Standby generator sizing starts with the loads you need during an outage, not the total number of appliances in your home. List essential equipment, check its running and starting requirements, then identify which items may operate at the same time. The largest motor’s startup demand can matter as much as the combined running load. Your final choice also needs to suit the property’s electrical supply, generator rating and transfer setup. A qualified electrician can verify those details and account for conditions that affect performance. A careful assessment helps avoid paying for unused capacity while protecting the equipment you rely on.
Generator capacity is the amount of electrical power the unit can provide at a given time. It is not the same as energy storage. A generator’s kW or kVA rating describes output, while how long it can run depends on its fuel supply, operating load and equipment specifications.
For sizing, two demand figures matter. Running load is the power an appliance needs after it has started. Starting load is the short-term demand some motors and compressors draw as they begin operating. Refrigerators, water pumps and heat pumps are common examples of equipment where the startup requirement deserves attention.
The generator must cope with the running loads that are active together and with a plausible startup event on top of them. A rating that looks adequate when you add only normal running watts may still struggle when a motor starts. Conversely, adding every appliance’s maximum starting demand can lead to an unnecessarily large estimate if those starts cannot happen together.
Before comparing generator capacities, choose an outage plan. Some households want a limited set of essential circuits; others want broad household coverage with controls that temporarily switch off selected high-demand appliances. These approaches lead to different load calculations and installation requirements.
| Backup approach | Loads considered | What it means for sizing | Best suited to |
|---|---|---|---|
| Essential circuits | Selected lighting, refrigeration, communications, pumps or other priorities | Capacity is based on a defined group of loads and their likely overlap | Households willing to leave some appliances unavailable during an outage |
| Managed household coverage | A broader set of circuits, with chosen loads controlled or shed when necessary | Controls can limit simultaneous demand, but their settings must reflect household priorities | People seeking flexibility without expecting every large appliance to run together |
| Unrestricted whole-home use | Potentially all connected household loads | The design must account for the loads that could run at the same time, including large appliances | Homes where broad coverage is a priority and the electrical design supports it |
“Whole-home” coverage does not automatically mean every appliance can operate at full demand at once. The generator, transfer equipment and load controls must be designed for the intended use. If the system relies on load shedding, ask which appliances will be switched off, under what conditions, and whether the controls restore them automatically.
Use manufacturer data where possible. An appliance’s input current cannot always be converted to real power by multiplying amps by volts alone: power factor and the electrical characteristics of the load can affect the result. For some equipment, the maker’s published starting information or an electrician’s assessment is more useful than a rough calculation from the label.
The figures below are assumed only to show the arithmetic. They are not typical appliance specifications. Use the actual values for your equipment, and have the final calculation checked for the site and generator under consideration.
| Example load | Assumed running demand | Assumed starting demand |
|---|---|---|
| Refrigerator | 0.30 kW | 0.90 kW |
| Lighting and network equipment | 0.25 kW | 0.25 kW |
| Water pump | 0.60 kW | 1.80 kW |
| Heat pump | 1.60 kW | 3.20 kW |
If all four example loads are running, their combined demand is 2.75 kW. If the heat pump starts while the refrigerator, lighting, network equipment and pump are already running, the example peak is 3.20 kW plus the other loads’ 1.15 kW, or 4.35 kW. The heat pump’s running demand is replaced by its starting demand in that scenario; it is not counted twice.
That 4.35 kW figure is not a generator recommendation. It does not account for the actual appliance specifications, other household loads, generator transient performance, site conditions or electrical configuration. It shows why sizing from running watts alone can understate demand, and why adding all startup figures together can overstate it. An installer should establish a suitable capacity rather than simply matching the example total.
Some loads are straightforward: a lamp or other steady appliance generally adds its operating demand when switched on. Motors and compressors require more care because their starting characteristics vary. A heat pump’s demand, for example, should be checked from its own data rather than estimated from the size of the room or the home.
Also consider what happens after an outage. Refrigerators, pumps and heating systems may restart without anyone operating a switch. If several controls call for power at once, their startup demands may overlap. An electrician can assess whether the generator can support that sequence or whether a delay or load-management control is appropriate.
Load management can make a smaller generator viable when it reliably prevents certain high-demand combinations. It is only useful if the household accepts the restrictions. Confirm which loads receive priority, which are interrupted, and whether a critical appliance could be shed unexpectedly.
Where a motor’s startup demand is the main sizing obstacle, ask whether a compatible soft-start or other manufacturer-approved measure is available. It is not suitable for every appliance or installation, and it should not be treated as a substitute for checking the generator’s capacity and compatibility.
Generator listings may show kW, kVA, or more than one rating. These figures are related but not always interchangeable; the relationship depends on power factor and the generator’s specifications. Compare like with like and confirm which rating applies to the intended supply and operating conditions. Do not choose a unit solely by a headline “maximum” figure if its specified continuous or standby output is lower.
The property’s electrical arrangement matters too. A single-phase generator cannot supply a three-phase load, and a three-phase generator may have limits on how much power can be drawn from one phase. The electrician needs to confirm the site supply, the circuits being backed up and how loads will be distributed.
The transfer switch or other approved transfer arrangement is part of the design, not an optional sizing detail. It determines how the generator connects to the home and helps prevent unsafe interaction with the incoming supply. In New Zealand, have the system assessed and installed by an appropriately qualified electrical professional familiar with local requirements. Do not connect a generator to house wiring in a way that can backfeed the supply.
Ask the installer to check whether site conditions or the selected fuel affect the generator’s available output. Confirm the exact model rating, its compatibility with the home’s supply, required transfer equipment and any load controls before accepting a proposal. A larger generator may also require changes to the installation or fuel arrangements, so capacity is only one part of the project decision.
These questions make it easier to compare proposals on the same basis. A useful quote should explain the intended outage loads and any operating restrictions, rather than presenting a generator capacity without showing how it was selected.
No. Bedroom count does not reveal the power demand or startup requirements of the appliances you plan to use. Base the assessment on actual equipment data, the circuits you want backed up and the loads that may overlap.
Usually, that would assume all appliances start simultaneously, which may not reflect how the home operates. Calculate credible combinations, including automatic restarts after an outage, and ask an installer to verify the generator’s ability to handle the largest likely event.
It may, if the generator and electrical design can support the heat pump’s running and starting requirements alongside the other loads you need. Check the specific heat pump’s manufacturer data; its size or general category alone is not enough to determine compatibility.
Not necessarily. More capacity can increase equipment and installation costs, while the home may not need the additional output. Choose a unit that covers the defined load plan with suitable performance at the site, rather than buying capacity without a load assessment.
No. Capacity describes how much power it can supply at a time; runtime depends on factors such as fuel supply and operating load. Plan both the electrical demand and the expected outage duration when choosing a backup system.
The most useful next step is to prepare a list of essential circuits and appliances, gather their actual electrical data and note which loads may run together. Have a qualified installer check startup demand, generator ratings, phase compatibility and the transfer arrangement. That load-based approach makes standby generator sizing more reliable and helps you avoid paying for capacity your outage plan does not require.
