How Many Watts Do You Actually Need Off-Grid? A Load-Calculation Guide

The question cannot be answered as asked, and the reason matters.

Watts measure a rate — how fast energy is being used at a given instant. Asking how many watts you need off-grid is like asking how many miles per hour you need to drive to Denver. The honest answer is that it depends entirely on how long you intend to drive, and the thing you actually want to know is the distance.

The distance, in electrical terms, is watt-hours per day. That is the number that sizes your battery bank and your solar array. Almost every off-grid system that disappoints its owner was sized on the wrong number, and almost every one that works was sized on this one.

This guide walks the calculation properly. It takes about twenty minutes with a notepad, and it is the single highest-return twenty minutes in the entire build.

The three numbers you actually need

There is no single figure that describes an off-grid electrical system. There are three, they are independent of one another, and each one sizes a different component.

1. Daily energy — watt-hours per day (Wh/day). The total energy your loads consume over 24 hours. This sizes the battery bank and the solar array. It is the number this guide is mostly about.

2. Continuous power — watts (W). The largest total draw that will be running simultaneously for a sustained period. This sizes the inverter’s continuous rating, the main DC cabling and the fuses.

3. Surge power — watts, for a few seconds. The momentary spike when a compressor, pump or motor starts. This sizes the inverter’s surge rating and nothing else, but it is what causes systems to trip.

These fail differently, which is why they need to be tracked separately. Size on daily energy but ignore surge, and the inverter will shut down every time the well pump kicks in. Size on surge but ignore daily energy, and you will have a magnificent 3,000W inverter connected to a battery bank that is flat by three in the morning.

Most people, if they calculate anything at all, calculate the second number. It is the least useful of the three.

Step one: build the load table

Every load in the system gets a row. For each one you need three things: the power it draws while it is actually running, how many hours per day it runs, and how many of them you have.

Wh/day = running watts × hours per day × quantity

Sum the column. That total is your daily energy budget. That is the whole method.

The difficulty is not the arithmetic. It is that two of the three inputs are routinely wrong.

The duty-cycle trap

Anything with a thermostat does not run continuously. A refrigerator, a freezer, a DC air conditioner, a water heater and a furnace all cycle on and off to hold a setpoint. The fraction of time the compressor or burner is actually running is the duty cycle, and it is the most commonly ignored figure in off-grid planning.

A 12V compressor fridge draws roughly 45–60W while the compressor runs. If you enter that as “50W × 24 hours” you get 1,200 Wh/day and conclude you need a battery bank the size of a suitcase. In practice the compressor runs perhaps 30–40% of the time in mild conditions, giving 360–480 Wh/day. That is a 3x error, and it is an error in the expensive direction.

The trap runs the other way in heat. Duty cycles on the same fridge climb to 50–70% at 90°F ambient, and higher again if it is installed in a badly ventilated cabinet with no airflow behind it. Owners who have metered their installations consistently report that improving ventilation around the unit drops the duty cycle by ten to twenty percentage points — free capacity, no hardware.

So: use running watts and a duty-cycle percentage for thermostatic loads. Use running watts and honest hours for everything else. And plan for the hot-weather number, not the mild-weather one, because the hot week is when you will care.

The load nobody counts

Before the table, the awkward one. Every off-grid system has a baseline parasitic draw that runs 24 hours a day and appears on nobody’s spreadsheet: propane and carbon monoxide detectors, the stereo’s standby circuit, a battery monitor, a control board, a fridge’s electronics.

Metered installations typically show 0.25–0.5A of constant background draw, which is 70–150 Wh/day before you have turned anything on.

Then there is the inverter. A mid-size inverter left switched on but with nothing plugged in still consumes 10–25W simply to exist. Over 24 hours that is 240–600 Wh/day — potentially more than the refrigerator. If your inverter has a search or standby mode, use it. If it does not, get in the habit of switching it off, or accept that a meaningful share of your array exists to power an empty inverter.

Realistic power draw for common off-grid loads

Running watts below are typical measured figures, not manufacturer optimism. Where a load is thermostatic, the duty cycle is given instead of hours.

The satellite dish rows deserve a note. Connectivity is now, for a lot of people, the largest single load in the system after refrigeration — and it is the one load that runs 24 hours a day at full duty. The July 2026 V5 hardware roughly halved what the standard dish draws, which changes the arithmetic materially for anyone still working from a guide written before it shipped. If a permanently-connected dish is in your plan, the sizing walkthrough for running Starlink off-grid covers the specifics, including the startup current spike that catches out first-time DC installs.

Three worked examples

The weekender

Two people, three nights, spring and autumn, no air conditioning, no permanent internet.

Under 800 Wh/day is a modest system. A single 100Ah lithium battery holds 1,280 Wh, of which roughly 1,024 Wh is usable — a day and a bit of autonomy with no charging at all. Around 300W of solar covers this comfortably outside winter.

The full-timer working remotely

Two people, permanent occupation, satellite internet running continuously, cooking mostly on propane.

Note what happened: the internet connection is 30% of the budget, and the refrigerator is 22%. Everything the owner consciously thinks about — laptops, lights, kettle — is the minority.

The rural cabin

Small permanent structure, well pump, domestic appliances, no air conditioning, inverter running continuously.

Turning watt-hours into a battery bank

Three adjustments stand between your daily total and the battery capacity you need to buy.

Inverter losses. Anything running on 120V AC passes through the inverter at roughly 85–90% efficiency. Add 12–15% to the AC portion of your load.

Usable capacity. You cannot use all of a battery’s nameplate capacity. Lithium iron phosphate is generally worked to about 80% depth of discharge; lead-acid chemistries to about 50% if you want them to last. That means a 100Ah lithium battery gives you around 1,024 usable Wh and a 100Ah AGM around 600 usable Wh — a distinction that dominates the real cost comparison and is worked through fully in the cost-per-cycle analysis of LiFePO4, AGM and lead-acid.

Days of autonomy. How long the bank must carry the load with no meaningful charging. One day is optimistic almost everywhere. Two is a sensible default. Three or more starts to cost more than simply adding panels.

Bank capacity (Wh) = daily Wh × days of autonomy ÷ usable depth of discharge

The full-timer at 3,160 Wh/day, two days of autonomy, lithium at 80%:

3,160 × 2 ÷ 0.8 = 7,900 Wh — around 620Ah at 12V, or six 100Ah batteries.

That figure surprises people. It is also why serious full-time systems increasingly run at 24V or 48V rather than 12V: the same energy at a quarter of the current, with correspondingly smaller cable. If your calculation lands above roughly 400Ah at 12V, stop and reconsider system voltage before you buy anything. Shoulder-season conditions push this number harder than midsummer does, and sizing a bank for shoulder-season use is a different exercise from sizing one for July.

Turning watt-hours into an array

Array watts = daily Wh ÷ derate factor ÷ peak sun hours

Derate factor. Panels do not produce their rated output in the field. Heat, wiring losses, controller efficiency, dust, imperfect angle and partial shading combine to give roughly 70–80% of nameplate on a good permanent installation, and closer to 65–70% on a flat-mounted vehicle roof that cannot be tilted. Use 0.7 unless you have reason to be optimistic. The reality of what a 400W setup actually runs is an object lesson in what happens when this factor is ignored.

Peak sun hours. The equivalent hours per day of full 1,000 W/m² irradiance at your location. The US national average is around 5.0, but the annual average is nearly useless for off-grid planning because winter figures run 25–50% below it. December averages around 2.4 peak sun hours in Boston, roughly 1.8 in Portland, about 2.7 in Austin and above 5.0 in parts of Arizona.

Take the full-timer’s 3,160 Wh/day at a derate of 0.7:

• At 5.0 peak sun hours (summer, most of the country): 903W of array
• At 3.5 peak sun hours (spring and autumn, mid-latitudes): 1,290W
• At 2.4 peak sun hours (December, northern US): 1,881W

The same load, the same battery bank, and the array requirement doubles between June and December. Which of those numbers you build to is the central sizing decision in any off-grid system, and it is worked through properly in the guide to whether panels or batteries should come first.

If you would rather not do this by hand, the off-grid load calculator runs the same arithmetic with the appliance list preloaded and shows its working.

Five mistakes that wreck load calculations

Using nameplate watts for thermostatic loads. Covered above. It is the single largest source of error and it always errs expensive.

Forgetting the inverter exists. Both its idle draw and its conversion losses. Between them they routinely account for 15–20% of a real system’s consumption.

Sizing on the average day. Your system does not fail on the average day. It fails on the fourth consecutive overcast day in October when the heater has been running. Size for the worst week you intend to be out in.

Ignoring surge entirely. A compressor or pump draws three to five times its running current for a fraction of a second at startup. It does not affect your daily energy figure at all, and it will absolutely trip an inverter chosen on continuous rating alone.

Assuming the previous owner’s numbers. If you have bought a rig with an existing system, the loads have almost certainly changed. Dishes, DC air conditioners and induction cooking have all arrived in this market recently, and every one of them is a step change in consumption.

One thing to sort out before you spend the money

A substantive electrical system — several thousand dollars of batteries, an inverter, roof-mounted panels and a lot of new cable — changes what your vehicle is from an insurer’s point of view. Standard policies frequently exclude owner-installed electrical work outright, and where they do cover it, actual-cash-value settlement on a self-installed system tends to be close to nothing.

This catches people out after a fire or a theft rather than before, which is the wrong order. Sort out insurance for a custom or converted vehicle before the build, not after it. Agreed-value cover is available, it costs less than most people assume, and it requires documentation you will find much easier to assemble while the work is in progress.

Frequently asked questions

How many watts do I need to live off-grid full time? As a daily energy figure rather than a wattage: most full-time setups without air conditioning land between 2,500 and 4,500 Wh per day. Add air conditioning and the figure can triple. The wattage that matters separately is inverter capacity, typically 2,000–3,000W continuous for a system in that range.

How many watt-hours per day does the average RV use? There is no useful average, because the spread runs from around 500 Wh/day for a weekend trailer to over 20,000 Wh/day for a full-time coach running rooftop air conditioning on battery. Any source quoting a single figure has not asked what you own or how you use it.

Do I size the system on running watts or starting watts? Both, for different components. Running watts and hours give you daily energy, which sizes batteries and panels. Starting watts size the inverter’s surge rating. Neither substitutes for the other.

How many batteries do I need for 2,000 watt-hours a day? With lithium at 80% usable depth of discharge and two days of autonomy: 2,000 × 2 ÷ 0.8 = 5,000 Wh, or roughly four 100Ah 12V batteries. With AGM at 50%, the same requirement takes 8,000 Wh of nameplate capacity — around seven 100Ah batteries, and about 450 lbs of them.

Does a bigger inverter use more power? At idle, yes — larger inverters generally have higher standby consumption. Under load the difference is small. Do not buy far more inverter than you need on the assumption that headroom is free, because the idle draw is charged to you 24 hours a day.

Should I measure instead of estimating? If you have an existing system, measure. A shunt-based battery monitor will tell you your true daily consumption in a week and will almost certainly contradict your estimate. Estimation is for systems that do not exist yet.