[jyl]

Got curious about combining time-of-day electricity rate with a home battery system. Idea is to charge the battery during off-peak hours and run the house from the battery during mid-peak and especially high-peak hours. Compare to cost of staying w/ base rate which is the same all hours.

Off-peak rate 9P-7A $0.0839/kWh, mid-peak 7A-5P $0.1577/kWh, high-peak $0.4111/kWh, base $0.1634. My house uses average 1,973 kWh/month, ranging from 2,925 kWh in August (inefficient central AC on all day/night) to 1,416 kWh in November (gas furnace heat). Average daily use is CORRECTED 63 kWh/day, but it varies widely, can be as low as 20 kWh (we must not be home) to 166 kWh (24H AC plus doing a zillion loads of laundry w/ electric dryer?). The highest days are maybe once a month, the lowest days are 3-5 days a month.

I don't know the cost of a battery bank plus controller/inverter smart enough to charge at off-peak and drain at high-peak first and then at mid-peak if its has charge left. I know it depends a lot on battery kWh. I also don't know how fast batteries can charge up.

But anyhoo, IF a system large enough to let me buy 80% of my power at off-peak costs $14,000 after incentives, and a system large enough to let me buy 95% of power at off-peak costs $28,000 after incentives, then I calculate the payback for the 80% system is probably 11 years and the payback for the 95% system is probably 13 years.

I'm assuming electric rates go up 5%/year (its actually been more like 7% lately) and cost of money is 4%.

So my questions are:
- What is the realistic cost of a 60 kWh battery + controller? of a 90 kWh?
- What is the expected life of these system?
- Anyone do this?
- What am I missing?

No solar involved. Just time-shifting.