Most homes need 10–20 kWh to keep essentials running through a typical outage. Whole-home backup is more involved. Your exact figure comes down to two things: what you want to power and how long the outage lasts. I’ve sized plenty of these systems, so let me give you a method you can use tonight with your latest power bill.
Why Getting Your Backup Capacity Right Matters
This decision has real money and real comfort riding on it. Undersize the battery and you’re in the dark before the grid comes back, with your fridge warming and your medical devices off. Oversize it, and you’ve spent thousands on what you’ll never touch. Getting the number right also protects the battery itself, because a system sized with a little headroom avoids the deep, repeated discharges that shorten its life. So this is worth calculating before you spend.
How to Calculate Whole Home Backup Capacity
My method takes two steps. Grab a recent utility bill and a list of the appliances that matter to you, and you’ll have your target by the end of this section.
Step 1: Decide What You Need to Keep Running
First, make a clear list of what must stay on during an outage. For essentials-only backup, that usually means the fridge, freezer, a few lights, Wi-Fi, phone chargers, and any medical devices. For whole-home backup, add larger loads like central AC, heat pumps, well pumps, and electric water heaters. Put each appliance into one of two groups: “must run” or “can wait.” Dryers, pool pumps, dishwashers, and EV charging can usually wait. The fewer loads you include, the less battery capacity you need.
Step 2: Use the Sizing Formula
Use this formula to estimate the battery capacity you need:
Average load in watts × Backup hours × 1.2 = Required capacity (Wh)
The 1.2 adds about 20% extra capacity for inverter losses and depth of discharge. Batteries should not be drained to zero every time, so this margin helps protect the system. For example, if your must-run loads average 1,500 watts and you want 24 hours of backup:
1,500 W × 24 h × 1.2 = 43,200 Wh, or about 43 kWh.
Other Key Factors That Affect Your Needs
Inductive Loads
Inductive loads deserve special attention. Central AC units and heat pumps can draw several times their running power at the moment they start up. If you plan to back them up, check whether the system’s output can absorb that startup surge without tripping. A system with sufficient rated (continuous) output but limited peak output may fail to carry a whole home once inductive loads kick in.
Pairing With Solar and Generator to Recharge Daily
Solar and a diesel generator can make a backup system last much longer. Without them, the battery is limited to the energy it already has stored. With solar, panels can recharge the battery during the day, so a 10–15 kWh system can support essential loads through a longer outage when sunlight is available.
It is important to know how much power you can safely use. Many homeowners are not sure whether they should run the high-draw appliances or save power for the night. With the ESYsunhome app, you can check battery state-of-charge and solar input in real time, making it easier to manage usage and avoid draining the battery too quickly.
Scale Your Capacity With a Modular All-in-One ESS
Right-Size Now, Expand Later (5–30 kWh / 10–90 kWh)
I see a lot of people size a battery for today, it feels like plenty, then a year later they add an EV or a heat pump, and it’s suddenly too small. Worse, adding more often means ripping out the whole system and starting over. So my recommendation is to choose a modular design. Install what your numbers call for now, and stack on more when your needs grow. That’s why our All-in-One residential energy storage systems run from 5 kWh to 30 kWh for single-phase systems and 10-90 kWh for three-phase ESS, so you can size right today and expand the same system later, without starting from scratch.
ESYsunhome HM Series for Whole-Home Backup
Once you’re backing up a whole house with HVAC, capacity alone won’t cut it. You also need enough continuous power to start and run heavy loads like central AC and heat pumps, which can surge hard the moment they kick on. That’s where our HM series is built to deliver. The three-phase HM10-H puts out up to 10 kW of continuous power and scales to 90 kWh of storage and is compatible with diesel generator, so it handles the surge from your compressors while carrying enough energy to ride out a long outage, and it grows with you as your needs change. If you are not sure which model fits your home, our home energy solutions overview can help you match capacity and output to your goal.
Conclusion
Sizing whole-home battery backup comes down to a simple chain: pick your loads, measure output power, multiply by your target hours, and add 20% headroom. Essentials usually land at 10–20 kWh, full-home coverage at a higher range, and solar lets you do more with less. Run your own numbers from your latest bill, then pick a modular system you can expand as your needs grow, so you’re ready for the outage that actually matters.
FAQs
What is the difference between kW and kWh?
kWh is capacity, and kW is output. Capacity shows how long the battery can run, while output shows how many appliances it can power at once.
Can a home battery run central AC?
Yes, if it has enough output. Central AC needs high startup power, so check both continuous output and peak output before adding it to your backup loads.
Do I need a transfer switch for home battery backup?
Yes, most hardwired systems need one. A transfer switch or backup gateway safely separates your home from the grid and sends battery power to selected circuits. ESYsunhome HM-series ESS has already integrated the transfer switch.
Will solar panels work during a power outage?
Only if your system supports backup operation. Many grid-tied solar systems shut down during outages unless paired with a compatible battery and backup inverter.
Should I back up the whole house or only critical circuits?
Critical circuits are usually more practical. They use less power, extend battery runtime, and still keep the most important appliances running.