How We Calculate

Every number on this site comes from straightforward, published electrical formulas applied to the inputs you provide. Nothing is hidden — this page documents exactly what each calculator does, the defaults it uses, and where the real-world limits of that math are.

Daily energy use (Wh/day)

For each appliance: watts × hours per day × quantity = watt-hours per day (Wh/day). For appliances that run through an inverter (AC loads), we divide this figure by an efficiency percentage (default 85%) to approximate the extra energy an inverter draws from the battery due to DC-to-AC conversion losses. DC (12V) appliances are not adjusted, since they typically bypass the inverter.

Trip energy and reserve

Daily Wh is multiplied by your number of days to get trip energy. We then add a reserve/safety buffer (default 20%, adjustable from 10–30% or a custom value) on top, to account for battery aging, temperature effects, and the gap between estimated and actual appliance consumption. The result is the recommended rated capacity shown in your results.

Rated vs usable capacity

A battery's rated (nameplate) capacity is not the same as its usable capacity. We apply a typical depth-of-discharge (DoD) factor based on chemistry — around 95% for LiFePO4, and roughly 50% for lead-acid types (AGM, gel, flooded) — to estimate usable capacity. These are typical figures, not guarantees for any specific product; always check a manufacturer's own usable capacity claim where available.

Inverter sizing

Recommended continuous output is your highest expected simultaneous AC load, plus a safety margin (default 20%). Surge/starting requirement estimates account for the fact that motors and compressors can draw substantially more than their running wattage for a brief moment at startup — this is an estimate, and you should check the specific surge rating of any motor-driven appliance where possible.

Solar sizing

Recommended solar wattage = daily Wh ÷ (peak sun hours × system efficiency). Default assumptions are 4.5 peak sun hours and 80% system efficiency, both adjustable. A solar panel's rated wattage is a lab figure under ideal conditions — real-world output is lower and varies constantly with weather, shading, angle, dust and temperature, which is why results are shown as a practical range rather than a single number.

Battery runtime

Runtime = usable capacity ÷ effective load, where effective load accounts for inverter efficiency on AC loads. This assumes a roughly constant load — for loads that cycle (like a compressor fridge), we use an average draw over 24 hours rather than the peak/running wattage.

Off-grid autonomy (days)

Days = usable battery capacity ÷ (daily use − daily solar generation). If solar generation meets or exceeds daily use, autonomy is treated as effectively indefinite for planning purposes, since the battery is no longer being net-depleted.

Limitations — please read this

• These are planning estimates based on the inputs you provide, not guarantees.
• Appliance wattages are typical estimates unless you enter a measured or nameplate-accurate figure.
• Real-world battery performance varies with temperature, age, and manufacturing tolerances.
• Solar generation varies constantly with weather and is inherently unpredictable day to day.
• This site does not provide electrical installation advice. Permanent installations — especially anything involving mains AC wiring in a caravan, RV or cabin — may require a qualified, appropriately licensed professional depending on your jurisdiction.