On Batteries for Whole Home Backup Power

In recent weeks I’ve twice given my presentation on an Affordable Strategy for Whole Home Backup Power. The first time was online using Zoom. The second time was live at the July meeting of the Norhill Neighborhood Association. Both went very well. But as ever, the presentation changed a little from one outing to the next. In particular, a Norhill resident had a comment about battery storage.

This inspired me to show a couple of slides I had originally skipped in the interest of time. Simply put, I’m not a fan of whole house battery backup. And I’d like to explain the logic my position.

For the love of sunshine

Our is a modest home. A hundred+ year old Craftsman Cottage built by William Wilson Realty Company. This kind of home has a roof that’s all angles. There’s just not much contiguous space upon which to locate solar panels.

Further, one of the truly great features of our older neighborhood are all the mature trees. The place is just rich with grand old Maples, Pecans, Oaks and Sycamores. Even the occasional Loblolly pine. Our property has a mature Pecan in the center of the back yard.

All these big ‘ole trees tend to shade single-story homes like ours. Those houses that do have solar installations are mostly two-story homes, that manage to peer out beyond the treetops.

So, we don’t have solar panels on the house. Many companies have tried to sell us on the idea, but in the end, it’s just not practical for this house, at this location. That makes this all about batteries, and batteries alone. Not solar+batteries.

What we know

It’s been three years since I installed the Shelly current monitoring device in our breaker panel. In that time we’ve come to have a quite a clear understanding of how much power we use.

If the weather is cooperating, and the outside temperature is close to room temperature for much the day, we might draw as little as 18 kWh. The following graph illustrates an example of when this occurred on March 30th. I’ve overlaid the outside temperature against the power drawn over the course of the day. Click to see a larger image.

In the peak heat of the summer things are quite different. The maximum power we’ve ever used in a 24 hour period is about 98 kWh. That implies the AC running continuously for much of the day. Again, click to see a larger image.

On such a day the outside temperature never falls to anything near room temperature. This day started at 85 F and peaked at about 105 F. The temperature spike at 6pm is when the sun directly shines on the outdoor sensor.

Even so, the house never uses more than 5 kW. That means that it’s well within scope for our Predator 9500 inverter.

EcoFlow

While this argument is broadly applicable, I’m going to pick on EcoFlow because I think their marketing sometimes makes claims their product cannot deliver. At least not in the context of our use case.

Let’s consider the top-of-the-line Delta Pro Ultra. It’s an expandable battery + inverter system. Each inverter can deliver 7.2 kW. Happily, it is capable of 240V power. They make specific claims about running “large appliances,” even central air conditioning. These claims are not unreasonable.

The simplest system is one inverter and one battery, providing 6 kWh of power for around $5,800. At the outset, this is more than we have spent on our existing inverter + inlet solution. But let’s set that aside for the moment.

Where their claims don’t match my expectation are runtimes. Even on a day when weather was very cooperative, this most basic system would only last us about 8 hours. On a hot day, only about an hour. To be practical, we’d need more battery storage.

The system is expandable. One inverter can have 5 batteries (pictured above) delivering 30 kWh for $18,000. That would be more practical, but still not see us through an entire summer day.

At its maximum dimensions, the system would have three inverters and fifteen batteries, capable of 96 kWh of storage. Giving it the benefit of the doubt, I expect that would carry us through a day-and-a-bit with loss of utility power. But to get there, we would have spent well over $50,000.

Backup Power vs UPS

Batteries do have their place. Back in 2007, I visited what was then a new facility built by Fox Networks (now Disney Digital Center 3) in The Woodlands, Texas. This facility was intended to be the uplink for all of their various network operations. It was vast, with space to provide not only all that Fox needed, but also space to be rented to numerous other broadcasters.

It did seem odd to me to have such a facility built here in Hurricane Alley. However, the relatively low cost of land and talented people sealed the deal.

Acknowledging the reality of their location, they had a very well considered backup power solution. In the parking lot outside the building there were three big diesel generators. Each was the size of a railroad locomotive. The trio of generators are serviced by a huge underground tank of diesel fuel.

Inside the building they had a large room filled with lead-acid batteries I wish I had been allowed to take a photo. Essentially, it was a large UPS (aka uninterruptible power supply.) The batteries could sustain the entire building for long enough to spin up and stabilize one or more of the big generators.

Hereabouts, I have a similar situation, just on a much smaller scale. Our network core is powered by an Eaton 9130 UPS. It can sustain attached loads for about 30 minutes. Long enough for me to roll out the generator and get it running. In truth, it mostly serves to allow the network to coast through minor outages, which are usually brief.

Both situations illustrate the fundamental difference between an UPS and backup power – runtime.

Days or weeks without utility power

There have been several times when our power went out for days at a time. Thirteen days after Hurricane Ike in 2008. Three-and-a-half days this year after Hurricane Beryl. Others here in Houston were without power for an entire week. That means even if we spend big $ on one of these battery systems, we’d still need to run a generator routinely to recharge the batteries.

Best case, we’d be able to recharge in the day, and not suffer the noise of the generator at night. That is, the noise of OUR generator. The neighbors would still be using theirs. And theirs is much noisier than our Predator inverter.

While I appreciate that these battery backup systems are interesting, they still fail to meet my personal standard for affordability. They simply cost too much for the runtime that would be provided.