Understanding home load, usage and battery sizing — and why "we weren't using much" can still empty a 10kWh battery before sunrise.Where did my battery go overnight?
HOURS
USE
SOLAR
PM
PEAK
HOURS
The modern Australian energy day. Cheap or free energy floods the middle of the day, the evening peak is the expensive part, and your battery has to bridge everything in between. Getting the size right starts with understanding what your home actually draws, hour by hour.
A quiet house can still be a hungry house
Most people assume their battery should last all night because they're "not using much." But a battery doesn't drain based on how busy the house feels. It drains based on how much power is being drawn, for how long, and at what time of day.
Two units do all the work in this conversation, and mixing them up is the single most common source of confusion:
How fast power is being used right now. Think of it as the speedometer. A kettle draws about 2kW while it's boiling.
How much energy has been used over time. Think of it as the odometer. Your battery's capacity and your bill are both measured in kWh.
The "modes" your home runs in
Rather than thinking appliance by appliance, it helps to think in modes — the typical combinations of loads a home cycles through across a day. Each mode has a very different impact on your battery.
"I'm running nothing" mode
0.3–0.6kWThe house feels off
Fridge, Wi-Fi, standby power, security, pool timers and other background loads.
Low draw — but it adds up overnightNormal evening mode
0.8–1.5kWTV, lights, dinner prep
TV, lighting, fridge, dishwasher and small appliances ticking along.
Steady, manageable battery useDinner time mode
3–6kWCooking and living
Oven, cooktop, kettle, microwave, rangehood — often with aircon on top.
Drains the battery quicklyLaundry catch-up mode
2–5kWWeekend cleaning
Washing machine, dryer, iron, hot water top-up.
Heavy drain if done at nightComfort mode
1.5–5kWHeating or cooling the home
Split systems, ducted aircon, heaters, dehumidifiers.
Depends on weather and run timeHot water recovery mode
2–5kWYou may not even notice it happening
Electric element hot water reheating, or a booster kicking in.
Big hidden load outside solar hoursPool mode
0.8–2.5kWBackground outdoor load
Pool pump, chlorinator — pool heaters add far more.
Best run during solar / free powerEV mode
2–22kWCar charging
EV charger — most home units are 7kW; 22kW needs three-phase power.
Too large for most home batteriesAll-electric house mode
5–12kW+Everything happening at once
Cooking + hot water + aircon + laundry + EV, all overlapping.
Needs careful solar & battery designWhat each appliance really draws
Typical power draw for common household loads. The bar shows the upper end of each range — short, sharp loads like a kettle matter less than long-running loads like ducted aircon or a dryer.
Figures are typical ranges — your actual appliances may sit above or below these depending on age, size and settings.
Why batteries go flat overnight
Here's a very ordinary evening in a family home — nothing extravagant, just a base load plus three one-hour spikes:
| Usage item | Load | Time | Energy used |
|---|---|---|---|
| Base load overnight | 0.4kW | 10 hrs | 4kWh |
| Cooking spike | 5kW | 1 hr | 5kWh |
| Hot water spike | 5kW | 1 hr | 5kWh |
| Aircon / heating spike | 5kW | 1 hr | 5kWh |
| Total | 19kWh |
So when a customer says "we only used the oven and the hot plate" — that may genuinely explain where the whole battery went. A 10kWh battery would have run dry well before morning in the example above.
A 10kWh battery is not "10 hours of power"
How long a battery lasts depends entirely on the load:
Batteries have a maximum discharge rate as well as a capacity, set by the inverter (commonly around 5kW for a single unit). If your home suddenly draws 7kW, the extra 2kW comes from the grid — even with a full battery. That's another reason big overlapping evening loads work against you.
Battery sizing guide by household type
These are starting-point ranges, not fixed rules — the right answer always comes from your actual usage data and lifestyle:
| Household type | Daily usage | Suggested solar | Usable battery | Why |
|---|---|---|---|---|
| Small home gas hot water / cooking | 8–12kWh | 6.6kW | 5–10kWh | Low overnight load |
| Average family home | 15–25kWh | 8–10kW | 10–15kWh | Covers normal night use |
| All-electric home | 25–40kWh | 10–15kW | 15–25kWh | Cooking, hot water, heating/cooling |
| Large home with pool | 35–60kWh | 13–20kW | 20–40kWh | Pool and HVAC lift demand |
| Home with EV | 40–80kWh | 15–30kW | 20–50kWh | EV charged mostly from solar / free windows |
| Heavy lifestyle home | 60kWh+ | 20kW+ | 40kWh+ | Sauna, spa, pool heating, EV, ducted aircon |
How free midday power changes the maths
Australia now has so much rooftop solar that midday electricity is often close to worthless on the wholesale market. Retailers and governments have responded with free midday power windows — and they genuinely change how batteries should be sized and used.
What's on offer right now
The Solar Sharer Offer is a government-mandated plan that went live on 1 July 2026. Larger retailers in NSW, South East Queensland and South Australia must now offer an opt-in plan with three hours of free electricity every day — 11am–2pm in NSW and SEQ, 12pm–3pm in SA — capped at 24kWh per day. You need a smart meter, and you don't need solar panels to join. Victoria gets its own version, the Midday Power Saver, from 1 October 2026.
Retail plans like GloBird's ZEROHERO (currently $0 usage 12pm–3pm, paired with battery export incentives) and FOUR4FREE (currently four free hours, 11am–3pm) go further again, and other retailers such as OVO and Origin run their own versions.
Free windows exclude controlled-load circuits — and many electric hot water systems sit on exactly that circuit. If yours does, moving it onto a standard circuit with a timer may be needed to actually capture the free power. Also note that rates outside the free window are often higher, window times have already shifted on some plans, and fair-use limits apply — always check the current fact sheet before switching.
Why this matters for battery design
Traditionally, your solar had to do three jobs: run the house during the day, recharge the battery, and export the excess. With a free midday window, the grid can help do some of that work. The battery can be topped up from free grid power at lunchtime — cloudy day or not — then discharged through the expensive evening and overnight.
| Time of day | Best strategy |
|---|---|
| Morning | Use the battery carefully; avoid heavy loads where possible |
| Free window (11am–3pm) | Run heavy loads, charge the battery, charge the EV, pre-heat or pre-cool the home |
| Evening peak | Run the home from the battery instead of the grid |
| Overnight | Battery covers base load and essentials until morning |
What it means for sizing
| Scenario | Solar sizing | Battery sizing | Result |
|---|---|---|---|
| No free-power plan | Must cover daytime load and recharge the battery | Sized for evening + night | Bigger solar usually needed |
| Free plan, no battery | Less critical | No cover for the evening peak | Savings limited |
| Free plan + battery | Can be moderate — battery charges in the free window | Sized to cover 3pm through to the next free window | Strongest bill reduction |
| Free plan + EV | Helps, but the free window is the star | Battery usually shouldn't feed the EV | EV charging shifted to the free window |
A simple sizing formula
To size a battery properly, add up the energy the home needs between charging opportunities:
| Item | Estimate |
|---|---|
| Evening usage | 6kWh |
| Overnight base load | 4kWh |
| Morning usage | 3kWh |
| Safety buffer | +20% |
| Recommended usable capacity | ≈ 15–16kWh |
Then allow for usable vs nameplate capacity. Most modern batteries let you use nearly all of their rated capacity, but some hold a reserve — so a 15kWh usable requirement may mean around 16–18kWh of installed battery, depending on the model's usable depth of discharge and any backup reserve you set aside for blackouts.
The fuel tank way to think about it
A battery is like a fuel tank. Solar and free grid power fill the tank during the day. Your appliances empty it at night. Small loads are a slow leak; big appliances are opening the tap.
That's why battery sizing is never just about the number of people in the home — it's about the home's lifestyle. The more of these you answer "yes" to outside solar or free-power hours, the larger the battery needs to be:
- Do you cook with electricity at night?
- Do you have electric hot water?
- Do you run ducted aircon?
- Do you have a pool?
- Do you charge an EV at home?
- Do you use a clothes dryer?
- Do you run a spa or sauna?
- Is anyone home all day, or is the house empty?
Not sure what your home actually draws?
TSBA can review your real usage data and design a solar and battery system around how your household actually lives — not a one-size-fits-all number.
Click here to see if you qualify