Solar Panel Sizing: A Practical Guide for Home, RV, Van, and Off-Grid

Step 1: Calculate Your Daily Watt-Hours

Every solar system starts the same way: figure out how much electricity you actually use in a day. Not a guess, not a ballpark. Actual watt-hours.

For a home, the easiest path is your electric bill. Divide monthly kWh by 30 for a daily average, then multiply by 1000 to get watt-hours. A home using 900 kWh per month runs 30 kWh per day, or 30,000 Wh.

For a van or RV, list every device you plan to run, its wattage, and hours per day. A 12V compressor fridge pulling 45W for 12 hours a day is 540 Wh. Add up every device. A typical van build lands between 1,000 and 2,500 Wh/day. A larger RV with a microwave, TV, and well pump can hit 3,500-5,000 Wh/day.

A kill-a-watt meter on each AC device during a normal week gives you the real number. Nameplate watts are usually higher than actual draw.

Step 2: Add the 25% Design Margin

Real-world solar systems lose energy at every stage. Wiring has resistance. Charge controllers are 85-98% efficient. Inverters burn 10-15% in the AC/DC conversion. Batteries lose a few percent on every charge cycle. Weather varies. Your usage varies day to day.

An RV using 1,500 Wh/day gets a 1,875 Wh/day design load. A cabin using 3,600 Wh/day gets a 4,500 Wh/day design load. A home using 30,000 Wh/day gets 37,500 Wh/day.

This margin covers system inefficiency (usually 10-15%), day-to-day variation, and occasional cloudy days that reduce production. It's not generous enough to cover a week of heavy overcast. That's what battery sizing handles.

Step 3: Size Panels, Battery Bank, and Charge Controller

Panel Sizing

The panel-sizing formula pulls together three numbers: design load, peak sun hours at your location, and a system efficiency factor.

The 0.78 factor accounts for all real-world losses between panels and loads: temperature derating, wiring, charge controller efficiency, battery round-trip losses, and inverter efficiency. It already includes inverter losses. A common mistake is to apply an additional inverter-efficiency multiplier on top of 0.78, which double-counts and oversizes the array by about 15%. Grid-tied systems without batteries can use 0.82-0.85 because there's no battery round-trip loss.

Example: RV with 1,500 Wh/day usage, 4.5 peak sun hours, 200W panels:
Design load = 1,500 × 1.25 = 1,875 Wh
Panels = 1,875 ÷ (200 × 4.5 × 0.78) = 2.67, rounded up to 3 panels.

Battery Bank Sizing

Battery capacity is based on raw daily usage, number of autonomy days, and depth of discharge. No design margin on storage.

LiFePO4 batteries allow 80% depth of discharge and handle 4,000+ cycles. AGM lead-acid allows 50% DoD and 300-500 cycles. LiFePO4 costs more up front but nearly always wins on cost per lifetime cycle.

Example: 1,500 Wh/day van, 2 days autonomy, LiFePO4:
Battery Wh = (1,500 × 2) ÷ 0.80 = 3,750 Wh, or about 310 Ah at 12V.

Charge Controller Sizing

Charge controller amperage uses a 1.25 cold-weather margin because panels can exceed their rated output by 10-25% in cold conditions. Cold sunny winter mornings produce the peak current your controller has to handle.

Example: 600W array on 12V: (600 × 1.25) ÷ 12 = 62.5A. Round up to a 70A or 80A MPPT controller. If calculated amps exceed 60A, most residential and RV controllers cap out and you need either a 48V system (lower current for the same wattage) or multiple controllers in parallel.

MPPT controllers harvest 15-30% more energy than PWM. The extra cost pays itself back in months on any system above 200W.

Step 4: Size the Inverter for Simultaneous Loads

Inverter continuous wattage has to handle the maximum AC load at any one moment. Not the daily total. Not the single largest appliance. The maximum combination of appliances running at the same time.

Think about the worst case. Microwave running, fridge compressor cycles on, laptop on the counter charging, lights overhead. What's the total wattage during that 30 seconds?

RV example: Microwave 1,000W + fridge compressor 150W + laptop 65W + lights 40W = 1,255W simultaneous. 1,255 × 1.20 = 1,506W. A 2,000W pure sine wave inverter handles this with surge margin for the fridge startup.

Cabin example: Microwave 1,000W + well pump 750W + fridge 150W + lights 80W + TV 80W = 2,060W. 2,060 × 1.20 = 2,472W. A 3,000W inverter, ideally an inverter-charger for generator integration.

Step 5: Size Wire Gauge and Fuses Correctly

Wire undersizing is the most common mistake in DIY solar, and it's a fire hazard. Every wire in the system carries different currents and gets sized differently.

Battery-to-Inverter Wire (the critical run)

This wire carries the highest current in the system. Size from inverter maximum input current, not from daylight-average current.

A 2,000W inverter on 12V draws up to 167A. That requires 2/0 or 4/0 AWG cable. Keep this run as short as possible, ideally under 3 feet. Sizing this wire from average daylight current instead of inverter max current undersizes it by 4-5x and the wire will overheat or melt under load.

Panels-to-Controller Wire

Size from panel array short-circuit current (Isc) with a 1.25 multiplier, then check against voltage drop at your actual wire-run length. Target is 3% voltage drop or less.

Wire resistance per foot (copper): 14 AWG = 0.00253, 12 AWG = 0.00159, 10 AWG = 0.000999, 8 AWG = 0.000628, 6 AWG = 0.000395, 4 AWG = 0.000249, 2 AWG = 0.000156.

Example: 600W array, 3 panels in parallel at 11A Isc each = 33A total. 33 × 1.25 = 41A. For a 15-foot run at 12V, 4 AWG gives 2.61% drop (passes). 6 AWG would pass at 2.96%. 10 AWG would fail at 7.49%.

Array Fuse (before the charge controller)

The 1.56 factor comes from stacking two NEC requirements: 690.8(A) requires 125% of Isc for continuous current, and 690.9(B) requires another 125% for overcurrent protection. 1.25 × 1.25 = 1.5625, rounded to 1.56. For a 33A Isc array: 33 × 1.56 = 51.5A. Use a 60A DC-rated fuse (next standard size up).

Battery-to-Inverter Fuse (use Class T)

This is the most critical fuse in the entire system. A short circuit between battery and inverter on a LiFePO4 bank can pull 10,000+ amps in milliseconds. Enough to melt cables and start fires before a standard fuse opens.

Example: 3,000W inverter on 48V: 3,000 ÷ 48 = 62.5A. Use an 80A Class T fuse. For a 2,000W inverter on 12V: 167A, use a 200A Class T.

NEC 690.12 Rapid Shutdown (for roof-mounted home systems)

Roof-mounted solar on buildings needs to reduce conductor voltage at the array boundary to 30V within 30 seconds of shutdown. Microinverters (Enphase) and DC power optimizers (SolarEdge, Tigo) meet this natively at the module level. String-inverter systems need separate rapid-shutdown devices installed per panel. Van and RV roof arrays are vehicle-mounted rather than building-mounted, so 690.12 generally doesn't apply to mobile installations; check your local code if uncertain.

Federal Tax Credit Status for 2026

The One Big Beautiful Bill Act (Public Law 119-21, signed July 4, 2025) terminated the Section 25D Residential Clean Energy Credit effective December 31, 2025. Systems placed in service on or after January 1, 2026 receive zero federal residential tax credit on direct purchase or DIY installation. Standalone residential battery storage lost its credit on the same date.

The commercial Section 48E credit still applies to third-party-owned residential solar (leases and power purchase agreements) through end of 2027 if construction begins by July 4, 2026. State and local incentives are now the primary lever for cost reduction. Check DSIRE (dsireusa.org) for incentives available in your state before budgeting.

Common Sizing Mistakes

• Double-counting efficiency losses. The 0.78 system efficiency factor already includes inverter efficiency. Applying a separate inverter-efficiency multiplier oversizes the array by 15%.
• Sizing inverter to the largest single appliance. Multiple devices can run simultaneously. Size to the sum of simultaneous loads plus 20%.
• Undersizing battery-to-inverter wire. Size from inverter max input current (continuous W ÷ battery V), not daylight-average.
• Using ANL fuse between battery and inverter on LiFePO4. Use Class T. Every LiFePO4 build needs one.
• Forgetting cold-weather Voc. Panels produce 10-25% more voltage in cold conditions. Controller max input voltage must exceed cold-weather Voc, not nominal Voc.
• Ignoring voltage drop on long wire runs. Wire that handles the current at 3 feet will drop 7%+ at 15 feet. Use the voltage-drop formula or a voltage-drop table.
• Panel series-parallel mismatch. Mixing panels of different voltages or wattages in the same string wastes current. Series strings need matched panels.