How many batteries do I need to run my home during a power cut in Sri Lanka?

It depends on your load and how long the cut lasts. For a typical 3-hour cut running lights, fans, a router and a TV (about 226 W), one 12 V 150 Ah tubular battery is enough. Add the fridge or extend the hours and you move to a 24 V or 48 V bank of two to four batteries. Enter your own appliances above for an exact count.

What size inverter do I need for a fridge and fans?

Size the inverter to your running watts divided by the power factor (about 0.8), then leave 20% headroom. A fridge (150 W) plus three fans, lights, a router and a laptop is roughly 519 W, so about 650 VA — buy an 800 VA to 1 kVA pure-sine unit whose surge rating clears the fridge start-up spike (around 819 W).

How long will a 150 Ah battery run lights and a fan?

A 12 V 150 Ah tubular battery holds about 1,800 Wh, but you can only use ~50% (900 Wh) and lose ~15% in the inverter, leaving ~765 Wh usable. Running four LED bulbs and two fans (156 W) that lasts about 4.9 hours; add a TV and router (226 W) and it drops to about 3.4 hours.

Is a lithium (LiFePO4) or lead-acid (tubular) battery better for home backup in Sri Lanka?

Tubular lead-acid is cheaper up front and widely stocked, but you can only use about 50% of its capacity and it lasts fewer cycles. LiFePO4 lithium costs more but delivers ~90% usable capacity, far more cycles and lighter weight, so it works out cheaper per usable kWh over its life. For daily power cuts, lithium usually wins long term.

How much does a home inverter and battery system cost in Sri Lanka?

Prices move with the rupee and import duties, so this tool does not quote live prices. A small 12 V setup — a 600 VA to 1 kVA inverter plus one tubular battery — is the entry point, while lithium banks cost several times more. Enter the shop prices you have been quoted and the calculator totals the indicative system cost.

Why does the calculator recommend a bigger inverter than my running watts?

Two reasons. First, inverters are rated in VA, and watts equal VA only at a power factor of 1 — a mixed home load runs near 0.8, so 226 W needs about 283 VA. Second, motors in fridges and pumps draw two to three times their running watts for a split second at start-up, so the inverter's surge rating must clear that spike, not just the steady load.

Can I use this to size a solar off-grid system?

Not directly. This tool assumes the grid recharges your battery between cuts, so it sizes the inverter and battery for backup only. It does not size solar panels, charge controllers or solar charge time. For that, use the Sri Lanka solar panel size calculator linked in the related tools below.

What backup duration should I plan for?

The default is 3 hours, matching the length of a typical rolling cut approved under CEB and PUCSL demand-management schedules. If your area sees longer or back-to-back cuts, set 4 to 6 hours so the battery bank is not sized on a best-case day. You can adjust the hours field to any value from 0.5 to 24.

Sri Lanka · Utility

Sri Lanka Inverter & Battery Backup Calculator

Work out the inverter size and battery bank your home or shop needs to keep essentials — lights, fans, router, TV, fridge — running through a power cut. Get the VA rating, battery count, bank energy and realistic runtime. No signup, sources cited below.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 11, 2026

Size your inverter & batterySri Lanka

SLSEA wattages · datasheet norms

Quick scenarios

Appliances to back up

ApplianceQtyWatts

LED bulbLED බල්බ

Ceiling fanසිවිලිම් විදුලි පංකා

WiFi routerWiFi රවුටරය

TV (LED)රූපවාහිනිය

Laptop chargerලැප්ටොප් චාජරය

Phone chargerදුරකථන චාජරය

Refrigerator (avg)surgeශීතකරණය

Water pumpsurgeජල පොම්පය

Backup needed (hours)

Battery type

System voltage

Battery unit (Ah)

Running load

226 W

Peak with surge: 226 W

Recommended inverter

500 VA

400 W cont · 800 W surge

Batteries needed

1 × 150 Ah

1 in series × 1 parallel

Battery bank energy

1.8 kWh

Required: 1.6 kWh (133 Ah)

Realistic runtime

3 h 23 min

Meets your backup target

Load headroom

43.5%

Spare continuous capacity on the inverter

Per-appliance load

Appliance	Qty × W	Load (W)	Share
LED bulb	4 × 9	36	15.93%
Ceiling fan	2 × 60	120	53.1%
WiFi router	1 × 15	15	6.64%
TV (LED)	1 × 55	55	24.34%
Total running load		226	100%

Wattages: Sri Lanka Sustainable Energy Authority appliance labels. Backup duration basis: CEB / PUCSL demand-management notices. Battery depth-of-discharge, inverter efficiency and power factor: manufacturer datasheet norms — all editable above. Cost figures are indicative and based only on prices you enter. Sizes components only; have a licensed electrician handle wiring and installation.

How it works

The calculator turns your appliance list into two answers: the inverter size (in VA) that can drive the load, and the battery bank (in Ah, kWh and number of batteries) that can sustain it for the length of the cut. Default appliance wattages come from Sri Lanka Sustainable Energy Authority appliance labels; the depth-of-discharge, inverter-efficiency and power-factor constants come from battery and inverter manufacturer datasheets, and every one of them is editable above.

Running load is the sum of quantity × watts across all appliances. This is the steady power the inverter must supply.
Peak / surge loadadds a start-up transient for motor appliances — a fridge or water pump briefly pulls about twice its running watts. This only sizes the inverter's surge rating; it does not increase the battery draw.
Inverter size is VA = running watts ÷ power factor. With a home power factor near 0.8, add 20% headroom and round up to the next standard size (300, 500, 600, 800, 1000, 1500, 2000, 3000 VA) whose continuous and surge ratings both clear your load.
Battery energy is Wh = (running watts × hours) ÷ (efficiency × depth of discharge). Dividing by depth of discharge means you only lean on the usable part of the battery; dividing by efficiency covers the inverter's conversion loss.
Bank capacity is that energy ÷ system voltage, giving the amp-hours needed. The number of batteries is series × parallel, where series = system voltage ÷ 12 V and parallel strings round up to reach the amp-hours.
Realistic runtime reverses the maths for the whole batteries you actually install: hours = (voltage × Ah × depth of discharge × efficiency) ÷ running watts. The engine cross-checks this against an independent energy-balance route (usable Wh ÷ load) so both methods agree to the same figure.

Tubular lead-acid is capped at about 50% usable depth of discharge to protect its cycle life, so a 150 Ah tubular battery gives roughly 900 Wh before inverter losses. LiFePO4 lithium safely delivers about 90%, which is why a smaller lithium bank often out-runs a larger lead-acid one.

Worked examples

Essentials through a 3-hour cut — 12 V tubular

Load: 4×9 W LED (36) + 2×60 W fan (120) + 15 W router + 55 W TV = 226 W
No motor loads, so peak = running = 226 W
Inverter: 226 ÷ 0.8 = 283 VA → +20% = 339 → recommend 500 VA (400 W cont)
Bank: 226 × 3 ÷ (0.85 × 0.50) = 1,595 Wh → 133 Ah at 12 V
Batteries: one 12 V 150 Ah tubular (series 1 × parallel 1)
Realistic runtime: (12 × 150 × 0.50 × 0.85) ÷ 226 = 3.4 h ✓

Home office + fridge, 4-hour cut — 48 V lithium

Load: 6×9 (54) + 3×60 (180) + 15 router + 65 laptop + 55 TV + 150 fridge = 519 W
Fridge start-up surge +300 W → peak = 819 W
Inverter: 519 ÷ 0.8 = 649 VA → +20% = 779 → recommend 800 VA (1,280 W surge ≥ 819) ✓
Bank: 519 × 4 ÷ (0.90 × 0.90) = 2,563 Wh → 53 Ah at 48 V
Batteries: 48 ÷ 12 = 4 in series, one string → four 12 V 100 Ah lithium
Realistic runtime: (48 × 100 × 0.90 × 0.90) ÷ 519 = 7.5 h ✓

Edge case — water pump start-up, 30-minute cut — 12 V tubular

Load: 1×750 W pump + 10×9 W LED = 840 W running
Pump surge +1,500 W → peak = 2,340 W
Inverter: 840 ÷ 0.8 = 1,050 → +20% = 1,260; 1,000 VA fails (1,600 W surge < 2,340)
Recommend 1,500 VA (1,200 W cont, 2,400 W surge ≥ 2,340) ✓
Bank: 840 × 0.5 ÷ (0.85 × 0.50) = 988 Wh → 82 Ah → one 150 Ah battery
Realistic runtime: (12 × 150 × 0.50 × 0.85) ÷ 840 = 0.91 h

Frequently asked questions

Sources & references

Default appliance wattages and engineering constants were last cross-checked on 2026-07-11. Battery depth-of-discharge, inverter efficiency and power-factor figures follow manufacturer datasheet norms for tubular lead-acid and LiFePO4 packs, and are all editable in the tool. Cost figures are indicative and based only on prices you enter.

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