How to Size a Battery for Essential Loads: A Practical Home Backup Guide

A battery should be sized from the energy your essential appliances consume during the required backup period, not simply from the inverter’s VA rating.

That distinction prevents two common problems:

• Buying a large inverter with a battery that gives very little backup
• Buying an oversized battery that the inverter or solar charger cannot recharge properly

Size your battery right to keep essential home loads running reliably during power cuts

This guide explains how to size a battery for essential loads in an Indian home, including inverter losses, battery depth of discharge, appliance starting surge and a complete worked example.

What Are Essential Loads?

Essential loads are the appliances you want to keep running when grid power is unavailable.

For a typical Indian home, these may include:

• LED lights
• Ceiling fans
• Wi-Fi router
• Laptop or desktop computer
• Mobile chargers
• Refrigerator
• CCTV system
• Television

High-power appliances such as air conditioners, geysers, induction cooktops, electric kettles, irons and water pumps are usually excluded unless the inverter and battery system has been specifically designed for them.

The first step is therefore not choosing a battery. It is deciding which circuits and appliances truly need backup.

Battery Capacity and Inverter Capacity Are Different

An inverter is mainly sized according to instantaneous power demand, measured in watts or volt-amperes.

A battery is sized according to energy demand over time, measured in watt-hours, kilowatt-hours or ampere-hours.

For example:

• A 1,000-watt load running for 15 minutes uses 250 Wh
• A 200-watt load running for five hours uses 1,000 Wh

The second load is much smaller in power, but it requires four times as much stored energy.

This is why battery sizing must consider both appliance wattage and backup duration.

Step 1: List Every Essential Appliance

Create a load table with the following details:

Appliance	Quantity	Running watts	Required backup hours	Duty cycle
LED light	4	9 W each	4 hours	100%
Ceiling fan	2	50 W each	6 hours	100%
Wi-Fi router	1	12 W	8 hours	100%
Laptop	1	65 W	3 hours	100%
Refrigerator	1	120 W while running	8 hours	40%

Use the appliance nameplate, manufacturer specification or an energy meter whenever possible. BEE’s Standards and Labelling programme covers appliances such as refrigerators, televisions, LED lamps and ceiling fans, making the label a useful starting point for comparing energy performance.

Do not rely entirely on generic internet wattage charts. Two ceiling fans of the same size can have very different power consumption, especially when comparing conventional induction-motor fans with BLDC fans.

Step 2: Convert Every Load into Watt-Hours

Use this formula: Energy in Wh = Quantity × Running watts × Backup hours × Duty cycle

A duty cycle is required for appliances that switch on and off.

For example, a refrigerator compressor may not run continuously. If it runs for an estimated 40% of an eight-hour outage: 120 W × 8 hours × 0.40 = 384 Wh

The complete example becomes:

Appliance	Energy calculation	Required energy
4 LED lights	4 × 9 W × 4 h	144 Wh
2 ceiling fans	2 × 50 W × 6 h	600 Wh
Wi-Fi router	12 W × 8 h	96 Wh
Laptop	65 W × 3 h	195 Wh
Refrigerator	120 W × 8 h × 0.40	384 Wh
Total		1,419 Wh

The essential loads therefore need approximately: 1,419 Wh, or 1.42 kWh

For refrigerators and other cycling appliances, a plug-in energy meter used over a normal 24-hour period will usually give a better estimate than guessing the duty cycle.

Step 3: Add Inverter Losses and a Design Margin

A battery must supply more energy than the appliances finally receive because the inverter, battery cables and internal electronics consume some energy.

For a preliminary calculation, use:

Required nominal battery energy = Load energy × Design margin ÷ (Usable depth of discharge × Inverter efficiency)

A practical design margin of around 15% to 25% can cover:

• Small loads that were forgotten
• Inverter standby consumption
• Changes in appliance usage
• Battery ageing
• Higher consumption during hot weather
• Measurement errors

For the worked example, we will use a 20% margin.

Step 4: Select a Usable Depth of Discharge

Depth of discharge, or DoD, is the percentage of the battery’s rated energy that will be used before recharging.

A 5 kWh battery operated to 80% DoD provides approximately:

5 kWh × 0.80 = 4 kWh of usable DC energy

The correct DoD must come from the battery manufacturer’s operating limits and warranty conditions. Repeatedly leaving a battery at a very low state of charge can reduce service life, particularly with lead-acid batteries.

For the example calculations below, the following are conservative planning assumptions rather than universal specifications:

• Lithium iron phosphate battery: 90% usable DoD
• Lead-acid battery: 50% usable DoD

Always replace these assumptions with the values in the actual battery datasheet.

Worked Example: Lithium Battery Size

Essential load energy: 1,419 Wh

Add a 20% margin: 1,419 × 1.20 = 1,703 Wh

Assume:

• Inverter efficiency: 90%
• Usable battery DoD: 90%

Calculation:

Required battery energy = 1,703 ÷ (0.90 × 0.90)

Required battery energy = 2,102 Wh

The calculated minimum is approximately: 2.1 kWh nominal

The sensible selection would normally be the next available battery size, such as a 2.4 kWh or 2.5 kWh lithium battery, provided its discharge-power rating, voltage and communication protocol are compatible with the inverter.

For a 48-volt battery:

Battery Ah = 2,102 Wh ÷ 48 V

Battery Ah = 43.8 Ah

A 48 V, 50 Ah battery stores approximately: 48 V × 50 Ah = 2,400 Wh, or 2.4 kWh

Worked Example: Lead-Acid Battery Size

Using the same 1,703 Wh design energy, assume:

• Inverter efficiency: 85%
• Usable battery DoD: 50%

Calculation:

Required battery energy = 1,703 ÷ (0.85 × 0.50)

Required battery energy = 4,007 Wh

For a 24-volt battery bank: Battery Ah = 4,007 Wh ÷ 24 V

Battery Ah = 167 Ah

The practical selection would be approximately a 24 V, 180 Ah to 200 Ah battery bank, subject to the inverter manufacturer’s supported battery range.

That could mean two identical 12 V batteries connected in series. Series-connected batteries must be the same type, capacity, model and preferably the same manufacturing batch and age.
  

Why Lead-Acid Batteries Need Extra Care in Sizing

Lead-acid battery capacity is commonly declared at a specified discharge rate, often C20. A 150 Ah battery rated at C20 is tested at a discharge current that would theoretically discharge it over 20 hours. Exide explains that available lead-acid capacity falls as discharge current increases.

This behaviour is described by Peukert’s law. Higher discharge rates reduce the effective capacity of lead-acid batteries more noticeably than lithium batteries.

Therefore, a 150 Ah lead-acid battery should not automatically be treated as delivering its full nameplate capacity during a heavy two- or three-hour discharge.

For real installations, check the battery datasheet for C20, C10 and C5 capacity ratings.

Step 5: Check Continuous Battery Power

Energy capacity alone is not enough.

A battery may store enough energy for five hours but still be unable to deliver the instantaneous current required by the inverter.

Check the battery’s:

• Maximum continuous discharge current
• Peak discharge current
• BMS current limit
• Recommended inverter size
• Number of batteries permitted in parallel

For example, a 48 V battery limited to 50 A can theoretically supply: 48 V × 50 A = 2,400 W DC

The usable AC output will be lower after inverter losses.

This is especially important when running refrigerators, pumps, compressors or other motor loads.

Step 6: Check Starting Surge

Some appliances briefly draw much more power when starting than while running.

Typical examples include:

• Refrigerator compressors
• Water pumps
• Air coolers
• Mixers and grinders
• Conventional motors

The inverter must support both:

1. The combined continuous running load
2. The highest likely starting surge

The battery and its BMS must also be capable of supplying that surge current.

Do not size the inverter solely by adding the normal running watts. Check the appliance data and the inverter’s surge-duration specification.

Step 7: Convert Watts to VA Correctly

Many home inverters are rated in VA or kVA, while appliances are rated in watts.

The relationship is: VA = Watts ÷ Power factor

A 600 W load at a power factor of 0.8 requires: 600 ÷ 0.8 = 750 VA

Select the nearest suitable inverter rating above the calculated requirement, while allowing room for surge loads. Inverter and battery compatibility also matters because an incorrectly matched charger can lead to poor charging or reduced battery performance.

Step 8: Choose the Correct System Voltage

Battery voltage must match the inverter’s DC input.

Common home backup configurations include:

• 12 V systems
• 24 V systems
• 48 V systems
• High-voltage battery systems used with specific hybrid inverters

At the same power, a higher battery voltage reduces current.

For a 1,200 W DC load before losses:

• At 12 V: 100 A
• At 24 V: 50 A
• At 48 V: 25 A

Lower current can reduce cable size, voltage drop and heating, but the battery voltage cannot be chosen independently. It must match the inverter and battery architecture.

Never connect a 48 V battery to a 24 V inverter.

Step 9: Verify Recharging Time

A correctly sized battery is not useful if it cannot recharge before the next outage.

Check:

• Inverter or charger maximum charging current
• Battery maximum charging current
• Available rooftop solar capacity
• Typical daily solar generation
• Grid charging availability
• Time between outages

A rough charging estimate is:

Charging time = Energy to be replaced ÷ Effective charging power

If 2 kWh must be replaced and the effective charging power is 800 W:

2,000 Wh ÷ 800 W = 2.5 hours

Actual charging time will be longer because charging power may vary, solar generation changes with weather and some batteries reduce charging current near full state of charge.

A Faster Battery Sizing Formula

For quick preliminary sizing:

Battery kWh = Essential load watts × Backup hours × Margin ÷ (1,000 × Inverter efficiency × Usable DoD)

Example:

• Average essential load: 300 W
• Backup required: 5 hours
• Margin: 1.20
• Inverter efficiency: 0.90
• Usable DoD: 0.90

Battery kWh = 300 × 5 × 1.20 ÷ (1,000 × 0.90 × 0.90)

Battery kWh = 2.22 kWh

Choose the next suitable standard battery capacity after checking discharge current and inverter compatibility.

Common Battery Sizing Mistakes

• Sizing Only from Inverter VA: A 1 kVA inverter does not tell you whether the battery will provide 30 minutes or six hours of backup.
• Using the Total Connected Load: Not every appliance runs continuously. Use realistic operating hours and duty cycles.
• Ignoring Inverter Self-Consumption: The inverter itself uses power, especially during long backup periods.
• Treating All Battery Chemistries the Same: Lead-acid and lithium batteries have different usable capacity, discharge behaviour, charging requirements and cycle-life characteristics.
• Ignoring Refrigerator or Motor Surge: A battery may have enough kWh but still trip its BMS when a compressor starts.
• Using Advertised Ah Without Checking Voltage: A 100 Ah battery at 12 V stores about 1.2 kWh, while a 100 Ah battery at 48 V stores about 4.8 kWh.
• Ah alone is not a complete measure of stored energy.
• Forgetting Future Loads: Adding one more fan, workstation or refrigerator later can reduce backup time substantially. Include a modest expansion margin rather than doubling the battery without reason.

Practical Recommendation for Indian Homes: For a small backup system, first separate the essential-load circuits in the distribution board. This prevents heavy appliances from being switched on accidentally during an outage.

Measure actual consumption wherever possible, especially for refrigerators, fans and home-office equipment. Then calculate battery energy using the required backup duration.

Finally, verify four items together:

1. Battery energy in kWh
2. Battery discharge-power rating
3. Inverter continuous and surge capacity
4. Charger and solar recharging capability

Battery state-of-charge estimates also depend on discharge rate, temperature and charging efficiency, so installed monitoring is more reliable than estimating backup solely from voltage.

Battery Sizing Checklist

Before purchasing, confirm:

• Essential appliance list
• Running watts of each appliance
• Backup hours required
• Duty cycle of cycling loads
• Total energy in Wh or kWh
• Inverter efficiency
• Battery usable DoD
• Design margin
• Continuous discharge current
• Starting-surge capability
• Inverter DC voltage
• Charging current
• Solar recharge availability
• Cable, fuse and isolator sizing
• Ventilation requirements
• Battery and inverter warranty compatibility

Battery installations can carry very high DC current. Final cable sizing, overcurrent protection, isolation and earthing should be completed by a qualified installer according to the inverter and battery manufacturers’ instructions.

Conclusion

To size a battery for essential loads, calculate the watt-hours required during the outage, add a realistic margin, account for inverter losses and divide by the usable depth of discharge.

For the example in this guide, essential loads consume around 1.42 kWh. After losses and margin, the requirement becomes approximately:

• 2.1 kWh minimum nominal lithium capacity
• 4.0 kWh minimum nominal lead-acid capacity

The final product selection should be rounded up to the next compatible standard size and checked for continuous current, motor surge and recharging capability.

A well-sized system is not necessarily the biggest battery. It is the battery that supports the required loads for the required time and can be safely recharged before it is needed again.