The Future of Energy Storage

Batteries are easily the greatest solar add-on item of all time, adding thousands or tens of thousands of dollars to a standard solar sale. While this explains why nearly every installer will include and recommend getting a battery, it does not answer the question of whether a homeowner should buy one. Although even asking that question is heresy in the solar world (especially for a Certified Powerwall and Certified Enphase Battery installer like ourselves), we will tackle it nonetheless by establishing batteries’

Financial benefits
Non-financial benefits
Current and future alternatives

The typical financial benefits for batteries are the avoidance of reduced feed-in tariffs and the avoidance of high-demand charges. For example, a system owner who pays 12¢/kWh for electricity but only receives 5¢/kWh when exporting solar power to the grid saves 7¢ for every kWh that the battery prevents exporting. Similarly, a homeowner that pays 25¢/kWh from 1:00-8:00pm, but only 10¢/kWh after 8:00pm can save 15¢ for every kWh that the battery can help defer usage of until 8:00pm.

Battery Savings From Reduced Import Under A Reduced Feed-In Tariff Scheme — Figure 1: Battery Savings from reduced import under a Reduced Feed-in Tariff Scheme

Because we have net metering in our region (as opposed to reduced feed-in tariffs) and do not have mandatory time-of-use rates, batteries do not provide any financial benefits in our region. In fact, because batteries lose electricity in the process of storing and returning energy, systems like the Tesla Powerwall II and Enphase IQ 10 which have roundtrip efficiencies of ~93%, will effectively reduce the production of the solar system.

Battery Savings From Optimal Import Timing Under A Time-Of-Use Rate Scheme — Figure 2: Battery Savings from optimal import timing under a Time-of-Use Rate Scheme

The other benefit of batteries which is awesome and undeniable is the ability to provide electricity when the grid is down. Snow, wind, and accidents can all cause the grid to go down, sometimes for days at a time. Not having to suffer through days of cold, dark boredom with a flooded basement and a fridge full of spoiled food is a fantastic luxury.

The capabilities of batteries in a grid-out situation boil down to two factors: what they can power (kW) and for how long (kWh). As an example, the Tesla Powerwall II has a 7kW peak output and 14kWh of energy capacity. For context, the average household in the US has a 48kW service size and uses 29kWh/day. Thus, a Powerwall II can power around 15% of a home’s load and has the energy to last around half a day, longer if sunny weather is available to recharge the battery. Admittedly though, outages are seldom accompanied by nice, sunny weather.

Price And Output Comparisons For Popular Backup Power Products — Figure 3: Price and output comparisons for popular backup power products

Manufacturers like Generac (Pwrcell), Tesla (Powerwall II), and Enphase (IQ Battery) disclaim against the use of their batteries to power devices like heat pumps or well pumps which usually have inrush amperages that are beyond what the batteries can handle. The power capability of a battery is like a small portable generator that is best used to run space heater, a fridge, and a few lights. Increasing to three or four batteries can create an output closer to that of a whole-house generator, albeit at a significant price premium (see Figure 3).

The frequent sticker shock homeowners experience when learning of the price of battery systems has led some inverter manufacturers to reach for alternatives. Enphase recently introduced a sunlight backup system that enables a home to be powered directly from its solar system when the grid is down, without the use of batteries. While this does eliminate the cost of the battery, it still requires the installation of proprietary transfer switches and combiners, specialized inverters, and circuitry rewiring that typically cost between $5,000-$7000. This is not to mention that Enphase limits the system to powering no more than eight (8) items, all of which will be subject to the on-again-off-again activity that is inherent with using an unstable power source.

The thought of spending tens of thousands of dollars on battery system that can’t power the air conditioner and might run out of juice if it’s too cloudy outside is not a pleasant one, nor is the idea of running outside in a thunderstorm to fill your noisy generator up with gasoline, even if it does come with a much lower price tag. Worse still, these systems are almost never designed to be compatible with one another, which means your solar will probably not work when your generator is running. Enphase and Generac are taking steps to enhance generator/battery capability, but it may prove to be too little, too late as a third option seems set to take over: EV backup.

EV backup, also known as bidirectional charging or vehicle-to-home (V2H), is quickly becoming the obvious solution for those interested in having backup power for their homes. As vehicle owners move towards electric vehicles that are cleaner, more reliable, and less expensive to operate, charging stations are being installed in homes across the US. EVs have batteries that dwarf even the largest home battery systems and charging stations designed for high power flow.

Take the F150 Lightning for example. The extended-range version of Ford’s electric truck has a 131kWh battery, over seven times bigger than the largest home battery currently available, the Generac PWRcell M6. That’s enough energy to power an average home running at full tilt with no solar generation for over four days. Its power output is 17.6kW, over three times the Enphase IQ Battery 10. Even in this technology’s infant stage, the figures are approaching a level where a homeowner could live unaffected: doing laundry, baking, running the dishwasher, etc.

Figure 4: Energy Capacity Of Popular Batteries And Evs With Backup Power Functionality — Figure 4: Energy capacity of popular batteries and EVs with backup power functionality

Additionally, EVs have the ability to “wake up” (microgrid in PV jargon) a solar system, unlike generators which do not have sufficiently clean power output (stable frequency and voltage) to activate solar inverters. In general, the software-based foundation of EVs makes them inherently more capable than hardware-based generator systems. So even vehicles without EV backup or ones already on the road have the ability for that feature to be added in the future.

The end result is a backup power system that is more powerful and longer lasting than a battery, quieter than a generator, fuels itself from the sun, and does all this with an item that has already been purchased. And while generator and battery manufacturers may currently overplay the benefits of their backup systems, they are also busy preparing for an EV-backup-dominated future with Generac purchasing microinverter manufacturer Chilicon Power last summer and Enphase completing their acquisition of EV Charger manufacturer ClipperCreek earlier this year., with their press release boasting “Accelerates the Enphase roadmap to enable bi-directional charging capability”

So what’s the recommendation for who want backup power now and aren’t interested in an F150 Lightning or waiting around for the widespread adoption of bidirectional chargers? That’s a question with far too many individual considerations about finances, hotel proximity, willingness to crank up a generator in the rain, and love/fear of being stuck playing board games under candlelight for us to dare attempt to answer. Regardless of what route you choose, knowing what’s around the corner can only aid in making a more-informed decision now.

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