What size inverter do I need?
The inverter is the only part of a solar system that is doing hard work every second of daylight, and it is the part most likely to be the wrong size. Get it wrong one way and you throw away generation. Get it wrong the other way and it trips every time the air conditioner starts.
The counter-intuitive bit: smaller than the array
New buyers almost always assume a 10 kWp array needs a 10 kW inverter. It usually does not. Standard practice is to fit an inverter rated at roughly 80–85% of the array's DC rating.
The reason is that the array's kWp rating describes laboratory conditions your roof will meet for a handful of hours a year, if ever: 1,000 W/m² of irradiance with the cells held at 25 °C. In reality, by the time the sun is strong enough to push the array near its rating, the panels are hot and have already lost 10–20% of their output to temperature. Add dust, wiring losses and a plane of array that is not perfectly aimed, and a 10 kWp array might touch 8 kW on its very best day.
Paying for 10 kW of inverter to catch a peak that happens twice a year is poor value. So you deliberately fit a smaller one.
DC/AC ratio, and deliberate clipping
| Ratio | What it means | Where it fits |
|---|---|---|
| 1.0 – 1.1 | Conservative; almost nothing clipped | Cool climates, premium panels, export-rich tariffs |
| 1.15 – 1.25 | The usual compromise | Most residential installs, most climates |
| 1.3 – 1.5 | Aggressive; noticeable midday clipping | Hot climates, east-west split arrays, battery charging |
Clipping is what happens when the array could produce more than the inverter can pass. The inverter simply holds its output at its maximum and the extra is never harvested. This sounds wasteful and people recoil from it, but the arithmetic is usually in its favour: a ratio of 1.25 typically clips well under 2% of annual generation, while the smaller inverter can be 15–20% cheaper. You are trading a sliver of your best few hours for a real reduction in capital cost.
Clipping also flattens your output curve, which is quietly useful. A flatter curve matches household demand better and is easier on an export limit if your utility imposes one.
When the array does not decide — backup does
Everything above assumes the inverter's job is to convert solar. The moment you add a battery and expect the house to keep running during an outage, a second and completely different constraint appears: the inverter must carry your peak household load, whether the sun is out or not.
A house with a 5 kWp array but a 6 kW peak load needs a 7.5 kW inverter, not a 4 kW one. The array would have suggested 4 kW; the backup requirement overrules it. When this happens your DC/AC ratio ends up below 1, and that is correct, not a mistake — you are buying inverter capacity for the outage, not for the sunshine.
The calculator tells you which of the two constraints decided your number, so you can see when you are paying for backup rather than for solar.
Surge: the specification people forget
Motors do not start politely. A compressor, a water pump or a washing machine draws three to seven times its running current for the first fraction of a second. A 1.5-ton air conditioner that runs at 1,800 W can demand 5,000 W or more at the instant it kicks in.
Inverters publish a surge or peak rating, usually "2× for 5 seconds" or similar. Check it against your worst realistic simultaneous start, not your average load. The classic failure is an inverter sized exactly to the running load that trips the first time the AC and the pump start within a second of each other.
Inverter-driven ("DC inverter") air conditioners and soft-start retrofits both dramatically reduce this problem, and are often cheaper than buying the next inverter size up.
String, hybrid or off-grid?
| Type | Battery? | Works in an outage? | Best for |
|---|---|---|---|
| Grid-tied string | No | No — shuts down by law | Reliable grid, good net metering, lowest cost |
| Hybrid | Yes, optional | Yes, on a backed-up circuit | Most people; lets you add a battery later |
| Off-grid | Required | N/A — there is no grid | No connection available at all |
A grid-tied inverter is required to disconnect during a blackout. This is anti-islanding protection and it exists so that your panels cannot energise a line that a utility worker believes is dead. It is not a defect and it cannot be disabled. If you want power during an outage you need a hybrid or off-grid inverter and a battery — panels alone will not do it, no matter how sunny the blackout is.
For most households a hybrid is the sensible default even with no battery on day one, because retrofitting a battery to a plain grid-tied system usually means replacing the inverter.
Microinverters and optimisers
A string inverter handles the whole array at once, so a shaded or dirty panel drags down everything in its string. A microinverter sits under each panel and converts that panel alone; a power optimiser is a middle path that conditions each panel before a central inverter.
- Choose per-panel electronics when your roof has unavoidable shade, faces multiple directions, or is broken into small awkward sections.
- Choose a string inverter when you have one clean unshaded plane — it is cheaper, there is one box to service instead of twenty, and nothing that fails is bolted under a panel.
The MPPT specification
Three numbers on the datasheet decide whether your panels can actually be wired to the inverter you bought:
- Maximum DC input voltage. Panel voltage rises as temperature falls, so the string's cold-morning open-circuit voltage is the figure that must stay under the limit. Exceeding it destroys inverters and voids warranties. This is the single most common design error in self-installed systems.
- MPPT voltage range. On a hot afternoon the string voltage sags; if it drops below the window, the inverter stops tracking and your generation falls off a cliff.
- Number of MPPT inputs. Each independent input can track its own string. Panels on different roof faces belong on different MPPTs; put them on the same one and the weaker face pulls the stronger one down.
A short checklist
- Size the array first, from your consumption and your location.
- Divide by 1.2 for the solar-driven number.
- Divide your peak load by 0.8 for the backup-driven number.
- Take the larger, round up to a real product size.
- Check the surge rating against your biggest motor start.
- Check cold-weather string voltage against the maximum DC input.
- Count your roof faces and make sure you have enough MPPT inputs.
- Decide hybrid versus grid-tied before you buy, not after.