How does an inverter or UPS work?

When the power goes out and the fans keep turning, thank the box in the corner. A home inverter has a simple job: store electricity while the mains is on, and give it back when it goes off.

It can't store electricity as it comes from the wall. Mains is AC: it flips direction 50 times a second. A battery only takes DC, which flows one way. So the box turns AC into DC to charge the battery (the charger), and DC back into AC to run your home (the inverter).

Inside you'll find a heavy transformer (most of the weight), a row of MOSFET switches on a heatsink, a control board that watches the mains, a change-over relay that clicks when the power goes, and a small fan. In many home inverters the same switches and transformer do both jobs: run one way they charge, run the other way they invert.

The tall battery is a 12 V lead-acid tubular type, about 150 Ah and over 50 kg. A computer UPS is the same idea shrunk down, with a small 12 V, 7 Ah battery inside.

Mains electricity is AC. In India it swings between +325 V and −325 V fifty times a second (we call it 230 V, which is its average "strength", the RMS value). A battery is DC: a steady 12 V, always the same way round. To charge it, the charger has three jobs.

1. Step down. A transformer turns 230 V into about 15 V AC.

2. Rectify. Four diodes, one-way valves for current, are wired in a diamond called a bridge rectifier. Whichever way the AC pushes, two of them steer the current out of the same end. The backwards half of every wave is flipped forwards: now it's bumpy DC, 100 bumps a second.

3. Smooth. A big capacitor fills up at each bump and tops up the gaps, like a water tank under a dripping tap. The bigger it is, the smaller the leftover ripple.

Then a control circuit charges the battery in stages: bulk (a steady current, about 15 A), absorption (held at about 14.4 V while the current tapers off), then float (about 13.5 V, just enough to keep it full).

A battery pushes current one way only. To make AC, the inverter uses four fast electronic switches called MOSFETs, wired in a square called an H-bridge. Switch on the top-left and bottom-right pair, and current flows through the transformer one way. Switch on the other pair, and it flows the other way. Do that 50 times a second and you have AC.

Flipping hard gives a square wave. Fans hum and some gadgets dislike it. Pausing between flips gives a modified sine, a bit closer. A pure sine inverter switches thousands of times a second, making pulses that are wide at the top of the wave and thin near zero (PWM), then smooths them with a coil and a capacitor into a clean sine wave, just like the mains.

The transformer lifts 12 V to 230 V: its high-voltage coil has about 19 times as many turns. Energy can't be made from nothing, so the current goes down 19 times. Power is volts × amps: 230 V × 1.3 A on one side needs 12 V × about 29 A on the other, plus losses.

Most UPSes for computers are offline (standby): while the mains is fine, your PC runs straight off it through a relay. The UPS watches the wave. When it disappears, it starts its inverter and the relay clicks over. That takes about 5 to 15 milliseconds. A home inverter in UPS mode does the same in about 10 ms.

A line-interactive UPS keeps its inverter ready and also evens out high and low mains voltage, so it switches in 2 to 4 ms. An online (double-conversion) UPS never switches at all: it turns mains into DC and back into AC all the time, so a cut changes nothing. That costs some energy, which is why it is used for servers and hospitals.

Why can a PC survive even 10 ms of nothing? Its power supply has a big capacitor charged to about 380 V. It keeps the computer running for a few milliseconds on its own: the hold-up time. The ATX design guide asks for about 17 ms at full load. A cheap supply with a small capacitor may not make it.

A battery's label gives its voltage and its amp-hours (Ah). Multiply them for the energy inside: 12 V × 150 Ah = 1,800 watt-hours. Run 180 W of fans and lights and you might hope for 10 hours. You'll get far less, for three reasons.

1. You can't use it all. Draining a lead-acid battery flat shortens its life, so the inverter cuts off with about 20% left: 80% usable. 2. The inverter isn't perfect. About 15% turns to heat, and it needs a few watts just to stay on. 3. Fast draining wastes capacity. The 150 Ah is measured over 20 hours. Pull current faster and less comes out: Peukert's law.

Inverters are sold in VA (volt-amps), not watts. Fans, TVs and chargers don't draw current perfectly in step with the voltage, so a load needs a bit more VA than W. The ratio is the power factor. A 900 VA inverter can run about 720 W.

The cheapest way to stretch your backup? Swap old 75 W fans for 30 W BLDC fans. With fans and lights alone, that more than doubles it.

An inverter battery is six cells in a row. Each has lead plates (grey, negative) and lead dioxide plates (brown, positive) standing in dilute sulfuric acid. Each cell makes about 2.1 V, and six in series make 12.6 V.

As it gives out current, both kinds of plate soak up sulfate from the acid and turn into lead sulfate. The acid gets weaker and more watery: Pb + PbO₂ + 2 H₂SO₄ → 2 PbSO₄ + 2 H₂O. Charging drives the reaction backwards. That's why you can check the charge with a hydrometer: strong acid is denser. A reading of about 1.26 is full; about 1.12 is flat.

Charging also splits a little water into hydrogen and oxygen, so the level drops. Top it up with distilled water only, never acid or tap water. Tubular batteries hold the positive paste in little fabric tubes, so it can't flake off. They last about 1,200 to 1,500 deep cycles, far more than flat plates.

LiFePO₄ (lithium iron phosphate) batteries use 3.2 V cells, four in series. They weigh about a third as much, need no water, can be used more deeply and last several thousand cycles. They cost more up front.