How to Build an RV Power Budget in Watt-Hours

A power budget is one number: the watt-hours you use in a day. Everything downstream depends on it, because both the array and the battery bank are calculated straight from it, and a budget that is wrong by 40% produces a system that is wrong by 40%.

The arithmetic is watts times hours per day, added up. The hard part is not the math. It is being honest about the hours, and remembering the loads that never occur to anyone.

Start with the numbers you can look up

Two loads have published specifications, and between them they usually account for a third of a small rig’s consumption.

The fridge. Dometic’s CFX3 45, a 46 litre portable compressor fridge, is specified at 1.03 amp-hours per hour on 12V, measured at 90F ambient with the box set to 39F. Over 24 hours that is 24.7Ah, or roughly 297 Wh at 12V nominal.

Read the conditions on that figure, because they matter. 90F ambient is a hot day, so a cooler week costs less. But Dometic publishes the one number, and we are not going to interpolate a cooler-weather figure and present it as a spec. Bigger fridges and built-in RV units draw more, and we have not verified a published figure for a specific large unit, so use your own model’s documentation rather than scaling this one.

The inverter, doing nothing. Victron rates the MultiPlus 12/1200 at 10W zero-load power, or 3W in search mode. Switched on and idle for 24 hours, that is 240 Wh a day. Your inverter can be the second largest load in the rig while powering nothing at all.

Those two alone are 537 Wh a day before you charge a single phone.

The inverter costs you twice

The idle draw is the first cost. The conversion is the second. Victron rates the MultiPlus 12/1200 at 93% maximum efficiency, and maximum means at its best operating point, not at every load. Treat 93% as a ceiling.

So 1,000 Wh of AC energy costs your battery at least about 1,075 Wh. Budget the AC side, then add the conversion, then add the idle.

There is a third cost that does not show up in watt-hours at all. Victron’s Gel and AGM documentation notes that effective battery capacity drops faster under a constant power load such as an inverter than under a constant current load. Heavy inverter use is harder on a lead acid bank than the raw watt-hours suggest.

Nameplate is not delivered

The number on the box is a test-bench figure in every part of this system.

The MultiPlus 12/1200 is a “1200VA” inverter. Its rated continuous output is 1,000W at 25C, and 900W at 40C. On a hot afternoon in a closed cabinet, a 1,200 badge delivers 900W.

That gap decides real questions. Our generator sizing tool uses 1,500W as a planning figure for an electric kettle. Five minutes of kettle is only 125 Wh, a small entry in a budget, but a 1,200VA inverter will not run it at all. Budget and capability are separate tests, and a load has to pass both.

The same is true of the panels, where a 200W module gives less than 200W on a hot roof: how much solar you need goes through that arithmetic.

Building the list

Work down the rig, item by item, and for each one write watts, hours per day, and where the number came from. That last column is the one that keeps you honest.

• 12V loads. Fridge, water pump, vent fan, lights, furnace fan, USB charging. If the spec is in amps, watts is amps times 12. These are your steady, all-day, unglamorous consumption.
• AC loads through the inverter. Laptops, induction hob, microwave, coffee, power tools. Multiply watts by the fraction of an hour they actually run, then divide by 0.93 for the conversion.
• The inverter itself. Hours switched on, times 10W, or 3W if you use search mode. This is the line everyone leaves out.
• Connectivity. A cellular booster or a satellite dish runs for hours and is not a small load. We have not verified published draw figures for those, so use the manufacturer’s, and see internet while boondocking for the wider tradeoff.

For AC appliance figures our generator sizing tool lists typical mid-range values for a microwave, coffee maker, toaster, kettle, and RV air conditioner. Those are planning figures, not datasheet values. Your appliance’s own label is the real number, and label figures vary widely within the same category.

A worked budget

Two verified lines and one estimate, for a van with a portable fridge:

Load	Figure	Source
Dometic CFX3 45 fridge	297 Wh	Dometic specification, at 90F ambient
Inverter idle, 24 hours at 10W	240 Wh	Victron MultiPlus 12/1200 specification
Everything else	you measure it	your labels, your hours
Running total before “everything else”	537 Wh

Switch the inverter off when it is not in use and the total drops to about 300 Wh, close to half. That is the single cheapest change available in this whole guide, and it costs nothing.

For scale, our solar calculator defaults to 1,500 Wh a day, which is a planning figure for a larger rig with an inverter in regular use, a bigger fridge, and real electronics. We have not verified a measured whole-rig figure, and you should not treat that default as one.

Then measure it

Everything above is an estimate, and estimates in this category run low. The honest method is a battery monitor with a shunt, installed on the negative side of the bank, read after a week of camping the way you actually camp. It will tell you your real daily consumption, including the loads you forgot, and it costs a fraction of the solar you would otherwise buy to cover your own guess.

Once you have that number, feed it into the solar calculator for the array, and into battery bank sizing for the storage. If the answer needs more roof than you have, the fix is almost always to cut the load rather than to buy more panel.

Planning estimates, not a system design. Have anything you intend to wire permanently checked by a qualified installer.