A home battery system should be sized around two things: the electricity your essential appliances use and how long you need them to run during an outage. A large capacity figure alone doesn’t show whether the inverter can start a pump, power several appliances at once or keep the house going until power returns. Begin by listing the circuits you need, estimating their energy use over your target outage period, and deciding what you can switch off. Then compare usable battery capacity, output limits, solar-recharging capability and installation costs. That order helps you avoid paying for capacity you won’t use—or discovering your backup falls short.
A battery sized for a few essential circuits overnight has a different job from one intended to run most of a home through a prolonged outage. Decide what “backup” means for your household before looking at product capacity or quotes.
Consider the outages you want to prepare for and the practical consequences of losing power. An interruption may affect refrigeration, internet access, lighting, water pumps, heating or cooling, and medical equipment. You might need to maintain all of those, or only a smaller set. Be specific about the minimum level of service you need rather than assuming the entire house must operate as usual.
Set a target duration, too. Think about whether you want the battery to cover a short interruption, an overnight outage, or a longer period when recharging may be limited. If outages can happen close together, account for how much time the battery has to recharge between them.
List the appliances and circuits you want backed up, then classify each as essential, useful or non-essential. This makes it easier to decide what the battery must support and what you could turn off to extend runtime.
| Load group | Examples | What to decide |
|---|---|---|
| Essential | Medical equipment, refrigeration, selected lights, communications equipment | Does it need uninterrupted power, and how long must it run? |
| Useful if capacity allows | A water pump, selected heating or cooling equipment, additional rooms | How often will it run, and can you use it for shorter periods? |
| Usually deferrable | Electric vehicle charging, clothes drying, cooking appliances or workshop equipment | Can you leave it off until grid power returns? |
These categories depend on the household. A pump may be essential where water supply depends on it, while another home may not need one at all. Heating and cooling also vary widely in electrical demand, so don’t assume that a battery capable of running lights and a fridge can run a heat pump for the same length of time.
For medical equipment, check its continuous and start-up requirements with the equipment supplier and discuss the backup arrangement with an installer. If an interruption could put someone at risk, plan a fallback as well; a home battery should not be treated as a guarantee of uninterrupted medical power.
Battery capacity is measured in kilowatt-hours. To estimate the energy needed for an outage, multiply each load’s average power by the number of hours it will operate, then add the results. For equipment that runs only part of the time, estimate its total operating hours rather than assuming it runs continuously.
For example, if your selected appliances average 0.45 kW together and you want to run them for 12 hours, they would use 5.4 kWh of delivered energy over that period (0.45 × 12). This is an illustration, not a household estimate. Actual use depends on appliance behaviour, operating conditions, how often equipment cycles, and which loads are running at the same time. Use measured or manufacturer-provided figures where possible, then ask the installer how the battery’s stated capacity translates into usable AC energy at your home.
A battery’s advertised or nominal capacity may not equal the energy available to household circuits. Some energy can be unavailable because of system settings, reserve levels or conversion from battery power to AC electricity. Ask for the usable capacity and the expected AC energy available in backup mode, rather than comparing headline kWh figures alone.
A battery can have enough stored energy for the night and still be unable to run the loads you need at once. Its inverter has a continuous output limit, and some appliances draw more power briefly when starting. Pumps, compressors and other motor-driven equipment can have start-up demands that differ from their normal running demand.
Ask for the system’s continuous output and its short-duration or surge capability, including how long any higher output can be sustained. Check those figures against the appliances that may operate together. If the system cannot support every load simultaneously, decide which ones take priority and whether automatic or manual load management is available.
Also confirm which parts of the home will receive backup power. Some installations supply selected circuits; others may be designed to support more of the house, subject to the inverter, wiring and connected loads. A “whole-home” arrangement does not necessarily mean every high-demand appliance can run together during an outage.
In New Zealand, ask an installer familiar with local electrical and network requirements to confirm how the system will disconnect from the grid and which circuits it can supply in backup mode. If your home has a three-phase supply or equipment with demanding start-up needs, raise that early: compatibility can affect the design and the loads that are backed up.
Use your inventory and energy estimate to create a few realistic operating plans. A simple plan might keep refrigeration, communications and selected lights on, while switching off water heating, cooking and other high-demand appliances. A second plan could include a pump or heating equipment for limited periods. Comparing these plans shows whether a smaller battery with disciplined load management could meet your needs, or whether you require more capacity and output.
These are planning options, not fixed system types. A hybrid arrangement may suit some properties, but it needs to be designed so the components work together safely and deliver the intended backup supply.
Solar can help recharge a battery, but having rooftop panels does not automatically mean they will operate during a grid outage. Many grid-connected systems stop exporting when the grid is unavailable. To supply a home safely while disconnected, the system needs suitable backup equipment and controls that can operate in that mode.
Ask the installer whether your proposed system can use solar to recharge the battery during an outage, whether it can supply household loads while charging, and what happens when solar production falls. Output varies with daylight, weather, season and household demand, so don’t count on a particular amount of recharging unless the design and its assumptions have been explained.
If you already have solar, check compatibility rather than assuming your existing inverter and a new battery will form a backup system. The answer can depend on the equipment, its configuration and the way the home’s circuits are arranged.
Once the target loads and duration are clear, quotes are easier to assess. Ask each installer to specify the same core details: battery usable energy, inverter output, backup circuits, reserve settings, expected operation during an outage, and any solar-charging capability. Otherwise, a lower quote may describe a smaller or less capable system rather than better value.
Installation cost can reflect more than the battery itself. The inverter and backup switching equipment, changes to the switchboard or wiring, circuit selection, integration with existing solar, and site conditions can all affect the work required. Ask for an itemised quote that identifies the equipment and installation tasks, along with any assumptions or exclusions.
Before accepting a proposal, get clear answers to these questions:
Have the installer explain any assumptions behind the runtime estimate. If it depends on switching off a major appliance, using a pump only intermittently or receiving solar input, make sure that operating plan is realistic for your household.
List the circuits you need, estimate their energy use across your target outage period, and compare the total with the system’s usable energy in backup mode. Then check that the inverter can support the loads that may run at the same time, including any start-up demand.
No. More capacity can extend runtime, but it won’t solve an inverter output limit or make an unsuitable backup-circuit design work. It may also add cost, so compare the extra capacity with a specific need in your outage plan.
That depends on the heat pump’s power requirements, the battery inverter’s output, the system configuration and the other loads running at the same time. Ask the installer to check both running and start-up demand and explain how heating affects estimated runtime.
Not necessarily. A grid-connected solar system may stop operating during an outage unless it has suitable equipment and controls for backup operation. Ask whether the proposed setup can supply loads and recharge the battery while disconnected from the grid.
Choose based on the service you need and the loads the system can support. Essential-circuit backup can suit households willing to manage usage, while broader backup may offer more convenience but needs careful checks on simultaneous demand, installation design and cost.
Before comparing battery brands or solar packages, write down the loads you need, the outage duration you want to cover, and the appliances that may run together. Use those requirements to compare usable energy, inverter output and backup-circuit design. That gives you a practical basis for choosing a home battery system—and for deciding whether a generator or a mixed backup setup better fits longer outages.
