Off-Grid Load Calculator — Appliance Power Audit

How a Load Audit Works

A load audit is the foundation of every off-grid design. Before you can size a single panel, battery, or inverter, you need an honest answer to one question: how many watt-hours do you burn in a typical day? Get that number right and the rest of the system falls into place. Get it wrong and you either run out of power at 2 a.m. or pay thousands of dollars for capacity you never use.

The math for a single device is simple: watts × hours per day × quantity = watt-hours per day. A 60-watt fridge that effectively runs 8 hours out of 24 uses 60 × 8 = 480 Wh/day. A 50-watt laptop used 6 hours uses 300 Wh/day. The calculator above does this for every row and adds them up, so you never have to reach for a spreadsheet.

The reason a dedicated tool beats a generic calculator is the preset library. Most beginners forget loads — the router that runs 24/7, the inverter's own idle draw, the well pump that cycles all day. Our categorized picker (Kitchen, Comfort, Work & Connectivity, Lights & Misc, Water) puts realistic default wattages and run-times in front of you so you build a complete list instead of a hopeful one. Every value is editable; the defaults are just a sane starting point drawn from manufacturer datasheets and field-typical duty cycles.

Running Watts vs Peak Watts vs Surge Watts

These three numbers trip up almost everyone, and confusing them is the fastest way to buy the wrong inverter. Here is the distinction in one sentence each.

• Running watts is the steady power a device draws once it is up and operating — the number on its nameplate or what a Kill A Watt meter reads after a minute.
• Peak (continuous) watts is the largest sum of running watts that realistically happens when several devices operate at the same time. This is what your inverter must supply continuously.
• Surge watts is the brief inrush a motor demands at the instant it starts — typically 2× to 7× its running watts for a fridge compressor, well pump, air conditioner, or power tool. It lasts a fraction of a second but the inverter must not fault during it.

The calculator models peak realistically: it takes the single biggest load running at full power and adds half of everything else, because in real life you almost never run every appliance simultaneously. For surge, it adds the single largest startup inrush on top of the running peak — the worst case is one big motor starting while the rest of the house is already drawing power. That is exactly the moment a cheap inverter shuts off.

Duty Cycles: Why a 60-Watt Fridge Uses 480 Wh a Day

A refrigerator's compressor does not run continuously. It cycles on to pull the temperature down, then shuts off until the box warms back up. Over 24 hours, a modern efficient fridge runs its compressor roughly 30 to 40 percent of the time — call it 8 effective hours at full draw. That is why a 60-watt fridge shows up as about 60 W × 8 h = 480 Wh/day rather than 60 × 24 = 1,440 Wh.

This "effective hours" trick is how you enter any cycling load: chest freezers, well pumps, mini-splits, and battery chargers all run part-time. Instead of guessing an on/off percentage, estimate the equivalent hours at full power. A chest freezer in a warm room might run 8 to 10 effective hours; the same freezer in a cool basement might run 5. Hot climates, frequent door openings, and a packed freezer all push the duty cycle up. When in doubt, measure with a Kill A Watt meter for a few days and divide the total Wh by the device's running watts to back out its true effective hours.

For deep detail on refrigerator and freezer power draw across climates and box sizes, see our off-grid refrigeration and appliance guide.

Phantom Loads — The Hidden Budget Killer

Phantom or parasitic loads are devices that draw power 24 hours a day, every day, whether you are using them or not. The usual suspects:

• Inverter idle draw — even with nothing plugged in, a typical inverter burns 10 to 40 W just staying on. Over a day that is 240 to 960 Wh.
• Starlink dish — the standard dish averages 45 to 75 W continuously, or roughly 1,080 to 1,800 Wh/day. The Starlink Mini is far gentler at 20 to 40 W.
• Router, modem, mesh nodes — 10 to 25 W, all day.
• Clocks, smart hubs, chargers left plugged in — a watt here and there that adds up to real energy over 24 hours.

Add it up and a constant 30 to 80 W of phantom load is normal, which is 720 to 1,920 Wh/day before you turn anything on. On many off-grid systems this is the single largest line item, larger than the fridge. That is exactly why the calculator above defaults these loads to 24-hour run-times: so they show up at their true daily cost instead of being quietly forgotten. If you can put the inverter into a low-idle or sleep mode overnight, or downgrade a standard Starlink to a Mini, you can often cut your battery bank by a full kilowatt-hour.

Worked Example: A Real Cabin Audit

Suppose a two-person cabin runs: an efficient fridge (60 W, 8 effective hours), LED lighting totaling 40 W for 5 hours, a 50-W laptop for 6 hours, a standard Starlink dish (45 W, 24 hours), and two 10-W phone chargers for 4 hours each. Row by row:

This cabin uses 2,140 Wh/day. Its running loads are 60, 40, 50, 45, and 20 W; the diversity model gives a peak of about 138 W (the 60 W fridge plus half of the other 155 W). Because none of these loads is a motor, surge watts equals peak watts at 138 W, so even a modest 300 VA inverter handles it — though most owners fit a 1,000 to 2,000 VA unit so they can occasionally run a microwave or coffee maker. Notice the single biggest number on the list is not the fridge or the laptop: it is Starlink at 1,080 Wh, more than half the entire budget. That is the phantom-load lesson in one row.

Feed that 2,140 Wh into the step-by-step system sizing guide or the system calculator and you get panel wattage, battery amp-hours, charge-controller amps, and inverter VA in seconds. The "Send to" buttons above pass your daily Wh automatically.

Appliance Power Reference Table

These are field-typical figures compiled from manufacturer datasheets. Running watts vary by model; surge multipliers apply only to motor and compressor loads. Use them as starting points and measure where you can.

For pump-specific sizing including surge and soft-start options, see the solar-powered well pump guide.

Measure, Don't Guess — Gear That Matches the Calculator

The single best upgrade to any load audit is real measurement. A plug-in energy meter on each major device for a week turns the estimates above into airtight numbers, and once you know your peak and surge watts you can buy the right inverter the first time. Here are the parts that pair with this tool.

These are category searches, not specific product endorsements — pick the model whose continuous and surge ratings clear your numbers above. As an Amazon Associate we may earn from qualifying purchases; this does not affect the figures in this reference.

What to Do With Your Number

Your daily Wh total is the input for every other sizing decision. Once you have it:

• Size the whole system: feed the Wh into the off-grid solar calculator to get panels, batteries, controller, and inverter at once. Use the "Send to System Calculator" button to carry your number over.
• Size the battery bank: pick autonomy days and depth of discharge in the battery bank calculator (the "Send to Battery Bank Calculator" button passes your Wh).
• Learn the full method: the how to size an off-grid solar system guide walks through every step by hand so you understand what the calculators are doing.

One honest caveat: always pad your final number. Add 15 to 25 percent on top of your audit for the loads you forgot, the appliances you will add later, and the bad-weather weeks. An off-grid system designed exactly to today's measured load will feel cramped within a year.

Frequently Asked Questions