How to size a DIY solar system (the simple 5-step method)

Almost every failed DIY solar build makes the same mistake: it starts by buying a panel. The right order is the opposite — you start with what you want to power, and let that decide everything else. Get this sequence right and your parts will actually work together. Here’s the five-step method we use.

Step 1 — Add up your loads (watt-hours per day)

List everything you want to run, its wattage, and how many hours a day you’ll use it. Multiply, then add it all up. That daily total in watt-hours (Wh) is the number the whole system is built around.

• Fridge: 60W × 24h (but it cycles ~⅓ of the time) ≈ 480Wh
• Lights: 20W × 5h = 100Wh
• Laptop + phone: ~150Wh
• Water pump, fan, etc.: add them in

Say that comes to 1,000Wh/day. That’s your target. Be honest — underestimating loads is the #1 sizing error.

Step 2 — Size the battery bank

Your battery has to store enough to cover your daily loads plus a reserve for cloudy days, and you can’t drain it all the way.

• Start with daily Wh, then multiply by your desired days of autonomy (1–2 for a sunny climate, more for cloudy).
• Divide by the usable depth of discharge: LiFePO4 gives you ~80–100%, lead-acid only ~50%.

For 1,000Wh/day, 1 day of reserve, on LiFePO4: about 1,200Wh of battery, or roughly a 100Ah 12V battery. This is exactly why LiFePO4 batteries win for DIY — you use nearly all of the rated capacity. See our pick, the LiTime 100Ah.

Step 3 — Size the solar array (watts)

Your panels have to refill the battery during the usable sun hours you actually get — call it 4 peak sun hours as a safe average.

• Solar watts needed ≈ daily Wh ÷ peak sun hours ÷ ~0.75 (system losses).
• 1,000Wh ÷ 4 ÷ 0.75 ≈ 330W of solar.

So three or four 100W panels covers our example. Round up — you’ll thank yourself in winter. Our starting-block pick is the Renogy 100W.

Step 4 — Size the charge controller (amps)

The controller sits between panels and battery. Size it by the charging current it must handle:

• Controller amps ≈ total panel watts ÷ battery voltage.
• 330W ÷ 12V ≈ 28A, so a 30A+ controller.

Choose MPPT over PWM — it harvests meaningfully more, especially in cold or cloudy weather. See charge controllers and our value pick, the Renogy Rover MPPT (step up to a 40A for the example above).

Step 5 — Size the inverter and wiring

If you need household AC, add an inverter rated above your largest simultaneous AC load, with headroom for surge. Then size your wire and fusing to the current — this is where beginners cut corners and start fires. Don’t. See how to wire it safely.

Put it together

Add a battery monitor and you’ll actually know what your system is doing instead of guessing from voltage.

The bottom line

Size from your loads outward — loads, battery, solar, controller, inverter — and every part will match. New to all of this? Start with solar for beginners, then come back and run your own numbers.