Sizing guide

Sizing your solar system: the honest math

Every DIY solar build comes down to four numbers: daily watt-hours of load, peak sun hours at your location, depth of discharge tolerance of your battery chemistry, and how many days of autonomy you want without sun. Get these right and the rest of the design follows naturally.

Step 1: Calculate daily watt-hours

For each appliance: watts × hours used per day = daily Wh. Sum them all up.

That's a typical van/RV daily load — about 2.7 kWh. A small cabin doubles it (microwave + bigger lights + heating fan). A tiny home with full appliances triples it.

Step 2: Size the battery bank

Formula: Battery capacity (Wh) = Daily Wh × Autonomy days ÷ Depth of Discharge (DoD)

Autonomy days = how many cloudy days you want to survive without solar. 2 days is typical for sunny climates; 3-4 for variable PNW/midwest; 5+ if you want zero-anxiety winter cabin life.

Depth of Discharge by chemistry:

• · LFP (LiFePO4): 0.80 (use 80% of rated capacity safely)
• · AGM: 0.50 (use 50% to preserve cycle life)
• · Flooded lead-acid: 0.50 (same, also requires venting)
• · NMC: 0.85-0.90 (but safety considerations limit DIY use)

Example: 2,726 Wh/day × 2 days ÷ 0.8 DoD = 6,815 Wh battery bank (typical 12V × 568 Ah or 24V × 285 Ah or 48V × 142 Ah).

Same load with AGM: 2,726 × 2 ÷ 0.5 = 10,904 Wh — over 60% more capacity needed and roughly 3x the weight. This is why LFP has taken over the DIY market.

Step 3: Size the solar array

Formula: Array watts = Daily Wh ÷ Peak Sun Hours × 1.3 (margin)

Peak sun hours by region:

• · Arizona, New Mexico: 6.0-6.5
• · California, Texas, Florida: 5.2-5.5
• · Colorado, Nevada, Utah: 5.0-5.5
• · Pacific NW (west): 3.5-4.0
• · Northeast (NY, MA, VT): 4.0-4.5
• · Midwest, Mid-Atlantic: 4.5-5.0

Example with same 2,726 Wh load in Colorado (5.3 sun hours): 2,726 ÷ 5.3 × 1.3 =669 watts of panels. Typically rounded up to 700-800W in practice (3x 300W panels or 4x 200W panels).

The 1.3 margin handles dust, panel aging, soft cloudy stretches, and the fact that "peak sun hours" is an average — December produces half what June does. For winter-heavy use cases, increase the margin to 1.5-1.8.

Step 4: Match the charge controller

Two things to size: the controller must handle the array's voltage (including cold-weather Voc derating per NEC 690.7) and produce enough current at battery voltage to actually charge the bank.

For our 800W array on a 24V battery: 800W ÷ 24V = 33A controller minimum. In practice, size up to 40A for headroom. For 48V battery: 800W ÷ 48V = 17A — go with a 30A controller for headroom and future expansion.

This is where the builder helps most. Our SolarControllerFinder builder runs the actual NEC 690.7 cold-weather Voc derating math for your specific location and panel configuration, and shows which controllers in our catalog can handle it.

Worked examples