Understanding critical loads sizing
Why this number is an estimate, not a quote
This calculator uses typical running wattages for common critical loads, not your specific appliance models or their startup surge. Real critical loads panels also depend on which circuits your home's wiring makes easy or hard to isolate. Use this number to start a conversation with an installer, not as a final subpanel design. If you want to check a single appliance in more detail, the refrigerator backup tool and medical equipment tool both dig into individual loads.
Why critical loads only saves real money
A critical loads only system frequently needs a single battery under 15 kWh, while whole home backup covering central air conditioning often needs 2 to 3 batteries stacked together. Since batteries are the single largest line item in most backup systems, narrowing your load list down to genuine essentials can cut total system cost substantially, consistent with cost breakdowns published by NREL's residential storage cost benchmarking. The cost per kWh comparison tool shows what that difference looks like in dollars.
What usually makes the critical loads list and what doesn't
Refrigerators, some lighting circuits, internet and networking gear, and any medical or safety devices are the most common inclusions, since losing any of these during an outage causes real problems within hours. Full kitchens, laundry machines, and central HVAC usually get left off a critical loads panel specifically because they draw enough power to force a much larger and more expensive battery system. The HVAC backup tool covers what it takes to add climate control back into the mix.
Why the subpanel matters as much as the battery
A battery and inverter can only back up the circuits that are physically wired into a dedicated critical loads subpanel, a distinction the Department of Energy's inverter basics guide covers as part of how backup-capable systems are wired. This is a licensed electrical job separate from the battery installation itself, and it is what makes selective backup possible instead of an all-or-nothing whole panel transfer switch. The hybrid inverter sizing calculator in our Inverters & Equipment category covers the inverter side of this same subpanel setup. Confirm subpanel scope with your installer before finalizing a battery size.
What to do with this number next
Once you know your critical loads battery size, check the solar recharge sizing tool to see how solar changes your outage math, or the battery expansion planning tool if you might add more circuits down the line. If financing this smaller system is the goal, the quick financing comparison tool in our Financing category gives a fast cost overview.