 hours, February
Reserve at restoration: 38 percent

The problem

The owners wanted backup power, not a payback story. Their essential load schedule came to 11.4 kWh per day: refrigerator, chest freezer, internet and networking, LED lighting through most of the house, two bedroom mini splits and a medical device that could not be interrupted.

The existing panel was full and the main service could not support a whole home transfer at the rating required.

What we did

Built a critical loads subpanel and moved seventeen circuits to it, leaving the range, dryer, water heater and main HVAC on the unbacked side.

Two 13.5 kWh modules for 27 kWh usable, which covers the 11.4 kWh daily schedule for two full days with no generation at all.

Wired and left space for the array from the start, so the solar install six months later needed no rework of the storage side.

What happened

A 31 hour outage in February drew the pack down to 38 percent with the array not yet installed. With solar now in place the same event would have recharged during daylight.

The medical device ran uninterrupted, which was the actual requirement behind the whole project.

After the array was added, modelled annual offset reached about 82 percent of the household's 11,700 kWh consumption.

What we took from it

Storage before solar is the right order when resilience is the goal. Doing it in this sequence also proved the load schedule with real data before we sized the array.

They measured every circuit with a meter instead of reading labels off the appliances. The runtime number they gave us turned out to be within an hour of what we actually got.

Marcus OyelaranHomeowner, Durham