Off-Grid & Hybrid Solar Calculator

How to size a complete off-grid solar system

An off-grid system has no utility to fall back on, so it must be designed to meet your needs from solar and storage alone — including through cloudy spells. That makes off-grid sizing more demanding than a grid-tied system: every part has to be matched, and the weakest link sets your reliability. The design flows from one number, your daily energy use, through four connected decisions: how big the array must be to refill the battery each day, how large the battery must be to ride out cloudy days, what inverter can run your loads, and how much margin to add for the real world.

Start with an honest energy audit

Everything depends on knowing your true daily consumption in watt-hours or kWh. List every load — lights, fridge, pumps, electronics, and any heavy items like air-conditioning or electric cooking — with its wattage and hours of use. Off-grid is where frugality pays directly in hardware cost: every watt-hour you trim shrinks both the array and the battery. Many successful off-grid homes start by reducing demand (efficient appliances, gas for cooking and heating) before sizing the system, because it's far cheaper to save a kWh than to generate and store one.

Size the array to refill the battery

The array must generate enough on an average day to cover your daily use plus the system losses, within your location's peak sun-hours. In winter, sun-hours fall just as heating and lighting loads rise, so off-grid arrays are usually sized to the worst usable month, not the annual average — otherwise the battery never fully recharges in winter. This is the key difference from grid-tied sizing, where annual averages are fine because the grid covers the gaps.

Size the battery for autonomy

The battery carries you overnight and through cloudy days. Off-grid systems typically target 2–5 days of autonomy, using the capacity formula: (daily use × days of autonomy) ÷ (depth of discharge × inverter efficiency). LiFePO4's high usable depth of discharge (80–95%) makes it the modern default; lead-acid needs roughly double the rated capacity for the same usable energy. Our battery bank calculator handles this step in detail.

Match the inverter and consider a generator

The inverter must handle your largest simultaneous load plus surge from motors and compressors. And because sizing a battery for, say, ten consecutive cloudy days is wildly expensive, most off-grid systems pair a sensibly sized battery (2–5 days) with a backup generator for the rare long spell. That hybrid approach is almost always cheaper and more reliable than trying to cover every worst case with batteries alone — an honest off-grid design plans for the generator rather than pretending it won't be needed.

Off-grid sizing checklist

• Audit daily energy use accurately — it drives everything.
• Size the array to the worst usable month, not the annual average.
• Size the battery for 2–5 days of autonomy at safe depth of discharge.
• Match the inverter to peak simultaneous load plus surge.
• Add 10–20% margin and plan a generator for rare extended cloud.

Why off-grid systems look "oversized"

People new to off-grid are often surprised that a cabin using a modest amount of energy needs an array and battery that seem far larger than a grid-tied home of similar consumption. The reason is that a grid-tied system can lean on the grid for cloudy days, night-time and seasonal swings, so it only has to match annual production to annual use. An off-grid system has to survive the worst realistic conditions entirely on its own: the darkest week of winter, several cloudy days in a row, and the inefficiencies of storing and reconverting every unit of energy it uses. Each of those margins multiplies the hardware. This is also why demand reduction is so powerful off-grid — trimming a heavy load doesn't just save one unit of generation, it shrinks the array, the battery and sometimes the inverter all at once. The most cost-effective off-grid systems are almost always the ones that started by cutting consumption before sizing anything.

Frequently asked questions

It depends entirely on your daily energy use and your local winter sun-hours. Enter your figures above for an estimate. As a rule, off-grid systems are sized to the worst-case season, so they look oversized compared with a grid-tied system of the same annual output.

Because off-grid has no grid to cover shortfalls. If you size to the annual average, the battery won't fully recharge during low-sun winter months and you'll run short exactly when loads are highest. Sizing to the worst usable month keeps the system reliable year-round.

Usually yes, as backup. Battery capacity sized to cover every possible cloudy stretch is hugely expensive; a moderate battery plus an occasional-use generator is cheaper and more dependable. Many off-grid owners run the generator only a handful of times a year.

You can, but AC is a heavy, sustained load that sharply increases both array and battery size. Many off-grid homes back up essentials and either limit AC, use high-efficiency mini-splits, or run cooling only when the sun is strong. Include it in your audit only if you genuinely intend to power it.