How to Size Home Battery Backup for Just Your Essential Circuits

Why size for essential circuits instead of the whole home

A battery has a fixed amount of stored energy. The fewer loads it feeds, the longer it lasts. Large loads like electric ranges, dryers, electric water heaters, pool pumps and central air conditioning can drain a battery quickly, so many homeowners leave them off the backed-up panel.

A backed-up essential-loads setup typically uses a dedicated subpanel or a battery system that can isolate selected circuits. Your electrician decides the best layout based on your existing panel, your wiring and local requirements.

The tradeoff is simple: whole-home backup offers convenience, while essential-circuit backup offers longer runtime per battery and often a smaller system.

Step 1: Choose which circuits are truly essential

Walk through your home and decide what you cannot go without during an outage. Common picks include:

Then decide on the borderline items: a single mini-split, a bedroom window unit, or one 240V circuit. Each borderline item changes the size of the system noticeably, so list them separately.

• Refrigerator and chest freezer
• Kitchen outlets for small appliances
• Lighting in main living areas, hallways and bedrooms
• Internet router, modem and a few outlets for phones and laptops
• Furnace blower or air handler fan (gas or propane heat still needs electricity to run)
• Well pump or sump pump, if you have one
• Medical equipment circuits, if anyone in the home relies on powered devices
• Garage door opener

Step 2: Find the wattage of each load

Check the nameplate or label on each appliance, which lists watts or amps and volts (watts = volts x amps). Your electrician can also measure real draw. Typical ranges are rough guides only, since models vary widely:

For motors (refrigerator compressors, well pumps, sump pumps, air handlers), note both the running watts and the startup, or surge, watts. The surge number matters for the power side of your sizing.

• Modern refrigerators draw a modest amount while running, but cycle on and off and surge briefly at startup
• LED lighting draws very little per fixture
• Routers and modems draw a small, constant amount
• Furnace blowers and well pumps draw far more and have a larger startup surge
• Central air conditioners and electric heat draw thousands of watts

Step 3: Calculate daily energy use in kWh

Energy is power multiplied by hours of use. For each load, estimate the watts and how many hours per day it actually runs, then divide by 1,000 to get kilowatt-hours.

Remember that cycling appliances do not run constantly. A refrigerator that is plugged in 24 hours may only run its compressor for part of that time.

These are made-up round numbers for demonstration, not measurements from your home:

That totals roughly 6 kWh per day. Tesla's Powerwall is commonly specified at 13.5 kWh of usable energy per unit, so one battery in this example would cover a bit more than two days with no sun and no grid. Confirm current specifications for the model being installed.

If you also add a mini-split running several hours a day, the daily total rises and the runtime falls, which is why borderline loads should be evaluated one at a time.

• Refrigerator and freezer: about 2 kWh per day
• Lighting: about 0.5 kWh per day
• Router, phone and laptop charging: about 1 kWh per day
• Furnace blower: about 2 kWh per day
• Well pump: about 0.5 kWh per day

Step 4: Check your peak power in kW

Energy tells you how long the battery lasts. Power tells you whether it can run everything at once. Add the running watts of everything that could be on at the same moment, then add the extra startup surge of the largest motor, since motors rarely all start simultaneously.

Compare that number to the continuous power rating of the battery system. Published Powerwall 2 and Powerwall 3 ratings differ noticeably, so the model matters. If your peak demand exceeds one unit's output, the usual options are:

A well pump is a classic example. It might use little energy overall but have a startup surge that stresses a smaller system.

• Add a second battery to increase power and energy
• Move a heavy load, such as a well pump or an air handler, off the backed-up panel
• Use load management, where certain circuits are shed when demand is high

Step 5: Decide how long you want to ride out an outage

Pick a realistic target. Some homeowners want overnight coverage, others want two or three days. Multiply your daily kWh by the number of days, then compare it to usable battery capacity.

Without solar: the battery is a fixed reserve. Once it is empty, it stays empty until grid power returns.

With solar: the panels can recharge the battery during daylight if the system is configured for backup operation. Runtime can be extended well beyond the battery alone, depending on weather, season and how much you consume. Note that solar without a battery typically shuts down during outages for safety reasons.

Also leave margin. Cold weather, cloudy stretches, a larger-than-expected furnace load, or guests staying with you can all increase consumption.

Step 6: Plan for the electrical side

Sizing is not only about the battery. A licensed electrician will evaluate:

These details can influence which circuits are practical to back up and what the project costs. Pricing varies by panel condition, wiring distance, battery count and local permitting, so a real quote requires a site visit.

• Your main panel's capacity and available breaker space
• Whether a dedicated backup subpanel is needed and where it should go
• Wire runs between the panel, the battery and any solar equipment
• Utility interconnection and permit requirements in your area
• Placement, such as garage or exterior wall, based on manufacturer and local guidelines

Common sizing mistakes

• Sizing only on kWh. A battery with plenty of energy can still trip off if the loads exceed its power rating.
• Ignoring motor surge. Pumps and compressors briefly draw several times their running wattage.
• Forgetting heat or cooling fans. Even gas furnaces need power for the blower and controls.
• Backing up too many circuits. Every added circuit shortens runtime.
• Planning for today only. If you may add an EV charger, heat pump or more solar later, mention it so the system can be designed with room to grow.

Get the numbers checked by Tesla-certified electricians

A spreadsheet gets you close, but a site visit gets you accurate. Charge Home Solutions works through a nationwide network of Tesla-certified, licensed and insured electricians who can walk your panel, identify the right essential circuits, and recommend a battery configuration that fits your backup goals. Estimates are free and in-home, and we match you with a local installer quickly.

Book your free in-home estimate

Ready to move forward? Charge Home Solutions matches you with a licensed, insured, Tesla-certified electrician in your area, with a free in-home estimate and no obligation.

Request your free estimate at chargehomesolutions.com/request-service or call 888-995-6044.