Essentials or Whole-Home Backup? Sizing a Solar Battery

The size of a solar battery isn't really about the size of your house. It's about what you want to keep running when the power goes out, and for how long.

This article walks through that choice, the two ways to wire a battery, and how to estimate what you need before you talk to an installer. For whether a battery makes sense at all, start with our guide to solar batteries and backup power.

What do you want to keep running in an outage?

Every battery design starts here. Think back to the last time you lost power for more than a few hours. What did you miss first?

For most New England homes, the list looks something like this:

• The refrigerator and freezer
• The furnace fan or boiler, so the heat stays on
• A few lights
• The Wi-Fi router and modem
• Outlets for charging phones and laptops
• The well pump, if you have one
• Any medical equipment someone at home relies on

Then there's everything else: central air, an electric range, the dryer, an electric water heater, the car charger. Deciding which side of the line each one falls on is the biggest sizing decision you'll make.

It also helps to think about how long your outages usually last. A home that loses power for an evening a few times a year needs less than one on a rural circuit that goes dark for days after an ice storm. If someone works from home or relies on medical equipment, put those needs at the top of the list.

What is a critical loads panel?

There are two common ways to connect a battery. The difference is how much of the house it covers when the grid goes down.

Essentials backup. An electrician moves the circuits you chose into a separate panel, often called a critical loads panel or backed-up loads panel. In an outage, the battery powers only that panel. Everything else stays off, so the battery's energy goes to what matters most.

Whole-home backup. The battery connects so the entire house stays powered in an outage. It feels just like having the grid, but everything you turn on draws from the same battery, so it needs more capacity and power, and some discipline about big loads.

Either way, the installer looks at your main electrical panel. Our article on electrical panel upgrades for solar explains when an older panel needs work first.

Capacity or power: which one matters more?

Both, for different reasons:

• Capacity (kWh) is how much energy the battery stores. It decides how long your essentials run.
• Power (kW) is how much the battery can deliver at once. It decides what you can run at the same time.

Power is where motors cause trouble. A well pump, central air or a heat pump needs a burst of power as its motor starts. Virginia Cooperative Extension notes that a motor can draw about three times its running current while starting. If the battery can't deliver that surge, the motor won't start, even when the battery is nearly full.

That's why battery spec sheets list more than one power figure: a continuous rating for steady use, and a peak or load-start rating for a few seconds. Your installer compares those ratings with the motors in your home.

Capacity and power work together. A battery with plenty of stored energy but too little power can't start your well pump. One with lots of power but little storage can start it, then run out early in the night.

How can you estimate your essentials yourself?

You don't need a calculator app to get a useful first estimate. The method from Virginia Cooperative Extension works for any appliance:

1. Find the wattage. It's usually printed on a label on the back or bottom of the appliance, or on its nameplate.
2. Estimate the hours it runs in a day. Be honest about what you'd use in an outage.
3. Multiply and divide by 1,000. Wattage times hours, divided by 1,000, gives kilowatt-hours per day.
4. Add up your list. The total is roughly what your essentials use in a day.

A few tips keep the estimate realistic. A refrigerator cycles on and off, so the extension guide suggests counting about a third of the hours it's plugged in. A deep well pump can range from about 250 to 1,100 watts while running, and it needs more to start. Phone chargers, a router and LED lights add little.

Write down the motors on your list separately. They matter more for the power rating than for the daily total.

When do you need more than one battery?

Home batteries can usually be installed in groups. Adding a second battery adds both stored energy and power output, so the system can run longer and start bigger motors.

More than one battery is worth a look if:

• You want whole-home backup rather than essentials.
• Your list includes a well pump plus central air or a heat pump.
• You lose power for days at a time and want more cushion between sunny days.
• You plan to add an electric car or heat pump in the next few years.

It's also why the first decision matters so much. Moving the dryer and the electric range off your list can be the difference between one battery and two.

Extra batteries also need space. Your installer checks where they can go, usually a garage, basement or outside wall, and how they connect to your panel.

How does an installer size your battery?

A careful installer works from your real home, not a rule of thumb:

• Your electric bill. A year of usage shows how much your home uses and when.
• Your circuits. The installer checks your main panel and confirms which circuits you want backed up.
• Your motors. Well pumps, air conditioners and heat pumps set the power the battery needs.
• Your plans. An electric car or heat pump changes what a battery has to handle. Our article on solar in an all-electric home covers that shift.

Most of this happens at the consultation and site visit. The design you get should list the battery's capacity, its power output and exactly which circuits are covered.

How does battery size affect program payments?

Battery programs pay for what your battery can share with the grid, so size plays a part. In Massachusetts, ConnectedSolutions pays $275 per kW of the battery's average contribution during summer events, which last up to 3 hours. A battery with more power and capacity can contribute more kW through an event.

In Connecticut, Energy Storage Solutions pays an enrollment incentive of $30 per kWh of rated capacity for new enrollments from April 1, 2026 ($130 per kWh on the most outage-prone circuits), plus performance payments of $300 to $550 per kW-year for 10 years.

Joining a program doesn't mean giving up backup. Events happen on summer afternoons, and the utilities try to avoid them when big storms are forecast. Ask your installer how much charge the battery keeps for backup during events. Programs shouldn't drive the size on their own. Size for backup first, then let the program payments help cover the cost. Questions about your home? Call Evergreen Solar at (508) 507-2908.

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