How Much Solar Do You Actually Need for an Off-Grid Start?

Most people size their first off-grid solar system by starting with a number that doesn't apply: their existing grid electricity bill. That number describes a life built around unlimited power. It tells you nothing about what you actually need to run off a finite battery.

Start with a load budget, not a bill

List every device you actually plan to run, its wattage, and how many hours a day it runs. Multiply watts by hours to get watt-hours per device, then add them up. A laptop, a few LED lights, a router and a small fridge is a genuinely different number from that same list plus a well pump, power tools and a chest freezer — and the difference is what should drive every decision that follows, not a bill that assumes grid convenience.

Be honest about which loads are daily and which are occasional. A power tool used for two hours once a week doesn't belong in your daily budget; it belongs in a separate "occasional load" line that you cover with headroom, not baseline capacity.

Three numbers you're actually solving for

Panel wattage has to produce your daily watt-hour budget in your worst realistic sun-hours day, not your best one. Four hours of usable sun in winter is a common planning assumption in temperate climates — check the number for your actual site rather than assuming a national average.

Battery capacity has to carry you through the load budget on days when the panels don't produce, with margin. A single very sunny day and a single overcast day are not the same battery-sizing problem; size for a run of cloudy days, not the average.

Inverter capacity has to handle your highest simultaneous load, not your daily total. A fridge compressor and a power tool starting at the same moment draws far more than either running alone — inverter sizing is about peak surge, not average draw.

The two mistakes that show up in year one

The first is undersizing the battery relative to the panels. A big array on a small battery just means you throw away midday power you can't store and still run dry every evening.

The second is ignoring seasonal production swing. A system sized around summer sun-hours can fall badly short in winter, exactly when heating and lighting loads are highest. Size for the worst season you'll actually rely on the system in, not the best one.

A sequencing note

If a grid connection is available at any price, most people are better off treating solar as resilience — communications, lighting, small loads — before scaling up to a fully isolated system. Grid-as-backup buys time to actually measure your real consumption before committing capital to a battery bank sized on guesswork.