Formula Used
Running watts = appliance watts × quantity
Average watts = running watts × duty cycle
Load energy = average watts × use hours
Total outage energy = load energy + inverter idle watts × outage hours
Nominal battery Wh = battery voltage × amp hours × battery count
Usable battery Wh = nominal Wh × DoD × battery efficiency × inverter efficiency × reserve factor
Runtime = usable battery Wh ÷ average load watts
Solar assist Wh = solar watts × useful solar hours × solar efficiency × inverter efficiency
Required Ah = net required Wh ÷ adjusted battery voltage factors
How To Use This Calculator
Enter the battery bank voltage, amp hours, and battery count.
Add depth of discharge, efficiency, reserve, and inverter ratings.
List each appliance that should run during an outage.
Use surge watts for motors, pumps, compressors, and refrigerators.
Set duty cycle for loads that cycle on and off.
Add solar input when panels can recharge during the outage.
Press calculate to view runtime, storage, inverter checks, and costs.
Download the CSV or PDF report after calculation.
Home Battery Planning Guide
A home battery is sized by energy, not only watts. Watts describe the live load. Watt hours describe how long that load must run. Good planning starts with essential circuits. Choose the refrigerator, router, lights, medical devices, pumps, and controls first. Large heaters, ovens, and dryers usually need a much larger system.
Why Usable Capacity Matters
The label on a battery shows nominal storage. You cannot use all of it safely. Lithium and lead acid batteries have recommended depth limits. Inverters also waste energy while changing DC power into household AC power. A reserve margin protects the bank from deep discharge. The calculator combines these losses before it reports runtime.
Load Timing Changes The Result
Many appliances do not run every minute. A refrigerator cycles. A sump pump may run only during heavy rain. Lights may be used for part of the outage. Duty cycle and use hours make the estimate closer to real life. Enter realistic values instead of nameplate watts for every hour.
Inverter Checks
The inverter must handle continuous load and short starting surge. Motors often need a brief surge when they start. Refrigerators, pumps, and compressors can exceed their normal watt rating. The surge check helps you see whether the inverter is too small, even when the battery capacity looks adequate.
Solar Support
Solar input can extend runtime during daytime outages. The estimate uses array watts, useful charging hours, and charging efficiency. Clouds, shade, battery limits, and seasonal sun can reduce output. Treat solar support as a planning allowance, not a guaranteed promise.
Cost And Safety Notes
Costs depend on battery chemistry, wiring, protection, permits, and installation labor. Cheap capacity can become expensive if it lacks monitoring or proper protection. Use listed equipment where required. Match cable size to current. Add fuses and breakers as specified. Keep indoor batteries in approved locations.
Practical Sizing Tips
Build a list of must-have devices first. Then add comfort loads only if the runtime remains strong. Keep a reserve for unexpected cycling. Check breaker ratings, transfer equipment, ventilation, and local electrical rules. A licensed electrician should review permanent home backup installations. Repeat the estimate after each new appliance change or battery upgrade for better planning accuracy.
FAQs
What size battery do I need for home backup?
Add the watt hours needed by your essential loads. Then divide by usable battery capacity after discharge limit, efficiency loss, and reserve margin. The calculator performs these steps for each appliance.
Why is usable capacity lower than battery label capacity?
The label shows nominal storage. Real backup power is reduced by depth of discharge limits, battery efficiency, inverter losses, and reserve margin. These factors protect the system and improve planning accuracy.
Should I include surge watts?
Yes. Motors, pumps, compressors, and refrigerators may need a short starting surge. Your inverter must handle that surge, even if the average battery energy looks acceptable.
Can solar panels extend battery runtime?
Yes, solar can add useful watt hours during daylight. Actual output depends on weather, shade, panel angle, battery limits, and charge equipment. Use conservative values for outage planning.
What is inverter idle load?
Idle load is the power an inverter consumes while it is on. It can matter during long outages. Add it because it slowly drains the battery even when appliances are light.
What duty cycle should I enter?
Duty cycle is the percentage of time a load actively runs. Use 100 percent for constant loads. Use lower values for cycling equipment like refrigerators, pumps, or controlled fans.
Is this calculator suitable for whole-house backup?
It can estimate whole-house backup if all loads are entered. Large appliances may require major inverter and battery capacity. Permanent systems should be reviewed by a licensed electrician.
Why does recommended battery count change with reserve?
A higher reserve leaves more stored energy unused. That reduces available watt hours. More batteries may be required to keep the same outage runtime while maintaining that safety margin.