Headroom (off-grid)

Where the number comes from

The simulator steps through the day at the chosen interval. At each step, surplus solar (anything not needed for live load) charges the battery up to its usable capacity. Once the battery is full, any remaining surplus has nowhere to go — there’s no grid to export to — so it’s curtailed and the inverter throttles back. Headroom totals up that curtailed energy across the day.

A headroom day means “the panels could have delivered more than your battery + load could absorb”. Bigger battery captures more of it; so does running heavy appliances during the daytime window.

What to do with a headroom day

Headroom is the cheapest electricity you’ll ever have access to off-grid — the panels are running anyway, the inverter’s on, and any extra load you flip on directly substitutes for what would have been curtailed. Practical uses:

For caravan / rv setups

• Top up phones, laptops, cameras, drone batteries, cordless tools.
• Run the air-conditioning a degree colder than usual, or earlier in the day, to bank cool inside the rig before evening.
• Cook a hot meal on the induction cooktop instead of saving the battery for it later.
• Charge a portable power station or a spare lithium battery you carry for shoulder weather.
• Run a 12 V or inverter-fed kettle for tea or coffee without thinking about it.

For off-grid home setups

• Run the washing machine, dishwasher, or any other high-power appliance you’ve been putting off.
• Pre-heat water in an electric storage cylinder (“solar dump” mode if your cylinder supports it).
• Run workshop tools — table saw, planer, welder, compressor — while the headroom is there.
• Pre-charge spare battery banks or off-the-shelf tools for use later in the week.

When there’s no headroom

A row with a dash (—) in this column means the day delivered exactly enough solar to cover load and refill the battery — no surplus, no shortfall. These are neutral days; treat them as routine. If most of your forecast days look like this, your system is well-sized for the load. If you’re seeing many headroom days back-to-back, you have spare capacity to add an appliance or two.