How to Design an Off-Grid Solar System from Scratch

Most off-grid solar systems fail not because of bad components but because of bad sequencing. People pick a battery size, then buy panels to match, then realize their inverter is undersized, and end up with a system that doesn't hold up when it counts.

The right approach is a specific order: loads first, battery bank second, solar array third, inverter last. Each step constrains the next. Do it in sequence and every number comes from a real input rather than a guess.

Step 1: Define Your Loads

Everything starts with what you're actually powering. Make an honest list of every load, its rated wattage, and how many hours per day it runs.

Total daily use in this example: ~2,595 Wh

A few things to do while building this list:

Separate DC and AC loads. DC loads (12V fridge, LED lights, water pump) run directly from your battery bank without an inverter. AC loads (laptop via charger, TV, kitchen appliances) require an inverter. Your inverter sizing in Step 4 comes from your peak simultaneous AC draw, not your daily total.

Account for surge draw. Motors — pumps, compressors, fans — draw 2–4x their rated wattage at startup for a fraction of a second. This matters for inverter sizing, not daily energy math.

Be honest about infrequent loads. Power tools, air conditioning, electric kettles — account for them even if you only use them occasionally. The system needs to handle them, even if they don't dominate the daily total.

Step 2: Size the Battery Bank

Once you know your daily energy use, you can size your battery bank. The formula:

Daily Wh × Days of autonomy ÷ Usable capacity = Bank size in Wh

Days of autonomy is how many days you want to run without solar input — cloudy stretches, deep winter, or a period of low sun. Most off-grid cabins and homesteads design for 2–3 days. Remote systems with no generator backup often go to 4–5 days.

Usable capacity depends on chemistry:

• LiFePO4: 95–100% usable — a 200Ah battery gives you ~190–200Ah
• AGM: 50% usable — a 200Ah battery gives you only ~100Ah

Running the example with LiFePO4 and 3 days of autonomy: 2,595 Wh × 3 ÷ 0.95 = 8,195 Wh needed

At 24V: 8,195 ÷ 24 = ~340Ah   At 48V: 8,195 ÷ 48 = ~170Ah

On system voltage: Off-grid systems are increasingly built at 24V or 48V rather than 12V. Higher voltage means lower current for the same power — which means thinner wire, less heat, and significantly better efficiency over long cable runs. For systems above about 2 kWh of daily use, 24V or 48V is almost always the right choice.

Step 3: Size the Solar Array

Your solar array needs to refill the battery bank in a reasonable number of sun hours. The formula:

Daily Wh ÷ Peak Sun Hours ÷ System Efficiency = Required panel watts

Peak sun hours is location-dependent — not total daylight, but the equivalent hours of full-intensity sun per day.

Use 0.8 as your system efficiency factor to account for wire losses, temperature derating, and controller efficiency. Running the example at 5 peak sun hours: 2,595 ÷ 5 ÷ 0.8 = 649W of panels minimum. Round up — a 4×200W array (800W) provides comfortable margin.

Step 4: Select the Charge Controller

Your MPPT charge controller sits between the solar array and the battery bank. Always use MPPT — it extracts 15–30% more energy from the same panels compared to PWM.

Match the controller's max input voltage to your panel string's peak open-circuit voltage, including cold-weather voltage rise. Exceeding the controller's input voltage limit destroys it.

Step 5: Size the Inverter

Size for peak simultaneous AC draw — the most you'd realistically pull at once. Add up your concurrent AC loads: microwave (800W) + laptop (65W) + lighting (60W) = 925W. If you add a kettle or air compressor to that window, you're looking at 2,500–3,000W peak.

A Victron MultiPlus-II in the 2,000–3,000W range covers most cabin load profiles and adds integrated shore-power or generator charging in one unit. Size up rather than down — you don't want to be rebooting the inverter because you started the pump while the kettle was boiling.

Putting It Together

Here's what a complete system looks like for our example cabin (2,595 Wh/day, 3-day autonomy, 24V):

The SmartShunt is not optional — without it, you have no reliable way to know your battery bank's actual state of charge. LiFePO4 batteries hold a nearly flat voltage curve for most of their discharge range, which makes voltage-based gauges nearly useless until you're almost empty.

Where to Go From Here

The four-step process — loads, battery, solar, inverter — gives you a defensible starting point. From there, the refinements are site-specific: shading analysis, seasonal sun variation, wire sizing and breaker selection, generator integration if you need it.

Browse the full solar panel, battery, charge controller, and inverter-charger collections at Blue Marine. Or schedule a free system consultation — we work through off-grid builds regularly and can sanity-check your numbers before you buy anything.

Related reading:
Off-Grid Battery Bank Sizing: Cabin, Homestead, and Tiny House
Victron MultiPlus-II Sizing Guide: Which Model Do You Need?
Generator vs Solar for Off-Grid: When Does Each Make Sense?