What is startup vs. continuous power?

As you compare options for energy storage systems, there are a few key technical specifications to focus on and questions to answer. How much energy can the battery store? How much of your home can the battery power, and for how long? Can the battery keep powering some or all of your home if the grid is down? And can your battery provide a momentary burst of power large enough to start up your largest appliances, like your air conditioner?

Energy storage systems have both a power rating, expressed in kilowatts (kW), as well as a usable energy capacity rating, expressed in kilowatt-hours (kWh). One useful analogy you can use is to think of your battery like water running through a pipe: the usable energy capacity is the amount of water available to push through the pipe, while power is the size of the pipe itself.

Larger pipes allow more water to flow through at once, which depletes the water faster. Similarly, a battery with a high power rating can deliver more electricity at one time. Still, it will burn through its available energy capacity faster, too, if you use it at its maximum power rating.

A battery's power determines which and how many appliances you can run from the battery all at the same time. The most popular batteries today have a standard power rating of 5 kW: this is the same for both the LG Chem RESU 10H and the Tesla Powerwall 2, two of the most installed batteries in homes in the US. As a result, a power rating below 5 kW can reasonably be considered small, and a power rating above 5 kW can be considered above average.

When considering which battery is right for your home and usage patterns, it's useful to take a look at the power draw from the appliances you plan to run with your backup battery. For instance, a clothes dryer can consume upwards of 4 kW for the duration of the load of laundry you're drying. Your refrigerator, on the other hand, only uses around 200 Watts. Knowing what you want to power and for how long is the best way to size your storage system.

Importantly, it's worth noting that some batteries can be stacked together to increase their power output, while others only increase the amount of energy you can store. For instance, adding a second LG Chem RESU 10H in a standard configuration doesn't necessarily mean that now you have 10 kW of power; rather, you'd need an additional, separate inverter in order to increase the capacity output of the system as a whole. With other batteries, however, the power output scales as you install additional batteries: for instance, a system with two Enphase IQ batteries would give you 7.68 kW of power.

Not all types of power supply are created equal, nor are all types of power demand created equal. In your home, you have appliances and devices that require a constant amount of power to operate whenever they're plugged in or turned on, such as your refrigerator or wifi modem. Other appliances, however, require a surge of startup power to even turn on before running at a more constant power demand from then on, such as an HVAC system or a sump pump.

This is where the difference between peak and continuous power comes into play: peak power is the amount of power that a battery can push out over a very short period of time to allow you to turn on some of those more power-hungry appliances. If you live in a place that experiences the loss of power at the same time as flooding–for instance, in a hurricane-prone area–having the backup power to get your sump pump back up and running is essential. Additionally, if you plan to be able to run your HVAC unit off of your energy storage system, you'll need to have enough peak power output to cover the surge associated with turning on your AC.

After the initial surge, most of these power-hungry loads and appliances return to a level of power demand that easily falls within the bounds of a battery's continuous power. But do keep in mind that running your HVAC or dryer will deplete your stored energy faster than just keeping your lights, WiFi, and TV on.

Here's a comparison of the peak and continuous power ratings for some of the most popular batteries quoted on EnergySage: the top batteries for peak and continuous power in our list include the Blue Planet Energy Blue Ion 2.0, sonnen eco 10, and Generac PWRcell M6. To learn more about each of these batteries and more, visit the EnergySage Solar Battery Buyer's Guide.

Peak/continuous power for popular batteries

EnergySage lets you compare quotes from multiple different installers in your area for free, all from the comfort of your home. With so many different battery options available today, it's easy to work with installers to find the system that fits your needs. And if you have any questions about your solar or storage offers on EnergySage, our team of Energy Advisors are here to help with unbiased answers to all of your questions. To get started, register for an account on the EnergySage Marketplace today!