How to Build a Real Boondocking Energy Budget, Not a Guess

Building a real budget means listing every load that runs off the battery in a typical day, then multiplying its amps by the hours it actually runs, not the hours it is merely turned on. A 12V water pump drawing 6 amps that runs for a total of 15 minutes a day across several short bursts uses 1.5 amp-hours, not 6 amps times 24 hours. Interior lighting drawing 2.5 amps used for four hours in the evening uses 10 amp-hours. Add every load this way, from the furnace blower to a laptop charger to a CPAP machine overnight, and the sum is the daily amp-hour budget the rest of this site's calculators are built around. The daily energy budget calculator does this same arithmetic once you have the amps and hours for each load, which is faster than a spreadsheet once the list is built, but building the list by hand once is what teaches you which loads actually matter.

Because almost nothing in an RV runs continuously at its full running wattage. A 12V compressor fridge rated at 45 running watts might only run its compressor about a third of the day, cycling on and off to hold temperature, so its real daily draw is closer to a third of what a naive 24-hour calculation would suggest. A furnace blower only runs while the thermostat calls for heat, not all night. Even lighting is rarely on for a full 24 hours. Treating a nameplate running wattage as a constant, all-day draw is the single most common reason a paper budget comes in far higher than what a shunt monitor actually measures over a real trip. The fix is not a different formula, it is honesty about actual hours of use, which is exactly what a few days of shunt monitor data gives you that a nameplate rating cannot.

Every AC load run off an inverter has to be converted to DC amp-hours before it belongs in the same budget as the 12V loads, and skipping the inverter's efficiency loss in that conversion is a common error. The conversion is watts divided by inverter efficiency divided by system voltage: a 1,000 watt AC load through an inverter running at a typical 85 percent efficiency pulls about 98 amps from a 12V bank, not the 83 amps a naive watts-over-volts calculation would suggest. That gap compounds fast on anything run for multiple hours, like a residential fridge's compressor cycles or a CPAP with a heated humidifier. The inverter sizing calculator on this site applies this same conversion when sizing the inverter itself, and the same math belongs in the daily budget for any load that passes through it.

A worked example makes the duty-cycle point concrete. The figures below use the published typical draws from the appliance power draw reference on this site, with duty cycles applied to loads that cycle rather than run continuously, which is what separates a real budget from a running-watts total.

Once you have a daily amp-hour figure, it becomes the input to two more calculations, not the final answer by itself. Multiply it by the number of days you want to camp without recharging, apply a depth of discharge convention for your battery chemistry, and the battery bank calculator turns that into a nominal bank size, covered in full in the battery bank sizing guide below. Separately, dividing the same daily figure by your local peak sun hours and a system loss convention gives a solar array size on the solar array calculator, and dividing a nominal bank by a steady draw gives an estimated run time on the battery run time calculator. All three calculators start from the same number built in this guide, which is why getting the daily figure right matters more than which formula you apply to it afterward.

Yes, and heating is usually the biggest swing. A furnace blower alone adds a moderate draw, but running electric space heaters instead of propane heat multiplies the daily budget several times over, since a space heater on its low setting alone draws close to what an entire evening of lighting uses. Cold weather also increases fridge compressor run time and can add a self-heating lithium battery's own internal heating draw if the pack is charging near freezing. Building one budget for typical weather and a second, higher budget for cold-weather camping avoids the surprise of a bank that worked fine in warm weather coming up short on a cold week. The propane versus electric heat calculator on this site is the fastest way to check whether a specific heating choice belongs in the electric budget at all or is better left on propane.

Sizing the budget around what a rig could theoretically draw at once, rather than what it actually draws across a full day. A rig with a residential fridge, a CPAP, an inverter and full lighting could theoretically draw well over 20 amps simultaneously for a moment, but almost never does so for more than a few minutes at a time. Basing a battery bank on that theoretical peak instead of the measured daily total produces a bank two or three times larger, and more expensive, than the actual usage requires. The corrective is the same one used throughout this guide: measure amps and hours per load, apply real duty cycles, and let a shunt monitor confirm the total over a few real days before committing to a bank size the theoretical peak would have oversized.

A calculator can size a bank from a number you supply, but only a shunt monitor tells you whether that number was right after a few real days off grid. Buying one before or right after a boondocking trip turns next season's budget from a guess into a measurement.