Sizing an Off-Grid Solar System from Your Actual Load List

Copying someone else’s off-grid setup is the most popular sizing method on the internet and the least reliable one there is. The YouTuber’s cabin runs on 1,200 watts of panels because of their loads, their latitude, and their willingness to ration power on a dark December afternoon — none of which transfers to your site, your appliances, or your tolerance for sitting in the dark. Off-grid sizing done properly starts from a written list of everything you intend to power, and the arithmetic that follows is genuinely simple, nothing beyond multiplication and division. What makes off-grid sizing hard isn’t the math; it’s that the math is unforgiving. A grid-tied homeowner who undersizes simply buys the shortfall from the utility and never notices, but an off-grid system has no such backstop — the grid isn’t there to quietly absorb your estimation errors. If your numbers are wrong, the lights go out, or the generator runs more than you planned, or the batteries get abused into an early grave. So the estimate has to be right, which means it has to start from your own loads rather than someone else’s. What follows is the full chain, worked through with a small-cabin example in which every single number is a labeled assumption you should replace with your own.

Start from a written load list, in watt-hours per day

The foundation of the whole system is a list of every device you’ll run, each converted into watt-hours per day. For each item, multiply its running wattage by the hours per day you actually use it, and you get its daily energy appetite in watt-hours. The wattage figure is where lists go wrong, so get it right: take watts from the nameplate label if you must, but far better is a $20 plug-in power meter, because nameplates state a maximum the device rarely sustains, and real average draws are often much lower. A refrigerator labeled “700 W” refers to the compressor at full tilt, but the compressor cycles on and off through the day, so metered over twenty-four hours that fridge might average only 60 to 100 watts. Sizing to the nameplate would roughly double the fridge’s real budget and cascade into a system far larger and more expensive than you need.

Here’s a sample cabin list, purely for illustration, showing the shape the exercise takes.

Load	Watts (running)	Hours/day	Wh/day
DC fridge	60 (averaged, cycling)	24	1,440
LED lighting (6 fixtures)	60 total	5	300
Laptop + router + phone charging	80	8	640
Water pump	300	0.5	150
Kitchen small appliances	800	0.4	320
Fans (summer) / misc	100	4	400
Total			~3,250 Wh/day

Three rules separate a load list that works from one that leaves you in the dark. The first is to list for your heaviest season, not an average day. If winter means longer hours of lighting and a fan that becomes a furnace blower, then size to that heavier version of the list, because an average-day system fails on the days that are worse than average — which are exactly the days you’ll remember. The second rule is that resistive heat is the budget-killer, and it deserves genuine fear. Electric space heating, electric water heating, and electric cooking can each single-handedly dwarf everything else on the list combined, and most workable off-grid builds move those heat loads to propane or wood instead. That’s not because electricity can’t make heat — it obviously can — but because the panels and batteries required to make heat electrically cost far more than a decade of propane, so it’s simply the wrong tool. The third rule is to include the phantom loads that don’t appear on any appliance: the inverter itself draws somewhere around 10 to 40 watts just idling, and standby draws lurk throughout the system, so add roughly 5 to 10% to your total for that overhead, or your carefully calculated 3,250-watt-hour system will mysteriously run at a daily deficit and slowly drain the bank. Separately from the daily energy total, note your largest simultaneous wattage — the pump kicking on while the kitchen appliances run, for instance — because that peak is a different spec entirely, one that sizes your inverter rather than your panels. The distinction between kW and kWh matters more off-grid than anywhere else, because here you have to size for both the daily energy amount and the instantaneous power peak, and getting them mixed up produces a system that’s either starved for energy or that trips offline when two motors start at once.

The hardest part of an honest load list is the psychology, not the arithmetic. It is genuinely tempting to build the list around the life you wish you’d live off-grid — always careful, always rationing, never running two things at once — rather than the life you’ll actually live once the novelty wears off. People consistently underestimate the loads they’ll add, because a cabin that starts as a weekend retreat quietly accumulates a coffee maker, a bigger fridge, a satellite dish, a space heater “just for the shoulder season,” and each of those lands on a system sized for the original ascetic fantasy. The discipline that separates a list that works from one that leaves you in the dark is to write down what you’ll really use on a normal, slightly indulgent day, not your best-behavior day, and then to revisit it honestly if your plans for the site are drifting from “occasional getaway” toward “somewhere we actually live.” A load list padded with wishful restraint produces a system that’s technically correct for a person who doesn’t exist, and the real occupant pays for the gap in generator runtime and dead batteries. Better to be honest on paper, where adding a few hundred watt-hours costs nothing, than to discover the shortfall on a dark December evening when fixing it means a new panel, a bigger charge controller, and a service call.

From daily watt-hours to panels, batteries, and deliberate margin

With a daily energy number in hand, sizing the array is a matter of dividing by how much sun you can count on — and the word “count on” is doing real work, because off-grid you use the worst month you’ll occupy the site, not the annual average a grid-tied system can safely lean on. Look up peak sun hours for your location and then find the figure for your design month. A site that averages 5.5 sun hours in July may deliver only 2.5 in December, and if you’re there year-round, December is what you design around, full stop. Panels also never deliver their rated wattage into your batteries: temperature losses, wiring resistance, charge-controller efficiency, dust on the glass, and imperfect angles all take a cut, so a common planning derate for battery-based systems is around 20 to 30%. Running the cabin’s 3,250 watt-hours through that math at an assumed 3 worst-month sun hours and a 25% derate: 3,250 divided by 3 gives about 1,083 watts of “perfect” panel, and dividing that by 0.75 to account for losses lands near 1,450 watts of installed panel — call it 1.5 to 1.6 kW. That might be four large residential panels or a larger count of smaller modules, and the panel-count calculator converts a target wattage into an actual number of panels for whatever module size you’re weighing. Notice what latitude just did to the answer: the identical cabin designed around a sunny, 5-sun-hour, summer-only season would need barely half that array. There is no general answer to “how big should an off-grid system be,” only this arithmetic applied to a specific place in a specific month.

Panels cover the daylight; batteries cover the nights and the weather, and they’re sized off a different question — how many sunless days the bank should carry the cabin through, your days of autonomy. Two to three days is a common target for a home someone actually lives in; one day may be acceptable if a generator stands ready as backup, and pushing beyond three days gets expensive fast, buying capacity for rare events that a generator handles far more cheaply. Battery chemistry then sets how much of the bank’s rated capacity you can actually use: lithium iron phosphate banks are commonly run to 80 to 90% depth of discharge, while lead-acid banks last dramatically longer when you keep them above the halfway mark, which effectively doubles the nameplate capacity you have to buy. For the cabin, assuming lithium at 85% usable and 2 days of autonomy, the math is 3,250 watt-hours times 2 days divided by 0.85, which comes to roughly 7,650 watt-hours, call it about 7.7 kWh of nominal battery. One honest add-on belongs here: in a genuinely low-sun winter month, back-to-back cloudy stretches will eventually exceed any battery bank a reasonable person would pay for. This is why most sustainable off-grid designs include a small generator, not as an admission of failure but as cheap insurance that lets you avoid oversizing panels and batteries for the single worst week of the decade. The generator covers the rare edge; the solar and battery cover the ordinary days, and each does the job it’s cheapest at.

Before you buy anything, run the whole chain again — load list, sun hours, derate, panel watts, battery capacity — through the solar panel size calculator with your own numbers, and see whether it lands near your hand-worked figures. A close match tells you your assumptions are at least internally consistent. A large disagreement is a gift, because it means an input is wrong somewhere, and finding that error now, on paper, is enormously cheaper than discovering it after you’ve mounted the wrong array on the roof. This cross-check is the single most valuable fifteen minutes in the process, and skipping it is how people end up living with a system that’s quietly 30% too small for the season they most need it.

Then add margin deliberately, as a specific decision, rather than by multiplying everything by some vague safety factor and hoping. There are concrete reasons the raw numbers understate what you’ll need. Loads grow over time and essentially never shrink — the cabin gains a chest freezer, a starlink dish, a second occupant — and a system sized exactly to today’s list is already undersized for next year. Batteries lose usable capacity as they age, so the bank that delivers its full rated energy on day one delivers less after several hundred cycles. And panels lose a little output every year to degradation, a slow and permanent decline that means the array producing your full budget when new produces a bit less a decade on. A cushion of 20 to 25% on the array, plus deliberate headroom in the charge controller and wiring for future panels, costs very little at build time and is expensive and disruptive to retrofit once everything’s installed and sealed up. The right time to leave room is before the first panel goes on the roof. Pull all of that together and the final cabin spec, built up from a 3,250-watt-hour daily list, comes out to roughly 1.8 to 2 kW of panel, about 8 to 10 kWh of lithium storage, an inverter sized to the peak simultaneous load rather than the daily total, and a backup generator for the worst stretches. Someone else’s identical-looking cabin two states north, or one running an electric water heater instead of propane, would land somewhere completely different on every one of those numbers — which is the entire reason the load list, and not the forum post, is where off-grid sizing has to begin.