How to Size a Solar System (The Complete Guide)
Solar system sizing is a four-step math problem: figure out how much energy you need, how much sun you get, how big your panels must be, and how much battery capacity keeps you running on cloudy days. This guide walks through every step with real numbers.
The Short Version
A typical RV or van running 2,000Wh per day needs 400 to 500W of solar panels, a 200Ah LiFePO4 battery at 12V, a 40A MPPT charge controller, and a 2,000W pure sine wave inverter. In most of the continental US (5 peak sun hours), 400W of panels recovers roughly 1,500Wh per day after system losses. Add 100W of panels for every 300Wh of additional daily load.
What a Solar System Consists Of
Every off-grid solar system has four core components. They must be sized to work together, and the weakest link limits the whole system.
Solar Panels
Panels convert sunlight into DC electricity. Their output is rated in watts at standard test conditions (STC), which means 25°C cell temperature and 1,000W/m² irradiance. Real-world output runs 75 to 85% of the nameplate rating due to heat, wiring losses, and angle variance. A 400W panel realistically delivers 300 to 340Wh per peak sun hour.
Batteries
Batteries store the energy panels produce during daylight for use at night or on cloudy days. Capacity is measured in amp-hours (Ah) at a specific voltage. A 100Ah 12V battery stores 1,200Wh total, but usable capacity depends on chemistry: LiFePO4 gives you 80% (960Wh), while AGM safely delivers only 50% (600Wh) before damage risk.
Charge Controller
The charge controller sits between panels and batteries, regulating voltage and current so panels do not overcharge or damage the battery. MPPT controllers extract 15 to 30% more power from the panels than PWM controllers by optimizing the operating voltage. For any system over 200W, MPPT is worth the price difference.
Inverter
An inverter converts DC battery power into 120V AC power for standard appliances. Pure sine wave inverters work with any device, including sensitive electronics and motor-driven appliances. Modified sine wave inverters are cheaper but damage some electronics and cause motors to run hot. Always use pure sine wave for RV, cabin, and van builds.
Step-by-Step Sizing Calculation
Step 1: Calculate Daily Energy Consumption
List every device you plan to run. For each device, record its wattage and how many hours per day you will use it. Multiply wattage by hours to get watt-hours (Wh). Add everything up.
| Appliance | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| 12V Compressor Fridge | 45 | 12 (duty cycle) | 540 |
| LED Lights | 30 | 5 | 150 |
| Laptop | 65 | 8 | 520 |
| Phone Charger | 10 | 3 | 30 |
| Fan (12V) | 25 | 8 | 200 |
| Total | 1,440 Wh |
Step 2: Apply the 20% System Loss Factor
No system runs at 100% efficiency. Wiring resistance, battery charge/discharge losses, and heat all reduce usable output. Multiply your raw daily Wh by 1.25 to account for losses (equivalent to assuming 80% system efficiency).
Step 3: Determine Peak Sun Hours
Peak sun hours (PSH) are the daily equivalent hours of full 1,000W/m² sunshine your location receives. This is not the same as daylight hours. A cloudy day might count as only 1 PSH while a clear summer day in the Southwest counts as 7.
Southwest US (AZ, NM, NV)
5.5 to 6.5 PSH
Southeast US (FL, TX, GA)
4.5 to 5.5 PSH
Midwest (IL, OH, KS)
4.0 to 5.0 PSH
Northeast US (NY, MA, PA)
4.0 to 4.5 PSH
Pacific Northwest (WA, OR)
3.5 to 4.0 PSH
Canada (most regions)
3.0 to 4.5 PSH
Step 4: Calculate Panel Wattage
Divide your adjusted daily load by your peak sun hours. This gives you the minimum panel wattage needed to replace that energy in one full sun day.
Round up to: 400W (two 200W panels)
Step 5: Size the Battery Bank
Decide how many days of autonomy you need the number of overcast days your system should handle without any solar input. Multiply daily load by autonomy days. Divide by depth of discharge (DoD) for your battery chemistry. Convert to amp-hours by dividing by system voltage.
LiFePO4 at 80% DoD: 2,880 ÷ 0.8 = 3,600 Wh capacity
At 12V: 3,600 ÷ 12 = 300Ah
Use: 2x 150Ah or 1x 300Ah 12V LiFePO4 battery
Step 6: Size the Charge Controller
Divide total panel wattage by system voltage to get the charging current. Apply the 1.25x NEC safety factor for overcurrent protection. Choose a controller rated at or above this value.
33.3A x 1.25 NEC factor = 41.7A
Use: 40A MPPT (if panels are 12V) or 50A MPPT
Step 7: Size the Inverter
The inverter's continuous watt rating must exceed your maximum simultaneous AC load. The surge rating must handle the highest-surge device in the system (usually a fridge compressor or pump motor, which draws 3 to 7 times running watts at startup).
Fridge compressor surge: 300W x 4 = 1,200W
Use: 1,500W inverter (continuous), 3,000W surge
Factors That Affect System Sizing
Geographic Location and Season
A system sized for summer in Colorado (6 PSH) will underperform in December (3 PSH). If you need the system to work year-round, size for your worst-case winter sun hours, not average annual. This often doubles the panel requirement for locations above 40 degrees latitude. If you only use the system in summer, size for summer and plan to supplement with a generator in shoulder seasons.
Panel Angle and Orientation
Panels fixed flat on an RV roof lose 10 to 15% compared to a tilted, south-facing array at the optimal angle. Tilting panels to latitude angle maximizes annual production. In summer, a shallower angle captures more energy; in winter, a steeper angle is better. For stationary installs, adjustable tilt mounts recover that efficiency. For mobile systems, flat-mounted panels are the practical trade-off.
Shading
Shade is the largest real-world source of power loss. One shaded panel in a series string reduces the output of the entire string, not just the shaded panel. A single tree branch casting shade on 10% of a panel can cut string output by 50%. Solutions include parallel wiring (shading one parallel string does not affect others), MPPT power optimizers on individual panels, or physically avoiding shade during installation.
Common Sizing Mistakes
Undersizing the battery bank
Sizing battery capacity to cover only one night of use leaves no buffer for consecutive cloudy days. A 2-day autonomy buffer is the minimum for any system used year-round. Mobile systems that move often can get away with 1.5 days. Permanent installs should target 3 days.
Ignoring surge watts when choosing an inverter
A fridge rated at 150W running may draw 600 to 1,000W at startup. A well pump rated at 750W may surge to 3,000W. If the inverter cannot handle the surge, it shuts off. Always identify the highest-surge device and size the inverter's surge rating above it.
Forgetting system losses
New builders often size for 100% panel output reaching batteries. Real-world losses add up: 5% wiring, 3% controller inefficiency, 10% battery round-trip, 10% inverter conversion. Without the 1.25x multiplier, the system will fall 20 to 25% short of expectations.
Using battery Ah as if it were 100% usable
A 100Ah AGM battery should not be discharged below 50% regularly doing so cuts cycle life from 400 cycles to under 200. A 100Ah LiFePO4 battery can discharge to 20%, but 80% is the practical maximum for longevity. When sizing, always divide by DoD to get the real capacity required.
Sizing panels for average annual sun hours
Your system will underperform in winter months if you size for annual averages. For a system that must work in December, use December's PSH value, which can be 30 to 50% lower than the summer peak. Alternatively, plan for generator backup during the winter months.
When to Use 12V vs 24V vs 48V
System voltage determines wire size, battery configuration, and component compatibility. Higher voltage moves the same power through smaller wire, reducing costs and heat losses on longer runs.
| Voltage | Best For | Panel Range | Notes |
|---|---|---|---|
| 12V | RV, van, boat, small cabin | 100W to 800W | Works with 12V appliances directly; most common component availability |
| 24V | Cabin, tiny home, large RV | 400W to 3,000W | Half the current of 12V at same power; smaller wire; requires 24V-to-12V converter for 12V devices |
| 48V | Whole-home backup, off-grid home | 3,000W+ | Quarter the current of 12V; most efficient for large arrays; components more expensive |
The rule of thumb: use 12V for anything under 1,500W total panel capacity and wire runs under 20 feet. Switch to 24V for systems between 1,500W and 4,000W. Use 48V for whole-home or heavy commercial applications above 4,000W.
Cost Expectations by System Size
Small (100 to 300W)
$400 to $900Weekend camping, CPAP, emergency backup
100 to 300W panels, 100Ah LiFePO4, 20 to 30A MPPT controller
Medium (400 to 600W)
$1,200 to $2,500Full-time RV, van life, summer cabin
2 to 3x 200W panels, 200Ah LiFePO4, 40A MPPT, 1,500 to 2,000W inverter
Large (800 to 1,500W)
$3,000 to $6,000Permanent cabin, tiny home, boat liveaboard
4 to 6x 200W panels or 2 to 4x 400W, 400Ah 24V LiFePO4, 60A MPPT, 3,000W inverter
Whole-home (3kW to 10kW)
$12,000 to $35,000Off-grid primary residence
Multiple 400W+ panels, 400 to 800Ah 48V LiFePO4, 80A+ MPPT, 5 to 10kW inverter-charger
Costs are for DIY builds using quality components. Professional installation adds 50 to 100%. Prices fluctuate with panel and battery costs.
Recommended Components for a Medium RV System
The following components make up a proven 400W system for an RV or van at 12V, covering 1,500 to 2,000Wh per day in average US sun conditions.
Renogy 200W 12V Monocrystalline Panel
2x these = 400W array. 21% efficiency, includes Z-brackets and cable.
LiTime 12V 200Ah LiFePO4 Battery
2,400Wh usable at 80% DoD. 4,000+ cycle lifespan. Built-in BMS.
Renogy 40A MPPT Rover Charge Controller
Handles up to 520W at 12V. Bluetooth monitoring. LCD display included.
Renogy 2000W Pure Sine Wave Inverter
4,000W surge capacity. Runs all AC appliances. USB ports included.
Frequently Asked Questions
How many solar panels do I need for a 2,000Wh daily load?+
How big a battery do I need for 2 days of autonomy at 1,500Wh per day?+
What is a peak sun hour and how do I find mine?+
When should I use 24V instead of 12V for my system?+
What is the 1.25x NEC safety factor for charge controllers?+
Calculate Your Exact System Size
Enter your appliances and location. Get panel wattage, battery size, charge controller, and inverter sizing in under 2 minutes.
Open the Calculator