Off-grid system sizing goes wrong in a predictable way: people start by choosing panels, then batteries, then discover in December that production has collapsed while consumption has not. The correct order is the reverse. Measure the load first, identify the worst production period for your location and array orientation, and design against that case. Everything else follows from those two numbers.
Start with a genuine load audit
The foundation is daily energy consumption in watt-hours, not a list of appliances. For each load, you need its power draw and the hours per day it actually runs, and the second figure is where estimates go wrong. A refrigerator cycles, a well pump runs in bursts, lights are on only in the evening, and standby loads run continuously. Standby consumption deserves particular attention off-grid, because a handful of devices drawing a few watts each around the clock can rival a major appliance over a day. A plug-in energy meter on each load for a week gives real data, and where that is impractical, published appliance figures are a reasonable starting point. Sum the daily watt-hours and you have the number the entire design serves.
Identify the design month, not the average
Solar production varies enormously through the year, and in most of the continental United States the December figure is a small fraction of the June figure at a fixed tilt. Averaging the year produces a system that fails for three months. The design case is the worst month you intend to operate on solar alone, and modeling tools that use historical weather data for a specific location, tilt and azimuth give far better numbers than a rule of thumb. Consecutive cloudy days matter as much as monthly totals, since that determines how much battery reserve you need. Steeper tilt favors winter production at the cost of summer, which is often the right trade for an off-grid system since summer already has surplus.
Deciding how much autonomy to buy
Days of autonomy is the number of days the battery bank can carry the load with no production at all. More autonomy means a larger, more expensive bank that spends most of the year underused and, for lead-acid chemistries, ages whether or not it is cycled. Less autonomy means a generator runs more often. There is no universally correct figure; it is an economic trade between battery cost and generator fuel and noise. Most practical off-grid systems settle on a modest autonomy figure combined with a generator for the stretches that solar cannot cover, because that is far cheaper than sizing batteries for the worst week of winter. Decide this deliberately rather than inheriting a number from a forum.
Reducing load is cheaper than adding capacity
Every watt-hour removed from the load reduces panels, battery, charge controller and inverter simultaneously, which is why efficiency work has a multiplied payoff off-grid that it does not have on the grid. Direct-current lighting, an efficient refrigerator, eliminating resistive heating, and dealing with standby loads routinely cut consumption enough to change the whole design class. Propane for cooking, water heating and space heating removes the loads that are most hostile to a battery system. Doing the efficiency work before specifying equipment, rather than after, is the single largest cost lever available, and it also reduces the generator run time that would otherwise be needed in winter.
Accounting for losses honestly
The energy that leaves a panel is not the energy that reaches a load. Panels produce less than their rating at real cell temperatures, wiring has voltage drop, the charge controller has a conversion efficiency, the battery returns less than it accepts, and the inverter has both a conversion efficiency and an idle draw. Multiply those together and the cumulative loss is substantial, and it is larger at low load where the inverter’s idle consumption dominates. Sizing from panel nameplate to load watt-hours without applying a system efficiency factor is the second most common sizing error after using the wrong month. Modeling tools apply a default loss factor; understand what it contains before adjusting it.
What to check before you act
- Measure daily watt-hours with a meter rather than estimating.
- Model production for your worst month, tilt and location.
- Decide days of autonomy as an explicit cost trade.
- Do the efficiency work before specifying any equipment.
Common questions
How many panels do I need for a cabin?
That question cannot be answered without the daily watt-hour load and a production estimate for your location in the worst month you plan to use it.
Should I just add a generator instead of more panels?
Often yes for winter. A modest generator covering a few weeks is usually cheaper than sizing an array and bank for December production.
The practical takeaway
Measure the load, model the worst month for your actual location and tilt, then decide autonomy as a deliberate trade against generator time. Spend on efficiency before spending on capacity, because every watt-hour removed shrinks four pieces of equipment at once. Apply an honest loss factor and the design will behave the way the spreadsheet promised.
References and further reading
- NREL PVWatts: modeling monthly solar production for a specific site and tilt
- Department of Energy: estimating appliance and electronics energy use
- ENERGY STAR: standby power consumption of household devices
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
