How long does a 120Ah battery last with 4 tube lights + 2 fans + billing computer?

The arithmetic, line by line

A retail counter that cannot stop billing when the grid drops. Duty cycle matters here: Battery Buddy multiplies each appliance by the minutes it actually runs in an hour, so a fridge that cycles 40 minutes an hour is counted at two-thirds of its nameplate wattage instead of full blast.

Why a 120Ah battery only gives you 504Wh

On paper a 120Ah 12V battery stores 120 x 12 = 1440Wh. You never get that. Lead-acid tubular plates are rated for roughly 50% depth of discharge, and the inverter loses more converting DC to AC, so the honest figure is 120 x 12 x 0.7 x 0.5 = 504Wh.

Popular short-tubular size for compact inverters. If you have already measured your own system with the efficiency estimator, put your real factor into the calculator — that single number moves the answer more than any other input.

What this result means for your outage

At 1 hour 12 minutes this is a High Load setup — Battery Buddy flags anything under 2 hours, because a single long outage will empty the bank before the grid returns.

• Step up the bank: a 150Ah battery on the same load gives about 1 hour 30 minutes.
• Swap induction ceiling fans for 30W BLDC fans — that alone removes roughly 40W per fan.
• Move the heaviest appliance off the inverter circuit entirely rather than trying to out-size it.

Will the inverter actually hold this load?

Runtime and capacity are two different questions. With every appliance on at once this profile draws 420W continuous, which sizes to (420W / 0.8) x 1.25 on average draw and lands on a standard 800VA inverter.

The start-up surge test is the one most calculators skip. Desktop Computer spikes to about 240W for a second or two when its motor kicks in, so peak demand is 420W continuous + 240W surge = 660W. Against a 800VA inverter that is 18% headroom — it holds.

Efficiency is the number that actually moves the answer

Battery age, electrolyte level, cable thickness and terminal corrosion all show up as one thing: a lower effective efficiency. Here is the same battery and the same load across the realistic range.

Run the two-hour test on the efficiency estimator to replace this guess with your own measured factor.

Other battery sizes against Small shop

If you are still choosing a battery, this is the whole shelf against exactly this load.

Other loads on a 120Ah battery

And this is the same 120Ah battery against every other load profile we model.

Frequently asked questions

How long will a 120Ah battery last with 4 tube lights + 2 fans + billing computer?

About 1 hour 12 minutes. The load averages 420W and the battery delivers 504Wh usable at 70% efficiency and 50% depth of discharge, so 504 / 420 = 1.2 hours.

Why does my 120Ah battery give less than 1.2 hours in practice?

Because 70% is an assumption, not your measurement. A battery two or three years old, a low electrolyte level, thin cabling or a charger that never completes a cycle in short grid windows all pull the effective factor down. At 55% the same setup gives 57 minutes. Measure yours with the efficiency estimator.

What inverter size suits 4 tube lights + 2 fans + billing computer?

A standard 800VA inverter. The sizing rule is (average load / power factor 0.8) x 1.25 safety margin, which on 420W gives 656VA and rounds up to the nearest retail tier.

Can I use the full 120Ah of the battery?

No. Lead-acid tubular batteries are rated for roughly 50% depth of discharge — draining past half state of charge sulphates the plates and shortens life sharply. That is why 120Ah x 12V = 1440Wh nominal becomes 504Wh usable.

How do I get more backup out of this setup without buying a bigger battery?

Cut the load, not the comfort. Replacing induction ceiling fans with 30W BLDC fans and any remaining CFLs with 9W LEDs typically removes 40-45W per fan and 6W per bulb. On this profile that is the difference between 1 hour 12 minutes and materially more, at a fraction of the cost of a second battery.

Does Battery Buddy sell batteries or recommend a brand?

No. Battery Buddy is a free, brand-neutral calculator from Flocci Technologies. It shows the formula for every number so you can check it against any dealer's quote, and it needs no sign-up.