Why OnSun exists
OnSun exists to help households use more of the solar they already generate. Solar that is not used at home is exported for a few cents a kilowatt-hour and bought back later at full price, so when you run things decides most of the saving.
That is harder than it sounds, because three things are usually unknown: which days will be the good solar days, how much each appliance actually uses, and what running it at the wrong time costs. Without those numbers, "use it during the day" is a rule of thumb you follow on the days you remember.
OnSun supplies the three numbers for your roof and your appliances. It does not switch anything on or off. The change it is after is in behaviour: knowing when to press start, and doing it.
Who built OnSun
Dr Steven Percy is an electrical engineer with a PhD in electrical engineering from the University of Melbourne, and has worked at CSIRO, Swinburne University of Technology and Victoria University. He built OnSun: the forecast pipeline that turns weather-model data into an hour-by-hour production curve for one roof, the scheduling algorithms that fit each appliance's run into the sunniest part of the week, the savings model behind every estimate the app shows, and the appliance load profiles the schedule is built on. He also built the two free calculators on this site, and he writes or models every article on the blog.
The forecast combines three inputs: numerical weather prediction data from national weather services (via Open-Meteo), the sun's exactly known path over a given roof, and the system details you enter (capacity, tilt, orientation and a performance ratio). A physics-based PV model converts expected sunlight into expected kilowatts at 15-minute to hourly resolution, depending on what the data for your region supports. It does not use satellite imagery.
The appliance profiles are calibrated against published energy-label test data from the Australian Energy Rating registry and the EU's EPREL database, so a dishwasher in the schedule draws like a dishwasher, heater spike included, rather than a flat block of power.
OnSun is published by Vireo Energy Pty Ltd, an Australian energy company that works on demand flexibility for large energy users. OnSun applies the same idea to a house with solar panels: use electricity when it is cheapest and cleanest, which for a solar home is while the panels are producing.
How the articles are written and checked
Every article on this site carries a named author and a visible "Updated" date that matches the page's metadata. The rules the articles are written to:
- Primary sources first. Tariffs, rebates and prices come from the regulator, tax authority or statistical agency (IPART, the Essential Services Commission, Ofgem, the EIA, Eurostat, the Bundesnetzagentur), and claims about physics, self-consumption or storage come from peer-reviewed journals, cited with a DOI. Installer and comparison sites are used only for indicative price ranges, and are labelled as such.
- Every figure names its market and its date. A number measured in Germany is not presented as a fact about Sydney or California.
- Charts carry a title, the geography of the data and a source line. Bars with no provenance do not get published.
- Every OnSun savings figure is an estimate, never a guarantee. The weather on the day always gets the final vote.
- Corrections are welcome. If a figure is out of date or a source says something different, email onsun.team@outlook.com. The article is fixed and its date is bumped.
What OnSun is, and what it is not
OnSun is a free app for iOS and Android. It forecasts a home's rooftop solar output up to 7 days ahead and recommends the best times to run flexible appliances such as the washing machine, dishwasher, dryer, EV charger and pool pump, so more of the solar is used at home instead of exported for a few cents. The 3-day forecast and two appliances are free; OnSun Pro unlocks the full week and unlimited appliances. It works with any grid-connected solar system, anywhere in the world.
What it does not do. OnSun never switches an appliance on or off, needs no smart plugs, meter access or inverter integration, and does not manage a home battery. Reminders arrive as notifications, and you press start.
Articles
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Is a Solar Battery Worth It in Australia in 2026? With and Without an EV
At published prices a 13 kWh battery takes a little over nine years to pay for itself on the east coast. Timing appliances is free and worth $200 to $500 a year.
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Solar Panel Output: Summer vs Winter and Cloudy Days
Mid-latitude panels make 2-3x more per day in summer than winter, and 10-25% of normal under heavy cloud. Includes a free estimator for your location.
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How Solar Forecasting Works (and Why It Cuts Your Power Bill)
Weather models, sun geometry and your panel specs become an hour-by-hour production curve, typically within 5-10% on a clear day.
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Feed-in Tariffs vs Self-Consumption: Where Your Solar Savings Really Come From
Exporting earns 4-8c/kWh; buying it back costs 25-40c. Why self-consumption is where the real savings live.
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The Best Time to Run Appliances With Solar Panels
Usually 10am-3pm, but the window moves daily with the weather and your roof. When to run the washing machine, dishwasher, pool pump, EV charger and hot water.
Free tools
- Solar output calculator: a typical summer day, winter day and cloudy day for any location, panel size, tilt and orientation, plus an estimated yearly total.
- Battery vs load shifting calculator: ten years of a home battery against simply running appliances in the sun, simulated hour by hour on a year of measured local weather, in 18 currencies.
Both run in the browser with no sign-up and can be embedded on other sites. All of them are listed on the tools page.
Contact
Email onsun.team@outlook.com, or find OnSun on LinkedIn, YouTube and Instagram. For help with the app itself, start with the support page.
