Battery, or better timing?
One costs thousands. The other costs nothing but a change of habit. Drop a pin on your location, answer four quick questions, and see which one actually leaves you better off. Free, no sign-up.
One costs thousands. The other costs nothing but a change of habit. Drop a pin on your location, answer four quick questions, and see which one actually leaves you better off. Free, no sign-up.
A home battery stores your spare solar for the evening. This tool helps you decide if you need a battery, if you should install solar, or if you should just shift your usage to the day.
How much power your solar system makes depend on where you live. Search for your suburb or town, or tap the map.
Tap the map to drop the pin.
These questions help estimate how much power you use each day.
Who lives here?
On a weekday, is anyone home?
How is the home heated and cooled?
How is the water heated?
An electric car?
A pool pump?
Most bills print an average daily figure. The named loads above stay as they are and the rest of the house is sized to match.
Are you thinking of installing a battery or solar? Enter details of your solar system, the battery you've been quoted, and what you pay for power. Leave them unchanged if you are not sure.
Do you have solar panels?
Skip these unless you know them. The roof is already estimated for your location: pitched near your latitude, facing the equator.
The battery you are weighing up
A typical 2026 price for that size, after any standard rebate. Type over it if you have a quote.
Energy costs
Off-peak runs 10pm to 7am every day. Weekends have no peak. The export rate above still applies.
Each appliance you time for daylight uses your own solar instead of selling it. Tap the ones you could realistically run then.
Most battery warranties run about 10 years. Give a battery longer and it has more time to earn its price back, but past its warranty you are assuming it still works.
Each line starts below zero by what that option cost up front, then climbs as the savings come in. Where a line crosses zero, that option has paid for itself. Above the line, you’re ahead.
What you would pay for power in each three-month season, and what each option would change. These are energy charges only, so your daily supply charge is not included.
This is the year the tool simulated, one hour at a time, starting today. Slide through it to see the appliances you moved shift into the middle of the day, and the battery cover what’s left in the evening.
Results are estimates for a typical weather year, not advice and not a quote. Terms of UsePrivacy
Weather. Five years of measured hourly radiation (global and diffuse) and temperature at your point, composed into a typical year the way a TMY file is: for each calendar month, the real days of whichever year sat closest to the five-year average. Day-to-day variety survives; the total stops depending on whether the last twelve months were a freak. That year is then laid over the twelve months ahead, starting today, so the dates and weekdays you see are the ones you are heading into rather than the ones the measurements came from – which also gives the household model a real, unbroken run of weekends. The clock offset is worked out for every day, daylight saving included.
Generation. The archive supplies the diffuse share directly, so each measured hour is separated into beam and diffuse without estimating either, then transposed onto your roof plane in local clock time and derated for cell temperature and system losses. Because the shape of every day is measured rather than assumed, a cloudy morning that clears at two in the afternoon is modelled as exactly that. Panels fade half a percent a year. Assumes an unshaded roof.
Household use. The rest of the house takes a typical daily shape set by who lives there and whether anyone is home on a weekday, with a separate shape for real Saturdays and Sundays. Heating and cooling are built day by day from the measured temperature: cooling for every degree the afternoon tops 25° (plus a little for a warm night), heating for every degree the daily mean sits below 18°, at a heat pump’s or a bar heater’s rate, with cooling landing in the afternoon and evening and heating at breakfast and dusk. Hot water is so much a day per household, more in winter, reheating overnight. A car and a pool sit on top of the typical figure. It is a typical home, not your own meter data.
Appliances. Each is a real load with a power draw and a run length, not a lump of energy: a dishwasher is 1.8 kW for 45 minutes. Every load the home has is booked into the day, in the sun or not, so the “before” evening genuinely contains the dishwasher. Discrete cycles chasing the sun are placed on the sunniest days of each week, against a budget that empties as they are booked; car charging and pool pumps fill the best hours of the best days; pre-cooling brings 40% of a hot day’s cooling into the afternoon. An oven or a dryer in a house that is empty on weekdays can only chase the sun at the weekend. Hot water is never moved, because nobody chooses when a tank reheats. Within a day the runs are laid down biggest first into whatever surplus is left, so they genuinely compete. A run the surplus cannot cover still happens – you buy the shortfall – and nothing is ever moved to 3am. Because nobody follows a plan every day, only 85% of the planned moves are counted.
Money. Imports are priced hourly, flat or time of use, exports at the feed-in rate, and the whole tariff rises together each year. The battery is dispatched hourly with a C/2 inverter limit and fades each year; a payback beyond its fifteen-year life reads as never. Nothing is discounted to today’s dollars.
If the roof is still bare. Answer “not yet” to the solar question and the comparison changes shape: the thing every path is measured against becomes a bill with no solar in it at all, the array’s installed cost sits in front of all four paths, and putting the panels up and changing nothing else becomes a path in its own right – the floor a battery has to beat. The battery is then judged only on what it adds once the array is already earning, so it is never credited with the sunshine. Install prices are pre-filled from a typical 2026 per-kW rate for your market and a per-kWh rate for the battery, after any rebate that is standard there; both are starting points, and typing over either one stops the tool touching it again.
What it leaves out. Supply charges, demand tariffs, controlled-load rates, wholesale pricing, virtual power plant payments, charging the battery from the grid overnight, the value of backup during an outage, finance costs and one-off subsidies, any of which can move the answer. Hourly resolution overstates slightly how much of a spiky load the sun covers. The market and currency are picked from the timezone of the pin you drop, and its cost defaults are broad typical figures rather than quotes; drop the pin where we hold no cost data and the tool says so instead of guessing.
Data sources. Historical weather from Open-Meteo, licensed CC BY 4.0 and derived from the ERA5 reanalysis produced by ECMWF for the Copernicus Climate Change Service. Place search by Open-Meteo and Nominatim, using © OpenStreetMap data. Map tiles © Esri. Simulation and appliance artwork by OnSun.
A battery and a well-timed dishwasher earn their money exactly the same way. Both take a kilowatt-hour that would have gone out to the grid for the feed-in rate and spend it at home instead of buying one. The value of that swap is the gap between what you pay and what you are paid, and nothing else. What separates the two paths is what you had to buy to get there.
That is why the third line matters. Shifting takes the easy kilowatt-hours first: the ones sitting in the middle of a sunny day, exactly where a battery would have been charging. Run the free path first and the battery is left with the harder, thinner surplus, so its payback stretches. Any comparison that leaves out the free option flatters the battery.
The grey days do the rest of the work in this model. A run of overcast weather caps both paths at once: the battery has nothing to charge from, and the pool pump has no surplus to soak up. Because the simulation uses real measured days for your point rather than an annual average, those runs show up where they really happened. And when the sun does not stretch to cover a whole cycle, the cycle still runs — you just buy the shortfall. That partial coverage is priced in, which is why the headline figure is the share of your appliance energy your own roof actually carried, not a claim that everything moved.
If there are no panels on the roof yet, say so and the whole comparison re-frames itself around the decision that actually matters. The reference stops being “your bill with solar” and becomes your bill with none, the array’s installed cost goes in front of every option, and a fourth line joins the chart: the panels on their own. That line is the floor. A battery has to beat it, and it is judged only on what it adds once the array is already earning — which is the number a combined quote almost never separates out.
Everything runs on your own device. When you drop the pin, the page asks the free Open-Meteo historical archive for five years of hourly solar radiation and temperature at those coordinates, and composes them into a typical year. That is the only thing that leaves your browser; your tariff, your appliances and your bill never do.
From there the model takes the measured direct and diffuse radiation for each hour, projects both onto your roof plane in local clock time, and derates for cell temperature. Then it dispatches the battery hourly, moves your ticked appliances into whatever surplus that particular day produced, and repeats the whole year for every year of the horizon with the battery fading and the grid price climbing.
The demand side gets the same treatment. Who lives there and whether anyone is home on a weekday set the shape of the day for the rest of the house, because a house that is empty from eight until six exports almost everything its roof makes at noon while a house with someone in it does not. Weekends get their own later-starting, home-all-day shape. Heating, cooling and hot water are then built up from the measured temperature rather than guessed: cooling grows with every degree the afternoon maximum tops 25° (the daily mean hides a 35° afternoon behind cool nights, which is why it is not used) and lands in the afternoon and evening, heating with every degree the daily mean sits below 18° and lands at breakfast and dusk, and a tank of hot water costs more to heat in winter. Appliances are modelled as real loads rather than lumps of energy. A dishwasher is 1.8 kW for 45 minutes; a 7 kW car charger is 7 kW for as long as you plug it in. Cycles chasing the sun land on the sunniest days of each week, chosen against a budget that empties as each one is booked, so the model does not have you doing the dishes, the laundry and the car on the same afternoon. Within a day, runs are placed biggest first into whatever surplus is left, which is what makes them compete: once the car has taken the middle of the day, the dishwasher is choosing from what remains. Only habits count: hot water is in the picture but is never moved, because nobody chooses when a tank reheats, and only 85% of the planned moves are credited, because nobody follows a plan every day.
For the reasoning behind the comparison, read is a home battery worth it on the Australian east coast and feed-in tariffs vs self-consumption. To find the best hours to run each appliance, see the best time to run appliances with solar, or check what your roof can produce in the first place with the solar output calculator.
OnSun builds a personalised production forecast for your exact system from real weather models and tells you the best time to run each appliance. The 3-day forecast is free; OnSun Pro unlocks the full week.