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Lumin now integrates with all Tesla Powerwall batteries -- including Powerwall 3 -- to read and respond to state of charge. Download the Lumin app to get started.

Lumin now integrates with all Tesla Powerwall batteries -- including Powerwall 3 -- to read and respond to state of charge. Download the Lumin app to get started.

How Energy Management Completes the Solar + Storage Package

Published at 17 Oct 2019
How Energy Management Completes the Solar + Storage Package

What are kW and kWh rates? The Limitations of Solar + Storage The Solution: Energy Management

In my last post, I highlighted a few reasons why energy storage is critical to the future of solar PV, and how each technology complements the other. While combining solar PV with energy storage creates a powerful, non-zero sum ecosystem, there are a few limitations of the resulting system that are best addressed by energy management technology.

In much the same way that energy storage is key to the future of solar PV, energy management is key to accelerating the future of energy storage.

What are kW and kWh rates?

To understand these limitations, it's best to start with the kW/kWh rating of the energy storage system. Every energy storage system has two important numbers associated with it - a kilowatt (kW) value and a kilowatt-hour (kWh) value. To keep things simple, think about it like this: the kW rating indicates how much power the storage system can push continuously, and the kWh rating tells you how much energy can be stored in the battery. So, a battery with a 5kW / 13.5kWh rating can push up to 5kW of power, and can sustain that pace for a little under 3 hours (13.5kWh / 5kW = 2.7 hours). This does not mean that the battery can only operate for 2 hours and 42 minutes - that's only the case if you are consuming the maximum amount of power that entire time. If you dialed back to a more conservative 1kW power demand, the same battery could last 13.5 hours.

The Limitations of Solar + Storage

1. kW Rating

Back to the limitations of solar + storage: the first limitation has to do with the kW rating - how much continuous power the energy storage system can push back into the home. Unlike the electricity grid, which can push as much power as a household could reasonably demand, energy storage systems cannot continuously push more power than their rated limit. That isn't a problem until it's a problem. To use an example, let's take the case of a home operating on battery backup power during a power outage. As long as consumption stays at a low level, the system will operate normally. If consumption were to spike above 5kW, however, the storage system would cease to operate and the house would effectively lose power again (even with solar + storage!). Since it's really hard to know how many kilowatts your house is currently demanding from your energy storage system, and because there's no point investing in an energy storage system if it is going to shut down during every grid outage, installers typically do one of two things: (1) offer to add multiple storage systems to increase that limit to the point where the system can power the whole home, or (2) install a backup loads panel, which has loads that have been pre-screened such that if they all turned on simultaneously, their total power demand would be less than the battery's discharge limit.

While both options have their pros and cons, they each leave something to be desired. The first option may end up providing whole home backup, but at a cost that is overbudget for most of the market. The second option can be frustrating for homeowners who didn't expect their hefty investment in solar + storage to result in just a handful of loads being backed up, and involves the added installation headache of wiring a new backup loads panel. Enter energy management. At risk of oversimplifying a complex topic, let it suffice to say that for the purpose of this illustration, energy management involves the measurement and coordinated control of electricity. Energy management gives the installer a third option: (3) back up the entire house with one or two storage systems, and use an energy management system to ensure that household power demand never exceeds the storage system's limit. With energy management systems in hand, installers can achieve the best of both worlds - protecting the storage system from excess power demand, while providing flexible backup to their customers, without the need for backup loads panels.

2. kWh Limit

The second limitation of energy storage systems pertains to the kWh limit, or the amount of energy that can be stored. During a grid outage, when a home is operating solely on battery power, it is critical that the power is not wasted or spent too quickly. The problem is, it's quite difficult to keep tabs on how much energy your home is consuming, how much energy is left stored in the battery, and what that means for your estimated off-grid runtime. This challenge can often be frustrating to navigate, and commonly ends in one of two ways: either (1) the available energy is depleted too quickly and the home runs out of power again, or (2) energy consumption is conserved too aggressively, and lots of stored energy is left unused at the end of the outage. With a reliable energy management tool in place, this friction can be avoided with the help of pre-programmed or automated settings that ensure that the level of energy consumption corresponds directly to the expected duration of the outage and other user preferences. This takes the guesswork out of budgeting backup power during a grid outage, and decreases the level of interruption caused by the grid outage.

The Solution: Energy Management

While energy storage is critical to the continued growth of solar PV, it is accompanied by some frustrating limitations. With a proper energy management technology, these frustrations can be erased. In much the same way that energy storage is key to the future of solar PV, energy management is key to accelerating the future of energy storage.

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