Solar Inverter Size Calculator

Two questions, two different answers

What size inverter do I need” hides two separate questions, and they rarely give the same number.
Sizing against the array asks how much solar power the inverter has to convert on a good day. This is the question if you are building a grid-tied system to cut your bill.
Sizing against the load asks how much the inverter has to carry when the grid is down. This is the question if you are buying backup, and it has nothing to do with how many panels you have.
A household with a 10 kW array that only needs to run fans, lights and a fridge during load shedding is answering both questions at once – and the answers point in different directions. The calculator above does each separately so you can see the gap, then reconcile it yourself. If you need both, take the larger number.

Your inverter should probably be smaller than your array

This sounds like a mistake and it is not. It is standard practice, manufacturers design for it, and being talked out of it is one of the more expensive things that happens to buyers here.
An array almost never produces its nameplate rating. Panels are rated in laboratory conditions – 1,000 watts per square metre of irradiance at a cell temperature of 25°C. On a roof in Multan in June the cells are running thirty degrees hotter than that, and output falls with temperature. Add dust, wiring losses and the angle of the sun through most of the day, and a 10 kW array spends the overwhelming majority of its life producing well under 10 kW.
So an inverter rated at the full array size sits underworked nearly all the time. That costs money twice: you paid for capacity you rarely use, and inverters are least efficient when running at a small fraction of their rating.
A smaller inverter runs closer to its efficient band for more of the day and costs less to buy. The trade is that on the brightest, coolest days the array may briefly produce more than the inverter can pass through, and the excess is clipped. Clipping sounds alarming and is usually trivial – it happens at the very top of a handful of days a year, and the energy lost is a fraction of a percent of annual yield.
Manufacturers plan for this explicitly. Every inverter publishes a maximum DC input alongside its AC rating, and that figure is almost always larger. Across the units in our catalogue that publish one, the ratio ranges from roughly 1.1 to 2.0 – meaning some inverters are designed to accept twice as much solar as they output.

The datasheet number that actually decides it

Most inverter sizing advice offers a rule of thumb – multiply by 1.2, divide by 1.25, and so on. We checked that against real datasheets before building this calculator, and no single ratio survives contact with the catalogue. The units that publish a maximum DC input span roughly 1.1 to 2.0, with the middle of the range sitting near 1.5.
Any fixed multiplier would therefore be wrong for most inverters on the market. So this calculator does not use one where it does not have to. Where a datasheet gives a maximum DC input, it matches your array against that manufacturer’s own figure. Where the datasheet is not on file yet, it falls back to the median of the ones that are – a number that improves on its own as we add datasheets rather than staying frozen at whatever seemed reasonable when it was written.
When you are comparing inverters yourself, maximum DC input is the specification to read. It tells you how much array that unit will actually accept, and it varies far more between models than the AC rating suggests.

Single phase or three phase – check before you order

This is the one sizing decision you cannot correct afterwards.
Your connection is either single phase or three phase, it is determined by your DISCO, and an inverter built for one will not work on the other. Larger systems generally require three phase, and moving from one to the other means a load enhancement application with your DISCO – its own cost, its own timeline.
Your phase is shown on your bill near the connection details. If you are unsure, look at your main breaker: a single-phase connection brings in two wires, a three-phase connection brings in four.
The calculator filters strictly on this. If you select single phase, units that do not declare a phase are excluded rather than assumed — an inverter recommended on a guess is worse than one not recommended at all.

What the continuous rating does not cover

An inverter’s headline figure is its continuous rating – what it can sustain indefinitely. Motors do not care about that number at the moment they start.
An air conditioner compressor, a water pump and a refrigerator all draw several times their running wattage for a fraction of a second at startup. A 750-watt pump can momentarily pull two to three thousand watts. If your inverter can only just cover the running load, it will trip when the pump and the air conditioner happen to start together.
This is why inverters publish a separate surge rating, usually around double the continuous figure for a few seconds. When you are sizing against backup load, the calculator adds 25% headroom to the continuous requirement – enough margin for sustained draw, but it is the surge rating on the datasheet that determines whether the unit survives a motor start.
If your backup load is motor-heavy – pumps, non-inverter air conditioners, older refrigerators – read the surge rating as carefully as the continuous one.

Work out your load first with the Solar Load Calculator – it gives you the connected-load figure this calculator wants.

Hybrid or grid-tied?

The battery question changes which inverters are even candidates, and it changes the price considerably.
A grid-tied inverter converts solar to AC and feeds your house and the grid. It is simpler, cheaper, and it shuts down during an outage – including when the sun is shining, for the safety of line workers. If your goal is purely to cut your bill, this is the correct and cheaper choice.
A hybrid inverter does the same and manages a battery as well, so it can keep running through an outage. It costs meaningfully more per watt, and it is only worth the difference if you are actually buying a battery – a hybrid inverter with no battery attached does nothing a grid-tied one would not.
Set the battery option in the calculator to match what you are genuinely buying, not what you might add later. If a battery is a someday plan rather than a this-year plan, price both and decide whether paying now for a capability you will not use for three years makes sense.

One more thing worth reading – MPPT count

Each MPPT is an independent tracker that can run a group of panels at its own optimum. Two MPPTs mean two roof faces, or two orientations, or one shaded section that will not drag down the rest.
If your roof is a single clean plane facing one direction, one MPPT is fine. If your panels will be split across two faces, or if a water tank or parapet shades part of the array for some of the day, more MPPTs will get you more energy out of the same panels. The recommendations above show MPPT count where the datasheet gives it.

Next step

Once the inverter is settled, the remaining questions are what it will all cost and what it will save.