More than one (or larger) electrical energy storage battery?

There are various reasons to want a second battery, and a second battery is less helpful to some of those reasons than others, so there’s no simple answer. But if I had to make a general recommendation, it would be to get a second one because it’s an important factor in going all electric; that is replacing gas appliances, reducing greenhouse gas emissions, and contributing to the reversing of climate change.

Possible Reasons

First let’s tease apart the various reasons that you might want a second battery:

1. Reduce grid usage / Increase financial benefit:
   • Reduce purchased energy: A battery reduces grid usage by saving excess solar produced during the day and using it later. This means less energy pulled from the grid, which may mean saving money (excluding net metering plans, for example).
   • Avoid more expensive energy: Further, you can also choose when to use the battery. In places where using electricity during peak times costs more (usually ~6 PM-9 PM), the battery can help avoid using the grid when it costs the most.
   • Arbitrage: Some places may allow you to charge the battery from the grid when energy is plentiful and cheapest, and sell it back at a higher rate when it’s most needed. The utility is essentially compensating you for helping to shift load.
2. Environmental impact: A battery can reduce energy needs from the grid, which often reduces the amount of energy that needs to be produced from fossil fuels — reducing the carbon impact of your energy usage — especially during the evening when both demand is higher and there’s less clean energy available.
3. Battery draw: Batteries are limited in how much draw (wattage/amps) they can support. Not having enough batteries can mean that running too many things shuts the battery down, and/or that the grid must also be used even when the battery has sufficient energy.
4. Power outage and backed-up loads: A battery can allow more appliances to be backed up and/or for the backup to last longer.

As you may have noticed, battery draw and backed-up loads are closely related. Installations are likely to exclude loads from being backed-up if they’re likely to cause overload during an outage, which would cause the battery system to shut down (in order to prevent problems such as voltage drops that could damage the system or some appliances). High-draw loads likely to be excluded include car chargers, air conditioning, and electric ranges.

My Situation

#3: Power outage and backed-up loads ⇒ Not a significant factor… yet

My utility has been very reliable (maybe two outages that were just several hours each during the past five years), so this dimension isn’t very significant for me.

Further, for nearly half of the year, the sun can provide enough for the house and one battery is enough to get us through the night (see ‘Page 4: A look at the production and finances of my system’). So, a second battery would only give us a handful of extra hours for about half of the year.

Currently, all of our loads except the car charger are backed-up.

#1: Grid usage and financial benefit ⇒ Not a significant factor

I’m not on a time-of-use rate schedule, I can’t engage in arbitrage with my utility, and a second battery would only reduce grid usage a couple months of the year (since one battery is enough in the summer, and solar production isn’t enough in the winter to charge one battery, let alone two; see ‘A look at the production and finances of my Tesla Energy system’ for more details).

So, I would SWAG that a second battery could save me up to another hundred dollars or so a year, which would take dozens of years to pay for itself.

#2: Environment ⇒ Not a significant factor (at least not directly)

Like grid usage, a second battery would have a marginal effect on grid usage, and thus not provide much of a benefit to the environment (i.e., not significantly reduce the amount of fossil fuel burned for our energy usage).

#3 Battery draw ⇒ Not a problem… yet

As mentioned above, all of our loads except the car charger are backed-up. I’m personally fine with not being able to charge our car during outages. In the very unlikely case that there’s an extended outage and I need to charge the car, I could do so elsewhere. I might also have the option to charge the car by plugging it into one of our backed-up outlets if the sun is shining.

So, that’s why we only had one battery. However…

Electrification!

Our current major electrical loads are just our air conditioner and our car charger (the latter of which is not backed up).

For a variety of reasons — but primarily because of climate change — we are electrifying more of our home. This includes replacing the following gas appliances with electric ones:

• Water heater: 15-30A
• Range (stove and oven): 50A
• Clothes dryer: 15-30A
• Heater (furnace): 20-40A?

Running a combination of these (along with other appliances, such as the A/C or microwave) is likely to exceed the power output of our Powerwall. While the sun is up, solar and one battery may provide enough power. Otherwise (insufficient solar), we would need to pull from the grid.

This also means that at least some of these appliances might not be able to be on the backed-up load.

So, we decided to add a second battery primarily to ensure that we can continue to use these appliances only powered by battery during peak hours (for about 7-9 months out of the year). It’s just a bonus that a second battery will allow us to have more of these appliances available during a power outage.

Recommendation

It probably doesn’t make sense financially

The best case is where the second battery is charged 100% and 100% of it is used at night every day.

For a battery with a 13.5kWh capacity (Powerwall 3), and avoided cost of $0.20/kWh (a pretty high average cost given the point is to avoid grid usage and this higher-tier energy cost), and a rough cost of $10,000, it would take nearly 10 years to pay for itself.

But there are several factors that would detract from the best case:

• Backup reserve: Most people will not let the battery drain to 0% so that there’s capacity available in case of outage, which reduces the amount of energy used to avoid grid energy cost.
• Solar production: Most locations and systems are likely not able to produce enough energy to charge a second battery every day of the year.
• Usage: Any day of reduced usage (that doesn’t use both batteries) eats into potential savings.

It may be marginally useful for power outages

Additional loads that can be backed-up

Questions for you to consider:

• What loads would a second battery allow you to back up that can’t be with just one?
  • Is it worth the cost to be able to use those for short (several hour) outages?

Extended power outages

A second battery may not make a big difference in summer because nights are shorter, and a single battery may suffice.

A second battery may not make a big difference in winter because there may not be enough sun to power the day, let alone to charge two batteries for the night.

Do if for the the environment (and the grid)

Unfortunately, (too) many of us are still using gas appliances, in which case one Powerwall is likely to suffice (as it has for us for several years).

However, I believe that replacing our gas appliances with electric ones an essential part of combating climate change.

So, I would generally recommend considering getting a second Powerwall — or whatever other additional / higher-capacity battery(ies) — so that:

• You can power more of your appliances with just battery power in the evenings, and perhaps through the whole night, more often — for your benefit and the community’s.
• You can have more of your appliances backed-up (i.e., available to use during power outages).