Off-Grid Solar Basics

This page is the orientation. It covers what the parts do, the vocabulary the rest of the guide assumes, and the order the decisions have to be made in. If you are new to this, read it before anything else.

What off-grid actually means

Off-grid means your electricity comes entirely from equipment you own, with no connection to the utility. Nothing is bought, nothing is sold back, and nothing covers you when your system falls short.

That last point is the whole difference. A grid-tied solar system treats the utility as an infinite battery: it exports surplus during the day and imports at night, so a system that under-produces simply buys the difference. Nobody notices, and nothing breaks.

Off-grid, a shortfall is darkness. Which is why off-grid systems are sized against the worst conditions rather than the average ones, and why the arithmetic matters so much more.

The four components

Solar panels convert light into direct current. Rated in watts. Cheap, durable, and the part least likely to cause you trouble. Covered in Solar Panels Explained.

A charge controller sits between the panels and the batteries, converting the array’s variable output into the correct charging voltage and managing the charge cycle. Without one, panels overcharge and destroy batteries. Covered in Solar Charge Controllers Explained.

Batteries store energy for night and bad weather. Rated in amp-hours or watt-hours. The most expensive component, the shortest-lived, and the least tolerant of a design mistake. Covered in Off-Grid Battery Basics.

An inverter converts stored DC into the AC that household appliances expect. Rated in watts. Covered in Off-Grid Inverter Basics.

Everything else is wiring, overcurrent protection, mounting, and monitoring.

The vocabulary

Six terms carry most of the weight.

Watt (W) is a rate of energy use. A 100 W bulb consumes energy at 100 watts for as long as it is on.

Watt-hour (Wh) is an amount of energy. That 100 W bulb running for 3 hours uses 300 Wh. A kilowatt-hour (kWh) is 1,000 Wh, and is the unit a utility bill uses.

The distinction matters because watts size your inverter and watt-hours size your batteries and panels. Confusing them is the single most common beginner error.

Volt (V) is electrical pressure. Off-grid systems are usually built at 12, 24, or 48 volts.

Amp (A) is rate of flow. Watts equal volts times amps, which is the most useful equation in the whole subject. It tells you that the same power at a higher voltage means less current, and less current means thinner cable.

Amp-hour (Ah) is how battery capacity is usually quoted. Multiply by voltage to get watt-hours: a 100 Ah battery at 12 V holds 1,200 Wh.

Peak sun hours is the one that confuses people. It is not hours of daylight. It is the number of hours equivalent to full-strength sun once weaker morning, evening, and cloudy light is accounted for. A site averaging 4 peak sun hours in December might have 9 hours of daylight, most of it weak.

The order of decisions

Every decision constrains the next one. Taken out of order, you buy equipment twice.

1. Work out your daily energy use, in watt-hours. List every appliance, its running watts, and realistic hours of use. This number drives everything downstream, so an error here scales through the entire design. The most common mistake is entering a fridge as running 24 hours a day, when a compressor fridge cycles and runs perhaps a third of that.

2. Choose a system voltage. Higher voltage means lower current for the same power, which means thinner cable, smaller fuses, and less energy lost as heat. Roughly: 12 V for small systems, 24 V for mid-size cabins, 48 V for full-time homes and anything with substantial loads. This is difficult to change later, because it determines which batteries, controller, and inverter you can use.

3. Size the battery bank. Daily watt-hours, multiplied by the days of autonomy you want without sun, divided by the usable depth of discharge for your chemistry. See battery bank sizing.

4. Size the array. Daily watt-hours divided by peak sun hours in your worst month, divided by about 0.8 for real-world losses. Sizing on an annual average produces a system that works beautifully in June and fails in December.

5. Size the charge controller, from array wattage and battery voltage, checking the maximum input voltage against the coldest temperature your site reaches.

6. Size the inverter, from the largest realistic simultaneous load plus the startup surge of your biggest motor.

7. Size the wiring and overcurrent protection, which follows from the currents everything above will actually carry. See wiring and safety.

Where the money goes

Beginners expect panels to dominate the budget. They usually do not.

Panels have become remarkably cheap per watt. On a typical off-grid build the battery bank is the largest single cost, often by a wide margin, and it is also the only component you will likely replace within the system’s life.

The practical consequence: reducing consumption is almost always cheaper than generating more. A more efficient fridge, LED lighting, or running some loads on DC to keep the inverter switched off will often save more money than the equivalent capacity in panels and batteries would cost. Work on the demand side first.

What usually goes wrong

• Sizing on the annual average rather than the worst month, producing a system that fails exactly when it is needed.
• Overestimating appliance run time, particularly for anything thermostatic, which inflates every component downstream.
• Choosing 12 V for a system that should be 24 or 48 V, then discovering the cable costs more than the voltage upgrade would have.
• Undersizing the battery bank to save money up front, then killing it early by discharging it too deeply.
• Buying panels first, because they are the fun part, before knowing what the system needs.

None of these are exotic. They are the ordinary path, and they are all avoidable by doing the arithmetic in order.