Inverter and Battery Sizing: Full Calculation Guide
Sizing an inverter and battery takes six calculations: total running watts, surge watts, inverter VA, backup hours needed, battery amp-hours, and system voltage. Most guides stop at the first and the fifth. The step almost everyone skips is surge, and skipping it is why a correctly sized inverter still trips when the refrigerator compressor kicks in. This guide runs the full calculation with a worked example, and tells you which number to round up and which to leave alone.
At a glance
Step | What you calculate | Common mistake |
|---|---|---|
1 | Running watts of your backup load | Including everything in the house |
2 | Surge watts | Ignoring it entirely |
3 | Inverter VA | Confusing VA with watts |
4 | Backup hours required | Guessing instead of checking outage history |
5 | Battery amp-hours | Forgetting the 50% depth limit |
6 | System voltage, 12V or 24V | Trying to run 1,500W on 12V |
Step 1: add up your running watts
List only what you actually want on backup. Everything you add costs money twice, once in inverter size and once in battery capacity.
Appliance | Typical running watts | Notes |
|---|---|---|
LED bulb, 9W | 9 | Negligible individually, adds up across a house |
Ceiling fan | 60 to 75 | BLDC fans draw 28 to 35 |
Tube light | 20 to 40 | |
Television, 43 inch LED | 80 to 120 | |
Wi-Fi router and set-top box | 20 to 30 | |
Laptop | 45 to 90 | |
Refrigerator, 200 to 250L frost free | 150 to 200 | Runs intermittently, not continuously |
Mixer grinder | 500 to 750 | Short duty, but counts toward surge |
Water pump, 0.5 HP | 375 | High surge |
1.5 ton inverter AC | 1,200 to 1,500 | Needs a 24V or 48V system |
Worked example, a 2BHK: 6 LED bulbs (54W), 4 fans (280W), TV (100W), router and set-top box (30W), refrigerator (180W). Running total: 644W.
Step 2: add surge watts, the step everyone skips
Anything with a motor or compressor draws 2 to 6 times its running wattage for a fraction of a second at start-up. The inverter has to supply that instantaneously or it trips on overload.
Appliance type | Surge multiplier | Example |
|---|---|---|
Resistive loads: LED, TV, laptop, router | 1x, no surge | 100W stays 100W |
Ceiling fan | 1.5x to 2x | 75W fan surges to 110 to 150W |
Refrigerator compressor | 3x to 5x | 180W fridge surges to 540 to 900W |
Water pump | 3x to 6x | 375W pump surges to 1,125 to 2,250W |
Air conditioner, non-inverter | 3x to 5x | Substantial. Plan the system around it |
You do not add every surge together, because appliances rarely start simultaneously. Take your largest single surge and add it to the running total of everything else.
Worked example continued: the refrigerator is the largest surge at roughly 720W (180W x 4). Running total without the fridge is 464W. Peak demand: 464 + 720 = 1,184W.
Step 3: convert watts to inverter VA
Inverters are rated in VA, not watts. Divide watts by the power factor, which is typically 0.8 on Indian home inverters.
Formula: inverter VA = peak watts ÷ 0.8
Worked example: 1,184 ÷ 0.8 = 1,480 VA. Round up to the next standard size, so a 1,500 VA inverter.
Inverter VA | Watts at 0.8 PF | Realistic household |
|---|---|---|
700 VA | 560W | 1BHK, lights and fans, no fridge |
900 VA | 720W | 1 to 2BHK, lights, fans, TV |
1,100 VA | 880W | 2BHK with a small fridge |
1,500 VA | 1,200W | 2 to 3BHK with fridge and pump |
2,500 VA | 2,000W | 3BHK-plus, heavier appliances |
3,500 VA and above | 2,800W-plus | Large house or light commercial |
Do not round this one down. An undersized inverter trips under surge, and repeated overload tripping damages both the inverter and the battery.
Step 4: decide how many hours of backup you need
Check your actual outage pattern rather than guessing. Most state DISCOMs publish outage data, and your own electricity bill or app often shows interruption history. Two weeks of observation is enough.
Outage pattern | Backup target |
|---|---|
Metro, occasional 30 to 60 minute cuts | 1 to 2 hours |
Tier 2 city, 1 to 3 hour evening cuts | 3 to 4 hours |
Semi-urban, 4 to 6 hour cuts | 5 to 6 hours |
Rural, long or unpredictable cuts | 8 hours-plus, consider solar charging |
Step 5: calculate battery amp-hours
Formula: battery Ah = (running watts × backup hours) ÷ (system voltage × depth of discharge × inverter efficiency)
Use 0.5 for depth of discharge on lead acid, 0.9 on lithium, and 0.85 for inverter efficiency.
Worked example, 644W running load, 3 hours backup, 12V lead acid: 644 × 3 = 1,932 Wh needed. 1,932 ÷ (12 × 0.5 × 0.85) = 1,932 ÷ 5.1 = 379Ah.
That is two 200Ah batteries, or a 24V system with two 200Ah units in series. A single 150Ah battery would give roughly 1.2 hours on this load, not three.
Load | Backup wanted | Lead acid Ah needed (12V) | Lithium Ah needed (12.8V) |
|---|---|---|---|
300W | 3 hours | 176Ah | 92Ah |
500W | 3 hours | 294Ah | 153Ah |
644W | 3 hours | 379Ah | 197Ah |
300W | 6 hours | 353Ah | 184Ah |
500W | 6 hours | 588Ah | 306Ah |
The lithium column is roughly half the lead acid column for the same delivered energy, because of the depth of discharge difference. That gap is the honest case for lithium inverters and batteries at higher backup requirements, where lead acid capacity becomes physically unmanageable.
Step 6: choose 12V or 24V
12V, one battery | 24V, two batteries | |
|---|---|---|
Practical load ceiling | 700 to 800W | 1,500 to 1,600W |
Current at 600W | 50A | 25A |
Cable thickness required | Heavier, more expensive | Lighter |
Conversion losses | Higher | Lower |
Upfront cost | Lower | Roughly double on batteries |
Failure behaviour | One battery fails, system stops | Weaker battery drags down the pair |
The current figures are the reason this matters. At 12V, a 600W load pulls 50A, which needs thick cable and generates real heat at the terminals. The same load at 24V pulls 25A. Above roughly 800W, 24V is not a preference, it is a requirement.
Common sizing mistakes, and what they cost
- Sizing the inverter on running watts only. The system works until the fridge compressor starts, then trips. Costs a replacement inverter or permanent frustration.
- Sizing the battery on nominal capacity. Forgetting the 50% depth limit means you get half the backup you calculated and deep-discharge the battery constantly, halving its life.
- Adding appliances after installation. A system sized for 644W and then asked to run a 1,000W geyser will fail. Size with 20% headroom for what you will add later.
- Putting a high-load appliance on backup because it is cheap to add now. Every 100W you add costs roughly 59Ah of extra lead acid capacity at 3 hours backup, which is not cheap.
- Mixing old and new batteries in a 24V pair. The weaker battery determines the performance of both and degrades the new one.
- Ignoring ambient temperature. Battery capacity falls in cold and life falls in heat. A battery in a 45°C utility room ages faster than the same battery in a shaded space.
Who needs what
Fit | Profile | System |
|---|---|---|
Strong fit, 12V single battery | 1 to 2BHK, load under 700W, cuts under 3 hours | 900 to 1,100 VA inverter, 150 to 200Ah battery |
Strong fit, 24V two battery | 2 to 3BHK with fridge and pump, cuts of 3 to 6 hours | 1,500 to 2,500 VA inverter, 2 x 150 to 200Ah |
Strong fit, lithium | Any of the above where space or weight is constrained, or backup above 4 hours | Lithium inverter matched to pack |
Marginal | Wants AC on backup | Technically possible at 48V with a large bank. Cost usually exceeds the value |
Not a fit | Load above 3,000W continuous | Consider a generator or a hybrid solar system instead |
FAQs
How do I calculate what size inverter I need?
Add the running watts of everything you want on backup, add the largest single surge load, then divide by 0.8 to convert to VA. For a 2BHK running 644W with a refrigerator that surges to 720W, peak demand is about 1,184W, which needs a 1,500 VA inverter. Round up, never down.
How do I calculate battery Ah for an inverter?
Multiply running watts by backup hours to get watt-hours, then divide by system voltage times depth of discharge times inverter efficiency. Use 0.5 depth for lead acid and 0.9 for lithium, and 0.85 for efficiency. A 300W load for 3 hours needs about 176Ah of lead acid at 12V.
What is the difference between VA and watts on an inverter?
VA is apparent power, watts is real power, and the ratio between them is the power factor, typically 0.8 on Indian home inverters. A 1,500 VA inverter supplies about 1,200W. Always size on the watt figure, because that is what your appliances draw.
Why does my inverter trip when the fridge starts?
Because the inverter was sized on running watts and not on surge. A refrigerator compressor draws three to five times its running wattage for a fraction of a second at start-up. A 180W fridge can momentarily demand 900W, and an inverter without that headroom shuts down on overload.
Should I choose a 12V or 24V inverter system?
12V for loads under about 700W, 24V above that. At 12V a 600W load pulls 50A, which needs heavy cable and creates real heat at the terminals; at 24V the same load pulls 25A. The 24V system costs more upfront because it needs two batteries, but it is more efficient and safer at higher loads.
How many batteries do I need for 4 hours of backup?
It depends on load, not on hours alone. At a 300W load, 4 hours needs about 235Ah of lead acid capacity, which is roughly one 220Ah battery. At 644W, the same 4 hours needs about 505Ah, which is three 200Ah batteries or a 24V pair of 250Ah units.
Can I add more batteries to my existing inverter later?
Only if the inverter is rated for that system voltage. A 12V inverter cannot run a 24V bank. Adding a second battery in parallel to a 12V system increases capacity but is generally discouraged, because the two batteries age unequally and the weaker one pulls down the stronger. Plan the final configuration at purchase.
Does an oversized inverter waste electricity?
Slightly. A larger inverter has higher no-load consumption, typically 15 to 40W depending on rating, drawn continuously while it is switched on. Over a year that difference is small in rupee terms, and it is a reasonable price for the surge headroom. Undersizing costs far more.
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