Average daily use can hide an overnight power shortfall. This tool does not check legal stay limits, road access, weather, propane, food, safe temperatures or essential-power reliability.
Illustrative example loaded. These values are not recommended allowances or battery settings. Replace them with your own measurements and manufacturer guidance, then calculate.
Your resource estimate
| Resource | Available above reserve | Daily use / net draw | Modeled duration |
|---|
Monitor actual levels and depart early enough to reach your confirmed services. A displayed “day” means 24 hours of averaged use, not a guaranteed bookable night.
How the calculation works
| Resource | Formula |
|---|---|
| Fresh water | (Usable water onboard − reserve) ÷ total household gallons per day. |
| Each holding tank | (Capacity − starting contents − reserved headroom) ÷ that tank’s daily inflow. |
| Battery above floor | Bank Ah × nominal volts × (starting charge − departure floor) ÷ 100. |
| Battery-side daily load | DC Wh + AC Wh ÷ (inverter efficiency ÷ 100). |
| Battery duration | Available battery Wh ÷ (battery-side daily load − delivered charging Wh), when that net draw is positive. |
The smallest modeled duration is the first estimated resource threshold. Complete days are rounded down; decimal days are displayed to one decimal place. Very close limits can tie. The no-charging comparison always divides available battery energy by the full daily load.
Enter values consistently
- U.S. gallons: do not enter Imperial gallons or liters. One U.S. gallon is approximately 3.785 liters.
- Whole household: daily use already includes everyone. Do not multiply it again.
- Battery bank: use the installed bank’s Ah and voltage, with a floor appropriate to your equipment. This is a simplified nominal-energy model, not a battery-chemistry simulation.
- No double counting: if a measured battery-side energy total already includes AC equipment and conversion losses, enter it as DC and set AC to zero.
- Charging: use delivered battery energy after charging losses. The model does not account for time of day, battery charge acceptance or wasted excess generation.
- Separate tank plumbing: do not combine two gray tanks if either can fill first independently. Run separate scenarios using each tank and its own inflow.
What zero means
Zero daily gray or black inflow excludes that tank from the duration minimum; it is not a prediction that you will never need waste services. Zero charging means no energy-replenishment credit. If charging equals or exceeds daily demand, the battery depletion date is not modeled; the tool still shows battery energy and the no-charging comparison.
If you use a cassette or portable toilet, account for its capacity and required servicing separately unless you can represent it consistently in the black-tank fields. Propane and other consumables remain separate limits.
Try the example, then stress-test your own plan
The example has 32 gallons above the fresh-water reserve and uses 8 per day: four days. Gray tank space allows five days and black tank space ten. Battery energy above the chosen floor is 1,024 Wh; a 400 Wh battery-side daily load minus 100 Wh delivered charging yields about 3.4 days. With no charging, the battery estimate drops to about 2.6 days. These are invented arithmetic inputs, not usage recommendations.
For your own plan, rerun with no charging, greater water use or a lower starting charge. If the trip only works under ideal assumptions, shorten it or arrange verified services. A charging surplus cannot establish that the battery will bridge a long cold night.
Privacy and keeping a copy
Calculations run in this page. This package sends no inputs to a server and does not save them in browser storage. Reloading or leaving the page loses your entries. Print after calculating if you want a record; browser print settings may affect the layout.
Read the full beginner guide · Return to the boondocking hub