Four things to do before you buy a battery for your rate

The window is small and expensive

On a residential time-of-use rate the on-peak window is weekday afternoons, three to seven, holidays excluded. That is about 1,040 hours a year, roughly twelve percent of the total, and at the sample rates we model it costs nearly three times the off-peak price per kWh.

Almost nothing that happens in those four hours has to happen in those four hours. That is the entire opportunity.

Move the car, then the appliances

A Level 2 charger at 11.5 kW pulling for three hours is 34.5 kWh. At the sample on-peak price that charge costs $8.20. After 9 pm it costs $2.88. Setting the schedule in the car takes ninety seconds and never needs doing again.

The dishwasher, the washer, the dryer and any pool or spa pump are all delay-start devices. Between them they commonly run 4 to 7 kWh a day, which is another 60 cents to a dollar of peak avoided daily if they had been running at five in the afternoon.

Pre-cool rather than fight

Running air conditioning hard from one until three and letting the house drift a couple of degrees through the peak moves real kWh into cheaper hours. How well it works depends entirely on the envelope. A sealed house at 4 ACH50 with R-60 above it coasts for hours; a 12 ACH50 house cannot do this at all, which is one more reason the audit comes first.

Then, and only then, price the battery

What is left in the peak window after all of that is what a battery would shift. For most Boulder County households we measure between 5 and 10 kWh a weekday.

The value of each shifted kWh is the on-peak price minus the off-peak price divided by round-trip efficiency. At the sample rates and 90 percent round trip that is 14.5 cents in summer and 11.3 cents in winter. Eight kWh a weekday across 250 weekdays at a blended 13 cents is about $260 a year, against an installed cost in the mid five figures.

That is not a payback. It is a partial offset against a resilience purchase, and anybody who presents it as a payback is selling rather than advising.

Every figure in this article is a sample for a demonstration website. Ask us for the workings on your own house rather than applying these numbers to it.