How to Size a Solar System for Off-Grid

Which System Fits Your Situation?

Pick the scenario closest to your situation. Each worked example later in this guide uses these exact loads and locations.

What's in an Off-Grid Solar System

Four components every off-grid system needs, and how they connect:

Step 1: Calculate Your Daily Load (Wh/day)

Every correctly sized solar system starts here. List every device you'll run, multiply wattage by daily hours of use, and sum the total. Then add 10% for inverter losses and phantom loads. Use the power needs assessment guide to build a complete load list before buying anything.

This is a mid-range cabin load. Always overestimate by 10–15% on individual appliances — measuring with a Kill-A-Watt meter ($25) is more reliable than label wattages, which often show maximum rather than running power.

Step 2: Find Your Worst-Month Peak Sun Hours

Peak Sun Hours (PSH) is not "hours of daylight." It's the number of hours per day your location receives the equivalent of full 1,000 W/m² irradiance. A location with 5 PSH receives the same total solar energy whether the sun shines weakly for 10 hours or strongly for 5. Always design for your worst-month PSH, not the annual average — your system must work when the sun is worst.

December worst-case PSH by US state (NREL PVWatts data):

Step 3: Size Your Solar Array

With your daily load and worst-month PSH, calculate the array wattage needed. The 0.75 system efficiency factor accounts for wiring losses (~3%), MPPT efficiency (~5%), temperature derating (~8%), and soiling (~5%). It's conservative, but real-world systems routinely validate it.

2026 Panel Technology: TOPCon is the Standard

TOPCon (Tunnel Oxide Passivated Contact) panels now hold over 65% of global market share. At 22–24% efficiency, a 400W TOPCon panel measures roughly 1.7m × 1.1m (67″ × 43″). PERC panels are still available but are being phased out of new production. Retail pricing in the US is $0.30–$0.50/W; wholesale (10+ panel quantities) is $0.26–$0.28/W.

Use the Solar System Calculator to run your exact numbers — it applies regional PSH data and the 0.75 efficiency factor automatically.

Step 4: Size Your Off-Grid Solar Battery Bank

Your battery bank must cover your loads for consecutive cloudy days — called "days of autonomy." Two to three days is the practical target for most off-grid cabins. For LiFePO₄, use 80% depth of discharge (DoD); for AGM, use 50% DoD.

Step 5: Choose Your System Voltage

System voltage (12V, 24V, or 48V) determines wiring gauge, charge controller compatibility, and inverter selection. Higher voltage = lower current = thinner, cheaper wire. Committing to a system voltage locks in all three major components — change it later and you're replacing everything.

A 12V system running 1,000W pulls 83A through your wiring. A 48V system running the same load pulls only 21A — meaning wire that's 1/4 the cost and 1/4 the weight. For anything over 800W, 24V or 48V is not optional.

Step 6: Select Your Charge Controller and Inverter

MPPT Charge Controller Sizing

Controller amp rating = (Array Watts × 1.25 safety factor) ÷ System Voltage. Round up to the next standard size (10A, 20A, 30A, 40A, 60A, 80A, 100A).

Victron Energy remains the reliability benchmark. The SmartSolar 20A (van builds) retails at $158 from EcoDirect; the 100A unit costs $612.85 from ShopSolarKits. Budget brands cost less but have shorter warranties and limited firmware support. Full controller guidance in the charge controllers and power management guide.

Inverter Sizing

Inverter wattage must cover your peak simultaneous load — not your daily average. A chest freezer motor draws 3–5× its running wattage on startup. An undersized inverter trips on that surge.

Worked Example: Van Build (200W System)

This works for summer western US travel. Winter full-timing in Oregon (1.90 PSH Dec, 1.62h effective with flat mount) makes solar-only impractical November through February. See the van conversion guide for roof layout and tilt solutions.

Worked Example: Off-Grid Cabin (1.1kW System)

Add $1,500–$3,000 for professional installation if not DIY. Texas freeze events (2021-style) can knock out solar production for 3–5 days — a 3,000W propane generator backup is strongly advised. See the cabin solar setup guide for wiring diagrams.

Worked Example: Full Homestead (4.5kW System)

Battery Chemistry Comparison: LiFePO₄ vs AGM vs Gel

For new off-grid builds in 2026, LiFePO₄ is the right choice in most situations. Here's the full comparison so you can verify that for your case:

2026 Off-Grid Solar System Cost Breakdown

Prices reflect Q1 2026 US retail, post-tariff. No federal tax credit applies. Wholesale panel pricing (10+ unit quantities) is $0.26–$0.28/W TOPCon.

Stacked by component: panels / batteries / controls + BOS. BOS = Balance of System (wiring, fusing, disconnects, mounting).

11 Common Off-Grid Solar Sizing Mistakes

1. 1
   Designing for average sun, not worst-month sun

   Your system must work in December, not July. Using annual average PSH means you run out of power for months at a time. Always use the worst-month PSH as your design number.

2. 2
   Removing the 0.75 system efficiency factor

   Wiring losses, MPPT inefficiency, temperature derating, and soiling combine to roughly 25% in real-world conditions. Dropping the 0.75 factor produces a system that underperforms on every cloudy day and dies completely in your worst month.

3. 3
   Sizing battery to nameplate, not usable capacity

   A 200Ah AGM gives you 100Ah usable (50% DoD). A 200Ah LiFePO₄ gives you 160–200Ah. If you buy both expecting identical performance and you run into AGM's 50% ceiling nightly, you'll reduce its 400-cycle life to under 200 cycles.

4. 4
   Using a PWM controller for systems over 200W

   PWM controllers waste 20–30% of available solar energy in most conditions. MPPT pays back its cost premium in 6–18 months on any system over 200W. It's not optional for cabin-scale and larger systems.

5. 5
   Charging LiFePO₄ below 0°C

   Charging LiFePO₄ below 0°C (32°F) causes permanent lithium plating that permanently reduces capacity. The manufacturer minimum is 0°C; practitioners set BMS charge cutoff at 5°C (41°F) as a safety buffer. In cold climates, this requires a heated battery enclosure.

6. 6
   Underestimating refrigeration loads

   A 12 cu ft chest freezer draws 150W but runs 8–14 hours/day (1,200–2,100 Wh/day). Refrigeration typically accounts for 30–50% of total cabin load. Getting this wrong makes your entire system calculation wrong.

7. 7
   Buying a modified sine wave inverter for sensitive electronics

   Modified sine wave damages variable-speed motor drives (HVAC, well pumps), causes computer power supplies to run hot, and voids warranties on many appliances. Any system with a pump, modern fridge, or computer equipment requires pure sine wave.

8. 8
   Choosing 12V for a system over 800W

   A 12V system running 1,000W pulls 83A through your wiring. The same load at 48V pulls 21A — requiring wire that's 1/4 the cost and weight. For cabin-scale and larger, 12V is the expensive choice, not the cheap one.

9. 9
   Not accounting for flat-mount losses in winter

   Panels on a van roof or low-pitch cabin with no tilt lose 15% of winter output vs. optimal tilt. In Oregon (1.90 PSH Dec), effective design PSH drops to 1.62h with a flat mount — making a solar-only van effectively impossible November through February.

10. 10
    Guessing appliance wattages from labels

    Manufacturers often list max power, not running power. A Kill-A-Watt meter ($25) measures actual consumption. Guessing 30% high wastes money on oversized components; guessing 30% low means a battery bank that dies every evening.

11. 11
    Budgeting for the 30% ITC on a 2026 purchase

    The Residential Clean Energy Credit (Section 25D) expired December 31, 2025. Budgeting as if a 30% rebate is coming — and it isn't — means you're $3,000–$10,000 underbudgeted on a homestead-scale system. No federal credit applies in 2026.

Frequently Asked Questions