Battery & storage

Whole home backup sizing calculator

Check off the major loads in your home, including HVAC and appliances. Get a realistic battery count for backing up everything at once.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your whole home loads
3 Backup duration
Whole home loads are power-hungry, so most systems target shorter windows than critical loads only.

How it works

Three steps to whole home sizing

Step 1

Check off every major load

Whole home backup means everything, including HVAC and laundry. If you'd rather trim this list down, the critical loads calculator shows what a smaller, cheaper system looks like.

Step 2

Pick a realistic duration

Whole home loads draw enough power that most systems target shorter backup windows than critical loads only setups. The 24 hour backup tool shows what a full day of whole home coverage would take.

Step 3

Plan for enough inverter capacity

Multiple batteries need an inverter rated to handle their combined output, especially with HVAC startup surge in the mix. The battery inverter pairing tool checks this specifically.

Understanding whole home backup sizing

Why this number is an estimate, not a quote

This calculator uses typical running wattages for common whole-home loads, not your specific equipment or its startup surge. HVAC compressors in particular can briefly draw 2 to 3 times their running wattage on startup, which affects inverter sizing more than total battery capacity. Use this number to understand the scale of whole home backup, then confirm exact sizing with an installer. The HVAC backup tool looks at that single load in more detail.

Why HVAC changes everything about this calculation

Central air conditioning alone can draw 3,000 to 5,000 watts while running, often more than every other household circuit combined. Removing HVAC from a whole home load list frequently cuts the required battery count by more than half, which is why some homeowners choose to back up everything except climate control, consistent with load-breakdown data from the Department of Energy's residential HVAC guidance. The peak shaving calculator covers a related strategy for managing high-draw loads without full backup.

How whole home backup compares in cost to critical loads only

Since batteries are typically the single largest cost in a backup system, going from 1 to 6 units multiplies the battery portion of your budget by roughly the same factor. Many installers recommend starting with critical loads only and treating whole home backup as a later upgrade if budget is a concern, a phased approach the quick financing comparison tool in our Financing category can help plan around. The cost per kWh comparison tool and single vs multi battery cost tool both help quantify this tradeoff.

What climate has to do with this decision

In hot, humid climates, losing air conditioning for multiple days during a summer outage carries real health risks for vulnerable household members, a risk Ready.gov's power outage preparedness guidance specifically flags for older adults and those with health conditions, which can justify the added cost of whole home backup. In milder climates, that same risk is much lower, and critical loads only often covers the actual need at a fraction of the price. The storm season planning tool factors regional risk into backup planning more directly.

What to do with this number next

Once you know your whole home battery count, check the battery inverter pairing tool to confirm your inverter can handle it, or the partial home backup tool if the full number feels like more than you need. The inverter needed to support a whole home load list is covered in the hybrid inverter sizing calculator in our Inverters & Equipment category.

Questions

Frequently asked

A typical whole home load with HVAC, a well pump, major appliances, and everyday circuits runs around 7.8 kW, which needs about 6 batteries at 13.5 kWh each for just 8 hours of coverage. This is why most whole home backup systems cost significantly more than critical loads only systems.

Central air conditioning alone can draw 3,000 to 5,000 watts while running, often more than every other household circuit combined. This single load is usually the deciding factor in whether whole home backup needs 2 batteries or 6.

It depends on your climate and tolerance for discomfort during outages. In hot climates where losing air conditioning for days is a real health risk, whole home backup can be worth the added cost, while in milder climates critical loads only usually covers the actual risk at a fraction of the price.

Yes, many installers design the system with this upgrade path in mind, leaving room in the electrical panel and inverter capacity for additional battery units later. The battery expansion planning tool walks through what that upgrade typically involves.

Related tools

Keep sizing your backup

Critical loads only sizing HVAC backup Battery inverter pairing Battery expansion planning Partial home backup Peak shaving battery size Single vs multi battery cost All battery & storage tools