Solar Panel & Battery Sizing Calculator
In short, Audit your daily energy budget, dial in your average sun hours, and instantly see exact requirements for solar panels, lithium battery capacity, and MPPT charge controller sizing.
1. Your Appliances & Daily Usage
Select appliances you plan to run. Adjust running watts or estimated operating hours per day to match your equipment.
2. System Architecture & Battery
3. Sunlight & Weather Reserve
How This Solar Calculation Works
Sizing an off-grid solar system isn't about guessing how many panels fit on your roof — it's an energy balance equation. You must harvest enough daily kilowatt-hours during the peak sun window to replace whatever you took out of the battery bank over 24 hours, plus system inefficiencies.
1. The Usable Energy Equation
Every electrical transfer loses energy. An MPPT charge controller operates at roughly 97% efficiency, wiring loses 2% to 3%, and charging a LiFePO4 battery consumes 5% in thermal losses (AGM loses closer to 20%). We apply an overall system efficiency factor of 0.80 to ensure your panels actually replenish the bank even on hazy days.
Solar Watts = (Daily Wh Needed / Peak Sun Hours) / 0.80
2. Battery Bank Usable Depth of Discharge (DOD)
A 100Ah lead-acid (AGM) battery can only deliver 50Ah safely. Discharging below 50% permanently damages the lead plates and causes premature capacity degradation. By contrast, modern lithium iron phosphate (LiFePO4) cells can be discharged to 85–90% daily for 3,000 to 5,000 cycles without significant degradation.
3. Typical RV & Campervan Sizing Benchmarks
| Vehicle / Use Case | Daily Energy (Wh) | Solar Panel Array | LiFePO4 Battery | Recommended MPPT |
|---|---|---|---|---|
| Weekend Campervan | 400 – 600 Wh/day | 150W – 200W | 100Ah (12.8V) | 15A – 20A MPPT |
| Full-Time Van Lifer | 1,200 – 1,800 Wh/day | 400W – 600W | 200Ah – 300Ah (12.8V) | 40A – 50A MPPT |
| Off-Grid Cabin / Heavy RV | 2,500 – 4,500 Wh/day | 800W – 1,200W | 400Ah+ (24V/48V) | 60A – 100A MPPT |
Frequently Asked Questions
Can I mix 100W and 200W solar panels together?
You can, but it requires careful wiring. If wired in series, the panel with the lowest current (Amps) throttles the entire string down to its level. If wired in parallel, the panels must have identical operating voltages (Vmp). We recommend keeping panel wattage and voltage identical across strings whenever possible.
Why does my 200W panel only produce 160W in real life?
Solar panels are rated at Standard Test Conditions (STC): 1,000 W/m² irradiance and a cell temperature of 25°C (77°F). Under direct summer sun, dark solar panels routinely reach 60°C (140°F). Due to the panel's temperature coefficient (typically -0.38% per °C), heat alone derates output by 12–15%. Adding atmospheric haze, dust, and flat mounting on an RV roof, 75% to 85% of nameplate rating is completely normal real-world performance.
Do I need an MPPT or PWM charge controller?
For any system above 150 watts, an MPPT (Maximum Power Point Tracking) controller is mandatory. A PWM controller forces the solar panel to match battery voltage, directly throwing away any voltage above 14V as lost heat. An MPPT converts excess voltage into extra charging current, yielding 20% to 35% more power on cold or partly overcast days.