How Much Battery Backup Do You Actually Need? A Load-by-Load Guide

The number one mistake homeowners make when sizing a battery backup system is guessing. They hear "one Powerwall" and assume that covers the house. It does not — a single 13.5 kWh battery running central air conditioning will be drained in four to six hours. Sizing a battery properly requires knowing your actual loads.

Start with priority loads — what you must keep running during an outage. For most households this is: refrigerator (150W continuous), key lights (10–15 LEDs at 10W each = 100–150W), phone and laptop charging (100W), internet router (15W), and any medical equipment. This essential tier typically draws 400–600W continuously and uses about 10–14 kWh per day — well within one Powerwall.

The second tier is comfort loads that you want but can manage without in a true emergency: central AC or heat pump (3,000–5,000W — by far the largest draw), electric range (5,000–8,000W), electric water heater (4,000W), clothes dryer (5,000W). Running central AC continuously is the single biggest variable in battery sizing. In a hot climate, AC alone can drain a 13.5 kWh battery in 4–6 hours.

Here is the sizing math: add up the watts of everything you want to run simultaneously (your "peak draw"), then estimate daily kWh by multiplying each load by hours used per day. Add 15% for battery efficiency losses. Divide by 0.90 if you want to preserve 10% minimum charge for battery longevity. The result is your required kWh.

Example for a 2,500 sq ft home in Texas wanting 24 hours of backup with AC: Fridge (3.6 kWh) + AC 8 hrs (24 kWh) + lights/plugs (4 kWh) + router/TV (1.5 kWh) = 33 kWh before efficiency. Adding 15% buffer = 38 kWh. That is nearly three Powerwall 3 units or three Franklin aPower units. This surprises most homeowners who assumed one or two would do it.

The alternative approach: drop AC from backup and your math changes completely. Without AC: Fridge + lights + plugs + router = roughly 10–12 kWh per day. One Powerwall comfortably covers 24+ hours of comfort living without climate control. Many solar+battery owners take this approach — pre-cool the house to 70°F before a storm, then coast on thermal mass for 6–8 hours without running AC at all.

Use our Backup Load Sizer to enter your specific appliances, quantities, and daily hours. It shows you exactly how many kWh you need and what runtime you get from 1, 2, or 3 batteries — so you can make an informed decision rather than trusting an installer who may be incentivized to sell you more hardware than you need.