How to Size Batteries for Overnight Loads
The dinner dishes are done, the air conditioner is still running, someone plugs in an EV and solar production has dropped to zero. That is the moment a battery earns its keep. To correctly size batteries for overnight loads, start with what your home actually uses between sunset and sunrise - not the biggest battery advertised and not your total daily bill.
For many Adelaide households, the evening peak is where expensive grid electricity bites hardest. A well-sized solar battery can cover those hours, store free or low-cost power when your energy plan allows it, and keep essential circuits running in a blackout if backup is part of the design. But capacity alone does not guarantee the result. The right answer depends on overnight energy use, the appliances running at the same time, your solar output through winter and the way you buy electricity.
What counts as an overnight load?
An overnight load is every appliance and device that uses power after solar generation falls away. Some are quiet background users: the fridge, modem, security system, standby power and a pool pump on a timer. Others are the heavy hitters: reverse-cycle heating and cooling, cooking, clothes dryers, electric hot water, EV charging and spa equipment.
The key is to separate energy from power. Energy is measured in kilowatt-hours, or kWh. It tells you how much battery storage you need over the whole night. Power is measured in kilowatts, or kW. It tells you whether the battery and inverter can run several appliances at once.
A household might use 12 kWh from 5 pm to 7 am, yet briefly demand 7 kW when the oven, kettle and air conditioner run together. A battery with enough stored energy but too little inverter output may still import electricity during that peak. This is why a sharp battery quote looks at both figures.
Start with your real evening usage
The most reliable starting point is interval data from your electricity retailer or smart meter. Look at at least two to four weeks of half-hourly or hourly use, focusing on the period after solar fades until it returns the next morning. Then compare a mild-weather period with the months when heating or cooling works hardest.
If you have existing solar monitoring, use it alongside the bill data. Your retailer shows grid imports, while solar monitoring can reveal how much solar was available before sunset and how much was exported instead of stored. Homes without detailed data can use electricity bills and appliance estimates, but this is less precise.
For a useful first estimate, add the likely overnight energy use:
- A typical efficient household with modest evening use may consume 6 to 10 kWh.
- A family running cooking, entertainment, heating or cooling can easily use 12 to 20 kWh.
- A fully electric home with ducted air conditioning, electric hot water, pool equipment or EV charging may need 20 kWh or more, depending on how those loads are scheduled.
- A house with a large overnight EV charge should treat the EV as a separate planning decision. Covering the car from a home battery can require very large storage and may not offer the best value.
Size usable capacity, not the headline number
Battery capacity is often quoted as a nominal figure. What matters for overnight use is usable capacity: the energy available to your home after allowing for the battery's operating limits and normal system losses.
A simple sizing calculation is:
Overnight kWh required ÷ expected usable battery capacity = number of battery modules or total storage needed
Say your home uses 14 kWh overnight and you want the battery to cover most of it. A system with around 15 kWh of usable storage may be suitable on paper. In practice, allowing a margin is often smarter. Battery charging and discharging is not perfectly lossless, overnight demand changes with the weather, and households tend to electrify more over time.
For that 14 kWh example, a 16 to 20 kWh battery could provide a more comfortable result, particularly if winter heating is part of the load. The right margin depends on budget, roof size and whether you can recharge from low-cost grid periods. Bigger is not automatically better if your solar system cannot regularly fill it, but undersizing can leave you buying peak-rate electricity every night.
Lithium iron phosphate, or LiFePO4, batteries are a popular choice for home storage because of their thermal stability, long cycle life and suitability for daily charging and discharging. The battery warranty also matters, including its throughput allowance and retained-capacity terms. Capacity is an investment for the long haul, so compare the usable kWh and warranty detail rather than shopping on a single price figure.
Match inverter output to the way your home runs
Capacity covers duration. Inverter output covers muscle.
A 5 kW inverter may suit a smaller household that avoids running several heavy appliances at once. A 10 kW hybrid inverter provides more room for families with larger air conditioning, induction cooking, pool pumps or future electrification plans. The goal is not to chase the largest number. It is to reduce grid imports when your normal evening loads overlap.
Single-phase and three-phase homes require different planning. Three-phase properties need particular care because a battery system may have limits around how backup power is supplied across phases. If blackout resilience matters, ask exactly what remains powered, whether the system backs up the whole home or selected circuits, and what transfer time to expect.
A battery may run essential loads brilliantly during an outage while not being intended to start every high-draw appliance at once. Fridges, lights, internet, selected power points and a garage door are usually more realistic backup priorities than unlimited ducted air conditioning, ovens and EV charging. Clear expectations make for a better system.
Solar production and tariffs change the best battery size
A battery does not need to receive every kWh from the roof. On some energy plans, charging from a free-power window or cheap overnight tariff can make financial sense, then reducing purchases during the costly evening peak. That can improve battery utilisation, especially through a grey Adelaide winter.
However, tariff charging is not a blank cheque. Check the plan's conditions, charge period, export rate and peak pricing. A battery repeatedly charged from the grid should still be cycled in a way that creates a genuine cost benefit after losses. Smart controls can also reserve capacity for the evening instead of discharging too early in the afternoon.
Solar array size matters just as much. A household with a 6.6 kW solar system and high daytime consumption may not have enough spare generation to fill a large battery on many days. Adding panels, shifting hot water heating into daylight, or using tariff charging may deliver a stronger outcome than simply adding more battery modules.
Avoid the two common sizing mistakes
The first mistake is sizing from total daily usage. A home using 30 kWh a day does not necessarily need a 30 kWh battery. Daytime solar may directly power washing, cooling and appliances, while only the evening and overnight portion needs storage.
The second is sizing purely for the annual average. Average figures hide hot nights, cold mornings and school-holiday usage. Review your highest-use periods, then decide how much grid use you are comfortable retaining. Chasing a literal $0 bill every day can mean paying for capacity that sits idle for much of the year. For some homes, covering 70 to 90 per cent of overnight use is the sensible value point.
A practical battery-sizing pathway
Start by choosing the outcome you want. Is it lower peak bills, better use of solar exports, blackout backup, EV integration or a combination? Then measure evening and overnight kWh, identify the simultaneous loads that set your inverter requirement, and check whether your roof or tariff can reliably charge the battery.
From there, compare scalable battery packages rather than treating storage as a fixed one-size product. A modular system can allow you to begin at a sensible capacity and expand if usage grows after adding an EV, heat-pump hot water or more electric heating. For homes across Adelaide, OG Trade can help match battery capacity, hybrid inverter output, phase compatibility and backup requirements to the way the property actually uses energy.
The best battery is not the one with the biggest sticker number. It is the system that still has useful energy left at breakfast, avoids expensive evening imports, and gives your household confidence when the grid goes quiet.