How to Build an Off-Grid Solar System: Panels, MPPT, Battery and Inverter

An off-grid solar system is four parts working together: solar panels, an MPPT charge controller, a battery bank and an inverter. You size each one from the same starting point, your daily energy use. This guide walks through the sizing math step by step, with a worked example for a small cabin, and explains when a portable power station is the simpler answer.

Product figures come from the Amazon listings in our US catalog. The design rules are general solar and electrical practice; for a permanent installation, have a licensed electrician check the plan.

What does each part do?

Panels make energy, the controller manages charging, the battery stores it and the inverter turns it into 120 V AC.

Part	Job	Sized by
Solar panels	Convert sunlight to DC power	Daily Wh and peak sun hours
MPPT charge controller	Charges the battery safely and efficiently	Panel watts and battery voltage
Battery bank	Stores energy for nights and cloudy days	Daily Wh and days of autonomy
Inverter	Converts battery DC to 120 V AC	Largest simultaneous AC load
Fuses, breakers, disconnects	Protect cables and allow safe maintenance	Current in each circuit

A portable power station puts the controller, battery and inverter in one box. An off-grid system splits them, which gives you flexibility and repairability in exchange for design and wiring work.

Step 1: How much energy do I use per day?

List every device, multiply watts by hours, and add it up. For AC devices, divide by 0.85 to account for inverter losses. Here is a small weekend cabin:

Device	Watts	Hours	Wh per day
12 V fridge (average draw)	50 W	24	1,200
LED lights (12 V)	30 W	5	150
Water pump (12 V)	60 W	1	60
Laptop and router via inverter (80 W ÷ 0.85)	94 W	5	470
Phones and small devices	–	–	50
Total			1,930

Round up to 2,000 Wh a day. Our solar calculator does this for you.

Step 2: How big a battery bank?

Daily Wh × days of autonomy × 1.2. Days of autonomy is how long you want to last without sun. Two days is a common starting point for weekend cabins; full-time homes often plan for more.

For the cabin: 2,000 × 2 × 1.2 = 4,800 Wh. At 24 V (25.6 V nominal for LiFePO4), that is 4,800 ÷ 25.6 ≈ 188 Ah. Two 12V 320Ah batteries in series give 25.6 V × 320 Ah = 8,192 Wh, comfortably above it; two 12V 100Ah in series give 2,560 Wh, too little.

Use the battery calculator and read how to choose a LiFePO4 battery for BMS limits, cold charging and series wiring.

Step 3: How many solar watts?

Daily Wh ÷ (peak sun hours × 0.75). The 0.75 covers real-world panel losses. Size for the worst month you actually use the system.

For the cabin with 4 peak sun hours: 2,000 ÷ 3 ≈ 667 W, so about 800 W of panels. In a northern winter with 2 peak sun hours, the same load needs about 1,333 W. Look up your region in how many solar watts you need.

For a fixed system, MC4 panels connect directly to standard controllers; the Renogy EFLEX 200W and Renogy 100W in our catalog both use MC4.

Step 4: What size charge controller?

Panel watts ÷ battery voltage × 1.25 gives the controller amps. For 800 W on a 24 V bank: 800 ÷ 25.6 × 1.25 ≈ 39 A, so a 40 A MPPT controller.

Also check the controller’s maximum input voltage. Panels in series add their open-circuit voltage (Voc), which rises further in cold weather. Keep the string’s cold Voc below the controller’s limit; our series vs parallel guide explains how.

Step 5: What size inverter?

Add up the AC loads that run at the same time and add about 25 percent. For the cabin, the laptop and router need under 100 W, but a microwave or a coffee maker would push that to 1,000-1,500 W. A 2,000 W pure sine wave inverter leaves margin.

Then check the battery can feed it. At 2,000 W, the inverter pulls about 2,000 ÷ 0.85 ≈ 2,350 W from the battery, or about 92 A at 25.6 V. A 12V 320Ah battery with a 200 A BMS handles that easily in a 24 V series bank.

Step 6: Which system voltage?

12 V for vans and small systems, 24 V for cabins, 48 V for larger homes. Higher voltage carries the same power at lower current. That means thinner cables, smaller controllers and fewer losses.

System voltage	Current at 2,000 W	Typical use
12 V (12.8 V nominal)	≈ 156 A	Vans, boats, small RVs
24 V (25.6 V)	≈ 78 A	Cabins, larger RVs
48 V (51.2 V)	≈ 39 A	Homes, large cabins

The GOKWH batteries in our catalog allow up to 4 in series, so 12 V, 24 V and 48 V banks are all possible with identical batteries.

Can I add wind or a generator?

Yes, both can charge the same battery bank. A small wind turbine helps on windy sites in winter and at night, when solar is weak. A generator with a battery charger covers long cloudy spells. If you have a steady stream, read is micro hydro worth it?.

Off-grid system or portable power station?

A power station is simpler; a DIY system scales further and is easier to repair.

• Choose a power station if you want plug-and-play power, need to move it, or use under about 3 kWh a day. The EcoFlow DELTA 3 Max Plus expands to 10,000 Wh and the Anker SOLIX F3000 accepts 2,400 W of solar, according to their listings.
• Choose a DIY system if you want a permanent cabin or RV install, 12 V circuits for lights and pumps, or the option to replace one part without replacing everything.

Compare ready-made bundles in the best solar generator kits and large units in power stations of 3,000 Wh and up.

What safety rules apply?

Fuse every circuit, add a battery disconnect and follow local electrical codes.

• Put a fuse or breaker close to each battery’s positive terminal and on each branch circuit.
• Size cables for the current and the length of the run.
• Never connect an off-grid inverter to your home wiring without a proper transfer switch installed by a licensed electrician.
• Permanent installations in buildings usually need a permit and inspection.