Battery sizing starts with the loads you actually need during an outage. Backing up the entire home can multiply cost and inverter size without improving resilience. Separate essential circuits first.
Core calculation
Required usable AC energy (kWh) = Σ(load watts × backup hours) ÷ 1000. Then adjust for inverter losses and the battery’s permitted usable depth of discharge. Add margin for ageing and surge, but stay within the inverter and battery manufacturer’s supported design.
| Load | Watts | Hours | Wh needed |
|---|---|---|---|
| Lights | Measured total | Desired backup | W × h |
| Fans | Measured total | Desired backup | W × h |
| Router / electronics | Measured | Desired backup | W × h |
| Refrigerator or motor load | Running + surge | Duty cycle estimate | Use realistic cycling |
Size the backup from loads to battery
List essential loads. Record running watts, starting surge and how many hours each must operate.
Calculate usable AC energy. Sum watt-hours and convert to kWh.
Account for system losses. Divide by realistic inverter efficiency and allowed depth of discharge.
Check power, not just energy. Confirm inverter continuous and surge ratings can start the largest simultaneous loads.
Add ageing and service margin. Size within manufacturer limits and plan for capacity fade, temperature and future replacement cost.
Cycle life is a usage assumption
A cycle-life claim only makes sense with depth of discharge, temperature and warranty conditions. Ask how the warranty defines retained capacity, eligible usage and replacement remedy instead of comparing one headline cycle number.
Backup-energy worksheet
Required usable AC energy (kWh) = sum of essential load watts × backup hours ÷ 1000. Divide by inverter efficiency and permitted battery depth of discharge, then add surge and ageing margin. Separate essential circuits; backing up every household load can make storage and inverter requirements unnecessarily large.
Size the battery from essential energy, not the inverter label
Start with the loads that must run during an outage. A 5 kW inverter does not mean you need 5 kWh of battery, and a 5 kWh battery does not necessarily deliver 5 kWh of usable AC energy. Usable capacity is reduced by permitted depth of discharge, conversion losses, temperature and battery ageing.
Build the load-hours worksheet
| Load | Watts | Quantity | Hours | Wh |
|---|---|---|---|---|
| Fans | W each | n | h | W × n × h |
| Lights | W each | n | h | W × n × h |
| Refrigerator | Use measured/estimated duty cycle | 1 | h | Average W × h |
| Router/laptop | W | n | h | Total |
Add the watt-hours, then divide by expected inverter/system efficiency and by the usable fraction of nominal battery capacity. Finally add an ageing/reserve margin. For example, if essential loads need 3,000 Wh and you assume 90% conversion efficiency and 80% usable depth of discharge, the theoretical nominal requirement is roughly 3,000 ÷ 0.9 ÷ 0.8 = 4,167 Wh before ageing reserve.
Power and energy are different constraints
The battery must supply enough energy for the required hours, while the inverter and battery must also support the instantaneous power and surge of loads such as refrigerators, pumps or motors. A battery can have enough kWh but still fail if the system cannot deliver the startup surge.
Do not size from one perfect-day solar assumption
If the battery is meant for outage resilience, decide what happens during cloudy weather and multi-hour grid failures. Separate “daily solar self-consumption” from “emergency reserve”. A system optimised to discharge deeply every evening may have little backup left when the grid fails at night.
Check the safety and warranty envelope
Use equipment and installation practices consistent with manufacturer specifications and applicable Indian electrical/safety requirements. Consult current guidance from MNRE, the Central Electricity Authority and relevant BIS standards. Battery chemistry, ventilation, protection, earthing and installation location are safety decisions, not only efficiency choices.
Decision rule: size from essential Wh, surge W, usable depth of discharge, efficiency and ageing reserve. If a vendor cannot show all five assumptions, the quoted battery size is not yet auditable.
Size one real household example
Suppose essential loads are four LED lights at 10 W each, two fans at 60 W, a 100 W refrigerator average draw and a 20 W router. Running load is roughly 280 W, but the refrigerator startup surge may be much higher. For four hours, simple energy need is about 1.12 kWh before losses.
If the inverter is 90% efficient and only 80% of nominal battery capacity is intended to be used, nominal storage must be greater than 1.12 kWh. Add ageing margin. Then separately ensure the inverter can handle refrigerator surge plus other simultaneous loads.
Why air conditioning changes everything
Adding a 1.5-ton AC can multiply both power and energy requirements. Instead of automatically sizing a huge battery, decide whether comfort loads can be excluded during outages. Essential-load subcircuits often produce a much cheaper system.
Check recharge time
A battery that lasts through an outage but cannot recharge before the next one may still fail your use case. Consider grid charging power and realistic solar generation.
Warranty metrics
Compare usable energy, cycle warranty, throughput where stated and end-of-warranty capacity terms. Do not compare batteries only by nominal kWh.
A good worksheet turns every appliance into watts, hours and surge, then tests both the worst outage and recharge period.
Use critical-load tiers when the budget is limited
Divide loads into Tier 1, Tier 2 and optional. Tier 1 might be lights, router, refrigerator and medical equipment. Tier 2 could add fans and selected sockets. Optional loads such as air conditioning or water heating can stay off during outages.
Size the first battery/inverter system for Tier 1, then check the incremental cost of Tier 2. This prevents a single heavy appliance from doubling the storage budget. It also makes the system easier to expand later.
Verify the installation after commissioning
Test an actual outage while the installer is present. Confirm essential circuits transfer correctly, the inverter does not overload during motor startup and monitoring shows expected battery state. Ask for shutdown and emergency instructions.
Keep a simple maintenance log with battery alarms, firmware updates and unusual capacity changes. A backup system is valuable only when it is ready during the outage; testing it periodically is part of ownership.