How the hourly usage profile is built

Under SCE’s net billing, a kWh sent to the grid at noon earns far less than a kWh bought at 7 PM, so the bill after solar depends on the hour. For LADWP homes the balance over each billing period counts, but the hours still decide how much solar the home uses directly and what a battery does. Either way Cali-Graf needs usage for each of the 8,760 hours of the year — not just monthly totals.

Where the hours come from

NREL’s ResStock study models hour-by-hour power use for a large, representative sample of U.S. homes — their size, appliances and local weather. Cali-Graf uses its single-family homes in Los Angeles.

• For each of those homes, Cali-Graf works out what its bills would be for the customer’s billing periods.
• It picks the 30 homes whose year has the most similar shape — the same balance of summer and winter.
• It scales each one so its bills match the customer’s bills exactly.
• It calculates the bill after solar for all 30 and shows the middle result. For SCE homes it also shows the range from the lower to the upper part of the group — see Why the first-year forecast is a range.

When the hours are more exact

• A Green Button file replaces the model completely: Cali-Graf uses the customer’s own hours. See Upload a Green Button file.
• SCE time-of-use kWh from the bills tell Cali-Graf how much the home used at peak and off-peak hours, and the hours inside each period are adjusted to match. See Enter time-of-use kWh from SCE bills.
• SCE TOU-D-PRIME customers usually charge an electric car on a timer at night, so for them Cali-Graf picks similar homes with an electric car and moves the charging to night hours.

With a yearly total only

With one number for the year there’s no shape to match, so Cali-Graf takes the typical year of homes that use about as much and splits the total by it. That’s why bills give a more accurate result than a yearly total.