Workspace
The calculator's own fields, action and results arrive with the verified pack when you load it. Nothing is computed in this page.
Hybrid & backup · the steady rating
Inverter continuous rating from load and margin
Size an inverter’s continuous rating from the load that genuinely runs at once, plus a spare-capacity margin for the appliances a household adds later.
Verified engine
What the engine returns
The figure that returns is the continuous rating, in kilowatts, to hold against datasheets — a machine rated at or above it carries the census with the chosen headroom intact. Read it alongside its twin before shopping: a circuit with a compressor or a pump on it is usually sized by its starting moment, not its steady evening, and the larger of the two verdicts is the one that buys the machine.
Inputs the result is built from
Connected continuous load
Spare-capacity margin
How the result is derived
MethodThe connected continuous load is scaled by one plus the spare-capacity margin, returning the continuous inverter rating in kilowatts; the declared reverse workflow solves for the load a fixed rating carries at a chosen margin.
StandardContinuous inverter sizing, rating kW = load kW × (1 + margin % / 100)
GuardA zero connected load is refused — the pack ships that refusal as a declared test vector. A census of nothing sizes nothing, and a zero here almost always means the tally was skipped rather than the circuit being empty.
How the inverter rating moves with the spare margin
Why the census of what runs at once, not the sum of every label, sizes an inverter
The load term is a census, not a catalogue. Adding up every rating plate in the house describes a moment no real evening contains — the kettle, the toaster, the vacuum and the washing machine do not conspire — so the figure that belongs here is the COINCIDENT draw: the worst combination that plausibly operates together, read from a meter, an energy monitor, or an honest walk through the circuit list.
The margin is bought headroom, and it earns its cost three ways. It absorbs the census being slightly wrong; it leaves room for the appliance the household has not bought yet; and it keeps the machine off its own redline, because electronics run continuously at their limit age faster and convert less efficiently near it. Entered as a percentage, it scales the whole census rather than padding one line of it.
The rating that returns is a CONTINUOUS figure — the thermal promise on the datasheet’s first page, the power the machine sustains indefinitely at its rated temperature. It is deliberately silent about the brief violent moments when a motor wakes; that is a different promise, quoted separately by manufacturers and sized by this page’s twin.
Oversizing has a price the margin should respect. A machine chosen far above the census idles through most of its life in the region where conversion is least efficient, and its own standing consumption is drawn from the bank every hour of an outage. The margin is a deliberate few tens of percent, not a doubling by reflex — and the declared reverse workflow audits the other direction, returning the census an inverter already owned can carry at a chosen margin.
The connected continuous load is scaled by one plus the spare-capacity margin, returning the continuous inverter rating in kilowatts; the declared reverse workflow solves for the load a fixed rating carries at a chosen margin.
When this calculation is used
- Choosing the inverter for a new backup or off-grid installation, once the essential circuit’s coincident draw has been measured or tallied.
- Checking an existing machine against a household that has grown — the declared reverse workflow returns the census it can still carry.
- Comparing quotes on equal terms: vendors pad differently, and restating each offer as census plus explicit margin makes the padding visible.
- Deciding what a circuit upgrade really requires before attributing the shortfall to the inverter at all.
Worked example
Take the pack’s anchor census: a backed-up circuit whose coincident evening draw tallies to 3 kW, sized with the default 25% spare-capacity margin.
The figure that returns is the continuous rating, in kilowatts, to hold against datasheets — a machine rated at or above it carries the census with the chosen headroom intact. Read it alongside its twin before shopping: a circuit with a compressor or a pump on it is usually sized by its starting moment, not its steady evening, and the larger of the two verdicts is the one that buys the machine.
The pack’s other declared duties show the margin as a lever: the same relation run at 20% over a 5 kW census, or at 15% over a fully metered 2.4 kW one. Tighter metering honestly buys a smaller margin — guessing does not.
What each input represents
Connected continuous load
The coincident draw of the backed-up circuit, in kilowatts: the worst combination of appliances that genuinely operates at the same time, not the sum of every rating plate. A whole-house energy monitor’s evening peak, or a tallied census of the essential circuit, is the honest source.
Spare-capacity margin
The headroom bought above the census, as a percentage. It covers census error, future appliances and the machine’s own preference for running below its limit. The supplied default is illustrative, not a recommendation — a household expecting to grow chooses more, a fully metered static circuit can justify less.
Assumptions and limits
- The census is a real coincident figure; the relation cannot rescue a load term built by summing every rating plate, it can only scale the error.
- The margin is flat across the whole census — no per-appliance diversity weighting is modelled, which is a finer instrument than a screening size needs.
- The rating is compared at the manufacturer’s stated temperature; hot enclosures and altitude derate real machines below their datasheet, and no such derating is applied here.
- Starting transients are out of scope by design: this page sizes the thermal promise, and the surge sibling sizes the violent one.
What the guards protect against
- A zero connected load is refused — the pack ships that refusal as a declared test vector. A census of nothing sizes nothing, and a zero here almost always means the tally was skipped rather than the circuit being empty.
- The margin is bounded between zero and one hundred percent: headroom beyond doubling is no longer a margin but a different design, and a figure above the band is far more likely a fraction-versus-percentage slip than a real intention.
- The load must sit below a utility-scale cap, so a watt figure entered where kilowatts belong — a thousandfold slip — is refused as a unit error rather than sized into an impossible machine.
Provenance
Continuous inverter sizing, rating kW = load kW × (1 + margin % / 100)
The connected continuous load is scaled by one plus the spare-capacity margin, returning the continuous inverter rating in kilowatts; the declared reverse workflow solves for the load a fixed rating carries at a chosen margin.
Screening and reference material, to be checked against the governing standard and a qualified engineer; not a design determination. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.