Battery System to Buy Home Power At Off-Peak Rates

[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.

[GH85Carrera]

Will the batteries themselves last 11 years? 13 years sounds like a real stretch for battery life. How much will just new batteries cost?

[Evans, Marv]

I'll keep track of this thread. I know the Tesla batteries cost around $13K apiece and store around 13 kWh of power. I don't know a lot else. I'm interested in the economy of charging the batteries (I probably need three) duriing the day with my solar panels. I'm ultimately hoping to get a good deal through some program for old people. The life span of the batteries is a factor to consider.

[jyl]

Yeah, I wonder. 11 years is 4,000 daily charge-discharge cycles.

I have downloaded my hourly use for a full year, and can model out exactly what kWh battery I need to be at 80% or 95% off-peak buying.

But I need to get a sense what the systems cost. If my guess above is way low, then it’s pointless to do this work.

The main reason I got interested in this is that I see our electricity rates going up at a too-high rate in the coming decade.

Nationally, the electric power grid - distribution, transmission, generation - needs a metric f-ton of investment. Something like 50% of the lines are at or near end of life. Power demand was flat for some years, but is now growing 2-3% a year - thank you, artificial intelligence and all the AI data centers being built that need city-level amounts of power 24/7 with perfect reliability. Something like $200 billion a year being invested in the national power system and that is nowhere near enough. Add wildfires out West which are forcing big investment to underground transmission lines.

The normal way this investment is funded is the utility companies get state permission to invest $X billion and make a return on that investment by raising rates. So everyone has to pay for Google or Microsoft to get enough power for their new datacenters. I’d think the companies should pay but they’ll make sure their friends in high places kill that idea.

If I run my model with power rates rising +7% year, the payback looks better.

[Cajundaddy]

For starters, a 25kwh server rack LFP battery is around $6500 so this may not pencil out right now. Battery prices are falling by around 40% per year so you may want to wait a bit. A quality LFP battery is probably good for 10 years but battery tech is still moving forward rapidly in terms of energy density and cost per kWh. Solar panels are really pretty cheap now so you may want a mixed system for the best energy leverage.

100kwh per day on average sounds like a lot. My house averages around 20kwh per day. Does it make sense to chase efficiency first and follow with a properly sized energy flex system?

[Evans, Marv]

I've discussed this and said this a few times on here. I originally got solar because my bill for my highest usage month went up by $100 from one year to the next - back around 2015. SDG&E charges (Ithink) the highest rates in the nation. I'm on the original net metering program, which means I feed excess generation back into the grid and get credit for it and paid at the wholesale rate (two to four cents per kWh) on my "true up" date. My system started generation in 2016, and had paid for itself by a couple of years ago. I didn't pay for any electricity from 2016 until 2022, when we got the EV in addition to powering the all electric house. Last year I paid $552 for electrical power running the house and charging the car. So I'd have to find a real deal on batteries for it to pencil out, but it woould be a convenience when they cut the power during high fire danger episodes. The generator is a PITA to deal with during those times, though it saves the stuff in the fridge and freezer.
'

[jyl]

I’m sorry! I typo’d. Daily average 63 kWh.

Yeah, I use a lot of electricity. The biggest offender is my AC, it is a central system but inadequate ducting (100 y/o house) means it really struggles to cool upstairs, and in the hottest months of the summer it runs 24/7. In winter, daily average is 43 kWh. The next offenders are some appliances - my dishwasher is a commercial high-temp unit, my fridge is a commercial unit, my espresso machine is a 240v commercial unit, and my clothes dryer is electric (going to gas whenever we replace it). All lights are LED, heating and hot water are forced air gas. Next year I’m installing minisplit heat pumps in all the bedrooms, which should help a lot in summer.

[Cajundaddy]

Yep, chasing efficiency is probably the low hanging fruit. I would get a meter on every major appliance and log the Kwh they use. Also increase shade or reduce heat transmission on the south facing side of the house. A modern ductless 22 SEER A/C system might cut your daily Kwh average in half so now you only need 1/2 the battery system. This starts to look a lot more viable.

[jyl]

Okay, so defer decision until after the minisplits are in.

Are there meters that can log the consumption of 240v appliances? I’m thinking clamp-on type?

EDIT I found this one, which seems absurdly cheap if it even sort of works. https://www.amazon.com/Current-Amperage-Voltmeter-Multimeter-Transformer/dp/B07JB9B2QL/

Actually this one looks better https://www.amazon.com/SIEMENS-Smart-Home-Electricity-Monitoring/dp/B0CTTSC7DR/

[cabmandone]

The only ones I know of are in line meters. The guy that did my energy audit a few years ago had something he could track energy consumption of everything in his house with and an app to track it on his phone. I don't know what that system was though. Look up whole home energy monitoring.

[Cajundaddy]

Originally Posted by jyl

Okay, so defer decision until after the minisplits are in.

Are there meters that can log the consumption of 240v appliances? I’m thinking clamp-on type?

EDIT I found this one, which seems absurdly cheap if it even sort of works. https://www.amazon.com/Current-Amperage-Voltmeter-Multimeter-Transformer/dp/B07JB9B2QL/

Actually this one looks better https://www.amazon.com/SIEMENS-Smart-Home-Electricity-Monitoring/dp/B0CTTSC7DR/

These modern tools look great.

In my last house I did it the old fashioned way with an amp clamp and then calculating the hours per day in use. My ancient HVAC from 1985 consumed 12,000w and struggled to keep the house cool on 100F days. This was a big clue. The first step is quantifying the problem, and then you have choices to address the problem from a clear-eyed cost/benefit analysis.

Our current home was finished in 2021 and is highly efficient with extensive insulation and very efficient appliances. We only pay 10 cents per Kwh on average here in ID with a lot of hydro and some of the lowest electrical generation costs in the US. As a result our energy cost is about 30% of what it was in SoCal with a 1965 home and essentially the same sq ft.

[Flat Six]

Time-Of-Use (TOU) rates are designed to influence when ratepayers consume electricity -- generally to incentivize use during nights and weekends, the off-peak hours.

If your utility provides it (we did at Portland General Electric when I was there ~6 years ago), you can download your electric usage down to the 15-minute increment. Dump it into Excel and it will give you a more granular picture of when (time of day) you're using more/less electricity. Combined with a line meter on your major appliances, you'll be able to identify which ones you can shift to using during off-peak rates (e.g., dishwasher, laundry -- especially w/electric dryer) and how to best program your HVAC to minimize on-peak rates. I'm sure you took advantage of your utility's programs re: mini-splits; if you haven't already you might want to look into any rebates on programmable t-stats.

Using residential storage to try to arbitrage off-peak vs. on-peak rates will likely not pencil out (or not be worth the hassle) compared to making EE improvements & modifying your electricity use patterns to take advantage of TOU rates.

[Deschodt]

If a battery captures X electricity at $x, will it truly send back X to the grid at the higher price, or are there losses involved in the "chemical storage" and restoration of it later, and is that factored into the calculation? I get that for a house use only even a % loss is worth is when the rate is 3x more ;-)

I wanted to do the above in lieu of a generator, time shift and free backup, but ended up going full solar in the end (glad I did as prices are way up) so it's moot for me, but curious about the math. I thought batteries in the act of converting power to a chemical state and returning it to power later for the grid/house would show losses in the transformation, and not send back exactly what they took in, a % that got more serious over time as the battery ages.. True/False ?

[jyl]

One thing that changes the payback a lot is cost of money. The let's say it is $14,000 spent on a system could be earning 4%-ish with no risk, that extends the payback period by a few years.