Power

What actually matters when choosing an inverter

The inverter is the single decision that shapes everything else in a power system. Get the type wrong and you are either buying equipment you cannot legally connect, or hardware that trips every time your borehole pump starts.

David Watts · Filed under Power

The three types of inverter, plainly

The word "inverter" is used loosely in the trade, and that looseness causes real problems. A retailer who says "solar inverter" may be selling any one of three fundamentally different devices. The type determines what you can legally connect, what you need from a battery bank, and whether a municipal official has to sign anything before you switch on.

Grid-tied inverter

A grid-tied inverter converts solar DC to AC and feeds that power into your municipal connection. There is no battery. When the sun is up you use less grid power; when it is dark or overcast you revert entirely to the grid. Because the inverter synchronises with the grid to export, it must shut down the moment the grid fails — this is a deliberate anti-islanding safety requirement, not a design flaw. The consequence is that a grid-tied system gives you no backup power during load-shedding. In most South African municipalities this topology now makes little sense unless paired with a battery, because the grid is unreliable enough that you will notice every outage.

Off-grid inverter

An off-grid inverter draws from a battery bank and generates its own AC — it is not connected to the grid at all. The inverter is the grid, as far as your appliances are concerned. Sizing is critical: the inverter must handle every load you intend to run simultaneously, including surge currents at start-up. This topology suits properties with no grid connection, or those that have made a deliberate decision to disconnect entirely. There is no municipal approval required for the inverter itself, because it is not connecting to municipal infrastructure.

Hybrid inverter

A hybrid inverter combines both functions: it can charge a battery bank from solar or from the grid, supply your loads from either source, and — if configured and approved — export surplus power to the grid. During a grid failure it switches to battery and continues running. This is the topology most South African households are installing for load-shedding relief, and it is the one that attracts the most compliance complexity.

Feature	Grid-tied	Off-grid	Hybrid
Works during load-shedding	No	Yes	Yes
Requires battery bank	No	Yes	Yes (for backup)
Can export to grid	Yes	No	Yes (if approved)
NRS 097-2-1 required	Yes	No	Yes
Municipal sign-off required	Yes	No	Yes
Typical use case	Bill offset only	No grid available	Backup + solar offset

Surge rating and why continuous rating is the wrong number to shop on

Every inverter has two power ratings: continuous and surge (sometimes called peak). The continuous rating is the load it can sustain indefinitely. The surge rating is the load it can sustain for a short period — typically two to ten seconds — to allow motors and compressors to start.

The problem is that motors draw a starting current that can be three to seven times their running current. A fridge compressor rated at 150 W running may demand 600–900 W at start-up. A borehole pump rated at 750 W running may demand 2,500 W or more at the moment the capacitor kicks. If the inverter surge rating cannot cover that starting current, the inverter trips on overcurrent and your pump never starts.

The correct approach is to list every motor-driven appliance — fridge, freezer, borehole pump, pool pump, air conditioner, washing machine — note the running wattage from the nameplate, and multiply by at least three to estimate starting surge. Add those surge figures and confirm that the inverter surge rating exceeds the worst-case scenario, which is two motor loads starting within seconds of each other. This is conservative and deliberate: the cost of sizing up one inverter bracket is far lower than replacing a unit that has failed on repeated surge trips.

What the nameplate does and does not tell you

Motor nameplates give running current and sometimes a locked-rotor (starting) current. If only running current is shown, a multiplier of three is a reasonable floor; for borehole pumps on long cable runs, use four or five. Soft-start modules — available for most borehole pump installations — reduce starting surge significantly and are worth discussing with an installer if your pump load is close to the inverter surge limit.

Why choosing an inverter is different in South Africa

Transfer time and what it means for your appliances

Transfer time is the gap between the grid failing and the inverter supplying power from its battery. It is measured in milliseconds. Most hybrid inverters advertise transfer times between 10 ms and 30 ms; some "online" or double-conversion inverters have zero transfer time because they are always running from the battery and the grid is merely charging it.

Whether transfer time matters depends entirely on what you are running:

• Lights, fans, resistive heaters — transfer time is irrelevant. These appliances tolerate a gap of several seconds without any consequence.
• Fridge and freezer compressors — a brief transfer gap does not harm them. The thermostat will restart the compressor when power returns. The surge requirement at restart is the more important consideration.
• Desktop computers and monitors without a UPS — a 20–30 ms gap will typically reboot the machine. Work is lost and file system integrity may be at risk. Either put the computer on a small UPS, or choose a faster inverter.
• NAS drives and home servers — same risk as a desktop, with the added consequence of a potentially corrupted array during a mid-write power cut.
• Medical equipment — confirm transfer time requirements with the equipment supplier. Do not assume a standard hybrid inverter is adequate for CPAP machines, oxygen concentrators, or infusion pumps.
• VoIP routers and fibre ONUs — most tolerate a 20 ms gap, but some reboot and take 30–60 seconds to re-establish a connection. Test before assuming.

The practical threshold for most households is 20 ms or below for anything with a processor. Above that, assume a reboot on every grid failure. Given that South Africa routinely sees four to eight grid interruptions per day during Stage 4 or higher load-shedding, a 30 ms inverter in an office environment is a meaningful daily nuisance.

What can go wrong when choosing an inverter

The failures we see repeatedly are not about the inverter quality — they are about the decision process that precedes the purchase.

Buying on continuous rating, ignoring surge

A 5 kW inverter sounds substantial. If its surge rating is only 8 kW and the household has a borehole pump plus a fridge plus an air conditioner starting in sequence, it will trip. The installer then returns, calls it a "load issue", and the customer buys a second inverter. Sizing for surge at the quoting stage costs nothing.

Installing non-compliant equipment

Equipment not on the NRS 097-2-1 approved list cannot be legally registered. Some installers fit non-listed equipment and apply for approval anyway, gambling that the municipality will not check. The municipality sometimes does check, the application is rejected, and the homeowner owns a system they cannot legally connect. Confirm that the exact model — not just the brand — is on the current approved list before signing a quote.

Assuming municipal approval is the installer's problem

The approval application is submitted in the homeowner's name. The homeowner is the registered party and is responsible for ensuring the system is legal. A reputable installer will manage the submission, but the legal obligation sits with the property owner. Ask explicitly: "Will you handle the municipal registration, and what does that include?" Get the answer in writing.

Confusing "solar-ready" with "hybrid"

Some inverters are marketed as "solar-ready" or "solar-assisted" but are fundamentally UPS units with a solar charge controller bolted on. They cannot run a house from solar during load-shedding; they can only extend battery life marginally. The distinction matters for sizing the battery bank and for understanding what the system will actually deliver. Ask the installer to show you the data sheet and confirm the topology.

Under-sizing the battery bank for the inverter

A large inverter drawing from a small battery bank will pull the battery voltage down rapidly under load, causing the inverter to protect itself by shutting off. The inverter then appears faulty when the fault is the mismatch. Battery sizing belongs in the same calculation as inverter sizing — they are not independent decisions.

What an installer should show you

A competent installer should be able to provide the following without being asked twice:

• The NRS 097-2-1 approval status of the specific inverter model being quoted — not the brand, the model.
• A load schedule: a list of your appliances, their running wattage, and the installer's surge estimate for motor loads.
• A single-line diagram of the proposed installation, showing how the inverter connects to the distribution board, the battery bank, and the grid.
• Confirmation of whether the installation will be submitted for municipal registration, and who is responsible for preparing and submitting the application.
• The name and registration number of the electrician who will issue the Certificate of Compliance.
• The inverter's continuous rating, surge rating, and transfer time from the manufacturer's data sheet — not from a brochure.

If an installer cannot provide a load schedule and a single-line diagram before you sign, that is a signal about the quality of their commissioning process, not just their quoting process.

Is it worth choosing a more expensive inverter?

The honest answer is: it depends on what you are protecting against. A cheaper inverter with a 25 ms transfer time and a five-year warranty may be entirely adequate for a household that runs lights, a fridge, and a television during load-shedding. The same inverter is inadequate in a home office where repeated reboots cost work hours.

The factors that justify moving up the price bracket are: a home office or server that cannot tolerate transfer gaps; motor loads that are at or close to the surge limit of a standard unit; three-phase loads (which require a different class of inverter entirely); and a desire to export to the grid under a municipal buyback scheme, which requires a more sophisticated grid interface. figure being sourced

What we would caution against is buying the cheapest available unit and then discovering, six months in, that it does not appear on the NRS 097 approved list and cannot be registered. The gap between a compliant unit and a non-compliant one in the same power bracket is not always large, and the compliance status is verifiable before purchase.

What we would do differently

• Start with the load schedule, not the inverter catalogue. List every appliance, note the motor-driven ones, and estimate surge before looking at any model.
• Confirm the NRS 097-2-1 approval status of the specific model on the NERSA or municipal approved-equipment list before signing a quote.
• Ask the installer for the single-line diagram and load schedule in writing before the deposit is paid — not after installation.
• If you run a home office or server, either choose an inverter with a transfer time below 15 ms, or put the sensitive equipment on a small UPS.
• Discuss soft-start options for borehole pumps before assuming you need a larger inverter surge rating.
• Clarify in writing who submits the municipal registration application, what documents are included, and what the expected timeline is.
• Keep a copy of the Certificate of Compliance and the municipal approval letter with the property documents — these are required when the property is sold.