Friday, March 15, 2019

Batteries cost less than a whole-house generator (+ Day #6 off-grid)

After the last hurricane, lots of people told me they'd get solar if they could use it in lieu of a whole-house generator.  I thought I'd use this post to explore the economics of solar + battery versus the whole house generator powered on "fossil fuel" (I hate that term - it's fossil hydrocarbon).

We've been watching the Tesla Powerwall batteries in action this week.  To find out about whole house generators, I consulted my generator gurus, Walter & Rosalie Goldberg.  I remember their generator coming on automatically moments after Hurricane Katrina knocked out the power to their house during dinner.  They insisted we spend the night, but clever me, I insisted we could make it home through a tropical storm (that was the forecast), and proceeded to drive home through a tree-wrenching hurricane so we could feed the dog.  A memorable drive it was.

The Goldbergs currently have a 20 kW air-cooled Generac, with 200A service, and 4 load-shed modules.  The generator is installed on 4” concrete pad with a 1000 gallon buried propane tank out back.  Cost was $15,000 including installation and the first load of fuel.  Liquid cooled generators much quieter, but also cost more.  The Kohler generator costs a bit more than the Generac and has much better customer reviews, but was unavailable where they currently live. 

Fuel cost

Liquid propane is the preferred fuel for on-site reliability.  An 800 gallon refill costs $2400.
The generator runs 20 minutes a week for maintenance purposes, and the fuel cost for that comes to $2.43/week, ~ $125 per year.  Running the house on propane costs $175/day.  If we assume a 10-day outage every 10 years, that's an average of 1-day emergency operation per year for $175/year.

Service cost

Failure to provide adequate service is why so many generators died in the days following the hurricane.  Installer Pat Porzio says, "After 24 to 48 hours of continuous use, get it serviced. After around 10 days, have a professional change the oil and the filter.”
A maintenance plan is essential to keep the generator running. A good plan includes immediate service if something breaks.  Our friends pay $325/ year. Parts & labor are extra, so add another $150/year.  Plus the propane to supply the weekly maintenance runs.

Cost summary

Total upfront cost $15,000
Yearly maintenance $600
Yearly emergency use $175

So one starts with a $15,000 generator and puts in an additional $7,750 over 10 years.
The 10 year cost of owning and maintaining a whole house generator is $22,750.

Price comparison: whole house generator vs. solar + batteries

We saw in a previous post that residential solar is a great financial investment under net-metering and the current tax credit (or break-even without the tax credit).

You don't buy house batteries to make money, but if you wish to keep the house running during a power outage, it's either batteries or the generator.

3 Tesla Powerwalls, have a combined output of a 21 kW generator, equivalent to the Generac in our example above. The batteries cost $23,275 installed, or $16,235 after the 30% tax credit.  That's significantly less than the 10-year cost of owning a whole house generator.

The Generac has a 5-year limited warranty, compared to 10 years for Tesla Powerwalls.  

Tesla Powerwalls should last 20-30 years if you only use them as emergency backup, whereas online reviews show generators lasting 5-10 years.  Do your own math on replacement costs.

Economics alone greatly favors the house batteries over the whole house generator.  Unlike lithium batteries, the generator also produces noise, bad smells, and air pollution (social cost of carbon!), and requires oil changes, fuel deliveries, and will likely needing servicing or fuel after a storm when you are depending on it.

Batteries beat the whole house generator hands down.

Day 6 off-grid

The batteries topped-off their charge and the battery controller turned off the solar array. A technician from Tesla convinced me that the Level 2 charger would work, so I plugged the car into the Level 2 charger and it charged up the car pronto, with the sun providing the largest contribution.  In the late afternoon, I tried out all the appliances to see how much each one drew.  I'm getting bored.

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