How to Size an Off-Grid Solar System

Start with the load, not the panels

The single most common off-grid mistake is to start by picking panels. The right starting point is your daily energy consumption in watt-hours per day (Wh/day). Add up each load as watts × hours used: a 60 W laptop for 5 hours is 300 Wh, a 40 W fridge averaging 12 hours of compressor time is 480 Wh, and so on. The total is the energy your system must deliver every day. Everything else is sized to serve that number.

Two more inputs shape the design: your site’s peak sun hours (the equivalent hours of full 1,000 W/m² sun, from 3.7 in the Pacific Northwest to 6.5 in Arizona) and your desired days of autonomy (how long the batteries carry you through cloudy weather). The peak sun hours table lists figures by US state.

Step 1 — size the solar array

The array has to replace the day’s energy during the available sun hours, with a loss factor for wiring, heat, dirt and controller inefficiency. A derate of about 0.75 is typical:

array W = Wh/day / (peak sun hours × 0.75)

For a 3,000 Wh/day load at 5 peak sun hours: array W = 3,000 / (5 × 0.75) = 3,000 / 3.75 = 800 W. The Solar Panel Size calculator turns that into a panel count.

Step 2 — size the battery bank

The battery bank stores enough usable energy for your autonomy, accounting for depth of discharge (DoD): lithium tolerates about 0.8, flooded lead-acid only about 0.5. In amp-hours at the system voltage:

battery Ah = (Wh/day × days) / (system V × DoD)

For 3,000 Wh/day, 2 days of autonomy, a 24 V bank and lithium: Ah = (3,000 × 2) / (24 × 0.8) = 6,000 / 19.2 = 312.5 Ah. The Battery Bank calculator and the battery-sizing guide go deeper on autonomy and DoD.

Step 3 — size the charge controller

The controller must handle the array current with headroom. Divide array watts by battery voltage and add 25%:

controller A = array W / system V × 1.25

For our 800 W array on 24 V: 800 / 24 × 1.25 = 41.7 A, so you choose the next standard size, a 50 A controller. MPPT controllers harvest more from the panels than PWM; the MPPT vs PWM guide explains the trade-off, and the Charge Controller calculator sizes it.

Step 4 — size the inverter

The inverter converts battery DC to AC and must cover the largest combination of loads that ever run at once, plus surge for motor starts. Size the continuous rating at about 1.25 times the running watts:

inverter W = peak running W × 1.25

If your simultaneous AC loads total 1,800 W, the inverter should be at least 1,800 × 1.25 = 2,250 W continuous, with extra surge capacity for a pump or compressor. The Inverter Size calculator handles continuous and surge.

Choosing the system voltage

Higher system voltage (24 V or 48 V instead of 12 V) means lower current for the same power, which means thinner cables, smaller controllers and less voltage drop. Small systems live happily at 12 V; once you pass roughly 1,000–2,000 W of array, move to 24 V or 48 V. The whole chain — array, battery, controller, inverter — is solved at once by the Off-Grid System Size calculator.

Measuring your real daily load

Every number downstream depends on the daily watt-hour figure, so it deserves more care than any other input. The reliable way to find it is to list each load with its wattage and the hours it actually runs per day, then sum the watt-hours. The trap is intermittent loads: a refrigerator is rated at a few hundred watts but its compressor only runs part of the time, so use its daily energy rather than its nameplate watts. Phantom loads from chargers and standby electronics add up quietly. The most accurate approach is to measure for a few days with an energy meter and use the real total, because a system sized to an optimistic guess will disappoint on the first cloudy week. When in doubt, round the load up; it is cheaper to add margin now than to expand the array later.

Peak sun hours, not hours of daylight

Newcomers often confuse peak sun hours with the length of the day, but they are very different. A peak sun hour is one hour of full reference sun at one thousand watts per square meter. A location might have fourteen hours of daylight yet only five peak sun hours, because early morning, late evening and any haze deliver far less than full intensity. Peak sun hours also vary by season and by panel tilt, and an off-grid system has to survive the worst month, not the average. If you size to a yearly average you will come up short in winter, so conservative designs use the lowest-month figure for the site and orientation, accepting some summer surplus as the price of winter reliability.

Where the derate factor comes from

The roughly seventy-five percent derate that appears in the array formula is not arbitrary; it is the product of several small losses that are unavoidable in a real system. Panels rarely hit their nameplate rating because they run hot, and heat reduces output. Wiring and connections lose a little, the charge controller is not perfectly efficient, dust and shading take their toll, and batteries lose energy on the round trip of charging and discharging. Stacking these factors lands near three quarters of the ideal, which is why dividing by sun hours alone would oversize the apparent harvest. Treat the derate as a realistic haircut on theoretical output, and use a lower figure if your installation runs especially hot or dirty.

Designing for the worst day

A grid-tied system can lean on the utility when production dips, but an off-grid system has no backstop except its own battery, so it must be designed around the worst realistic conditions rather than the typical ones. That means the lowest-sun month, the longest expected stretch of cloud captured in the autonomy days, and a battery deep enough to ride through both without over-discharging. Many builders also add a generator or a way to charge from a vehicle as insurance for the rare extended outage, so the solar array does not have to be sized for an improbable extreme. The art of off-grid design is balancing a comfortable margin against the cost of the extra panels and batteries that margin requires.

Sanity-check the result

Good designs are balanced: an array that can refill the battery in the available sun, a battery that carries the autonomy you want, a controller rated for the array, and an inverter that covers the loads. If any one element is undersized the system fails on the worst day. Build in a margin, measure your real consumption rather than guessing, and revisit the numbers once you have lived with the system. These are planning estimates based on stable engineering figures (derate, DoD, peak sun hours stated as ranges); your actual harvest varies with weather, shading and temperature.