What happens when the sun disappears for two days and your cabin battery is already empty?
That’s the question you should answer before buying solar panels.
An off-grid solar system isn’t simply a few panels connected to a battery. You need enough solar generation to replace the electricity you use, enough battery storage to survive periods of poor weather, and an inverter capable of handling your appliances when several of them operate at once.
For an RV, the calculation gets even more interesting because roof space is limited. A cabin gives you more room for panels and batteries, but it may also have much larger loads such as refrigerators, water pumps, heating systems and power tools.
The good news?
You can calculate most of the system yourself.
In this guide, I’ll show you exactly how to estimate your daily energy consumption, solar-panel size, battery capacity, inverter size and backup days for an off-grid cabin or RV.
NREL’s solar modeling tools use location, system size, orientation and other factors to estimate PV production, while off-grid system sizing also needs inputs such as operating hours and days of operation.
What Is an Off-Grid Solar System?
An off-grid solar system produces and stores electricity without relying on the utility grid.
A typical system contains:
- Solar panels
- MPPT charge controller
- Battery bank
- Off-grid inverter
- DC/AC wiring
- Fuses and disconnects
- Monitoring equipment
- Mounting hardware
The basic energy flow looks like this:
Solar Panels → Charge Controller → Battery → Inverter → Appliances
During sunny hours, the panels produce electricity.
Some of that power runs your appliances immediately. Any available surplus can charge the battery.
After sunset, the battery becomes the main energy source.
That’s why battery sizing is just as important as panel sizing.
The Department of Energy explains that batteries allow solar energy to be stored and used later when the sun isn’t producing electricity.
The Off-Grid Solar Calculator
Before buying equipment, write down these six numbers:
- Daily energy consumption
- Peak power demand
- Available peak-sun hours
- System efficiency
- Desired backup days
- Battery chemistry and usable depth of discharge
From those numbers, you can estimate:
Solar panel size
Battery size
Inverter size
Charge-controller size
Let’s calculate each one.
Step 1: Calculate Your Daily Electricity Usage
This is the most important step.
Don’t start with solar panels.
Start with your appliances.
Create a table like this:
| Appliance | Watts | Hours/Day | Daily Energy |
|---|---|---|---|
| LED lights | 40 W | 5 h | 200 Wh |
| Refrigerator | 100 W average | 10 h | 1,000 Wh |
| Laptop | 60 W | 4 h | 240 Wh |
| TV | 100 W | 3 h | 300 Wh |
| Water pump | 500 W | 0.5 h | 250 Wh |
| Phone charging | 20 W | 3 h | 60 Wh |
| Total | — | — | 2,050 Wh/day |
The basic formula is:
Watts × Hours = Watt-hours
So:
100 W × 5 hours = 500 Wh
And:
1,000 Wh = 1 kWh
Therefore, the example above uses:
2.05 kWh per day
This number becomes the foundation of the entire solar design.
Don’t Forget Hidden Loads
This is where many first-time solar buyers make mistakes.
Your appliance label might say:
100 W
But that doesn’t necessarily mean the appliance consumes 100 W continuously.
Refrigerators cycle on and off.
Water pumps run intermittently.
HVAC systems can have changing loads.
Electronics can consume standby power.
Even an inverter itself consumes some electricity.
So your real-world energy use can be higher than the simple appliance-label calculation.
Leave some margin.
For a preliminary design, adding roughly 10–25% to your calculated daily load can provide a reasonable planning cushion, but actual system design should use measured consumption when possible.
Step 2: Calculate Your Daily Energy Requirement
Let’s assume your cabin uses:
3,000 Wh/day
That’s:
3 kWh/day
Now add a 20% planning margin:
3,000 × 1.20 = 3,600 Wh/day
Your design target becomes approximately:
3.6 kWh/day
This doesn’t mean the cabin will always consume exactly 3.6 kWh.
It means you’re giving the system some room for losses and real-world variation.
Step 3: Find Your Peak Sun Hours
Now we need to know how much useful sunlight your location receives.
This is where location matters.
A solar system in Arizona won’t necessarily produce the same amount of electricity as an identical system in Washington, Maine or Alaska.
Weather, season, shading, panel orientation and temperature can all affect production.
NREL’s PVWatts tool is designed to estimate solar production using location and system characteristics, making it useful for preliminary planning. NREL also warns that modeled production is an estimate rather than a guarantee of actual future output.
For a simple manual calculation, let’s assume:
5 peak sun hours/day
Step 4: Calculate Solar Panel Size
Use this basic formula:
Required Solar Watts = Daily Energy ÷ Peak Sun Hours ÷ System Efficiency
Suppose:
- Daily energy = 3,600 Wh
- Peak sun hours = 5
- Overall planning efficiency = 80%
Then:
3,600 ÷ 5 ÷ 0.80 = 900 W
So you’d want roughly:
900 W of solar panels
In practice, I’d round upward rather than designing right at the mathematical minimum.
A practical target might therefore be:
1,000–1,200 W of solar
That gives the system more breathing room.
Why You Shouldn’t Size Solar Panels Exactly to the Formula
The sun isn’t a laboratory light source.
Real systems experience:
- Clouds
- Dust
- Heat
- Shading
- Panel mismatch
- Wiring losses
- Charge-controller losses
- Battery losses
- Inverter losses
- Seasonal changes
NREL notes that PV production estimates are subject to uncertainty and that actual production can differ from long-term modeled values.
So if your calculation says:
900 W
don’t automatically buy exactly 900 W.
You might choose:
1,000 W
or:
1,200 W
depending on your location, winter requirements and available space.
Step 5: Calculate Battery Capacity
Now comes the second major calculation.
Suppose your cabin uses:
3.6 kWh/day
and you want:
2 days of autonomy.
That means:
3.6 × 2 = 7.2 kWh
You need approximately 7.2 kWh of usable battery energy.
But usable capacity isn’t necessarily the same as the battery’s advertised capacity.
That’s important.
Battery Size Formula
A simple planning formula is:
Battery Nominal Capacity = Daily Energy × Backup Days ÷ Usable Battery Fraction
For example, assume:
- Daily energy = 3.6 kWh
- Backup = 2 days
- Usable fraction = 80%
Then:
3.6 × 2 ÷ 0.80 = 9 kWh
So you’d want approximately:
9 kWh of nominal battery capacity
If you’re using a lithium battery with a manufacturer-approved usable capacity close to its nameplate capacity, your calculation can be adjusted to the manufacturer’s specifications.
Always use the battery manufacturer’s stated usable-energy and operating limits.
Lithium vs Lead-Acid Batteries
For most modern mobile and cabin systems, lithium batteries are attractive because of their energy density, cycle characteristics and usable capacity.
Lead-acid batteries can still make sense for some budget applications, but they generally require more careful depth-of-discharge management.
Here’s a simplified comparison:
| Feature | Lithium | Lead-Acid |
|---|---|---|
| Usable capacity | Generally higher | Generally lower |
| Weight | Lower | Higher |
| Cycle life | Usually longer | Usually shorter |
| Upfront price | Higher | Lower |
| Maintenance | Low | Can be higher |
| RV suitability | Excellent | Possible |
| Cabin suitability | Excellent | Possible |
Don’t choose solely based on purchase price.
Look at usable kWh and expected lifetime.
Step 6: Decide How Many Backup Days You Need
This depends heavily on where and how you use the system.
1 Day
Good for:
- Sunny climates
- Weekend cabins
- Light RV usage
- Emergency backup
2 Days
A good middle ground for many small systems.
3 Days
Better for:
- Remote cabins
- Cloudier locations
- Winter operation
- Full-time RV living
4+ Days
Usually requires significantly more battery capacity and/or another generation source.
At this point, you should seriously consider whether adding a generator or other backup source makes more economic sense than endlessly increasing battery capacity.
Cabin Solar System Calculator: Example
Let’s build a realistic small off-grid cabin.
Assume the cabin uses:
3.6 kWh/day
Desired autonomy:
2 days
Peak sun:
5 hours/day
Planning efficiency:
80%
Solar Array
3.6 ÷ 5 ÷ 0.80
= 0.9 kW
Recommended planning range:
1.0–1.2 kW solar
Battery
3.6 × 2 ÷ 0.80
= 9 kWh nominal battery
Inverter
Suppose the maximum simultaneous appliance load is:
2,000 W
You wouldn’t want a 2,000 W inverter operating permanently at its maximum.
A reasonable starting point might be:
3,000 W pure-sine-wave inverter
provided the appliance startup loads and manufacturer’s requirements are compatible.
Cabin Example: What Could a 1.2 kW System Run?
A 1.2 kW solar array can potentially support a small cabin with loads such as:
- LED lighting
- Refrigerator
- Wi-Fi router
- Laptop
- Television
- Phone charging
- Small water pump
- Small kitchen appliances
But it may struggle with high-energy appliances such as:
- Electric resistance heating
- Large air conditioners
- Electric water heaters
- Large electric ovens
- Heavy power tools
That’s because energy consumption isn’t the only issue.
Power demand matters too.
Step 7: Calculate Inverter Size
Solar panels are rated in watts.
Batteries are commonly rated in watt-hours or kilowatt-hours.
But appliances require power, measured in watts.
Your inverter needs to handle the appliances you expect to operate simultaneously.
Let’s say your cabin has:
- Refrigerator: 150 W
- TV: 100 W
- Lights: 60 W
- Laptop: 70 W
- Water pump: 500 W
- Microwave: 1,200 W
If everything operates at once:
150 + 100 + 60 + 70 + 500 + 1,200 = 2,080 W
A 2,000 W inverter would be too close to its limit.
A larger inverter may be more appropriate.
But there is another issue.
Starting surge.
Motors and compressors can briefly demand substantially more power when starting.
The inverter must be capable of handling the relevant surge according to its specifications.
Pure Sine Wave Inverter: Worth It?
For cabins and RVs, I’d generally prefer a pure sine wave inverter when the budget allows.
Why?
Modern electronics, motors, chargers and appliances are designed around clean AC power.
The inverter’s job is to convert the battery’s DC electricity into AC electricity used by most household appliances.
Cheap modified-sine-wave inverters may cost less, but compatibility problems can make the savings disappear quickly.
Step 8: Size the Charge Controller
If you’re building a system with a separate charge controller, you need to consider:
- Solar-array voltage
- Solar-array current
- Battery voltage
- Controller maximum PV input
- Temperature effects
- Future expansion
For example, suppose you have:
1,200 W solar
on a:
12 V battery system
A rough current calculation is:
1,200 ÷ 12 = 100 A
But this doesn’t mean you should simply buy a 100 A controller.
Actual charging voltage, controller efficiency, array configuration and manufacturer specifications matter.
That’s one reason 24 V or 48 V systems become attractive as system size increases.
Higher battery voltage can reduce current for the same power.
12V vs 24V vs 48V Solar Systems
| System Voltage | Best For | Main Advantage |
|---|---|---|
| 12V | Small RVs, tiny cabins | Simple and widely available |
| 24V | Medium cabins/RVs | Lower current |
| 48V | Larger cabins | Much lower current and easier scaling |
12V
Great for small loads.
It’s also convenient because many RV appliances and accessories already operate at 12 V.
24V
A useful middle ground.
You can reduce current compared with 12 V while keeping the system relatively simple.
48V
Excellent for larger off-grid cabins.
At higher power levels, reducing current can simplify conductor sizing and reduce resistive losses, though proper electrical design is still essential.
RV Solar System Calculator
Now let’s switch from cabins to RVs.
RV solar has one major problem:
Limited roof space.
You might have a perfect theoretical system that simply won’t fit on the roof.
That’s why RV solar requires more attention to efficiency.
Example RV Daily Load
Let’s assume a travel trailer or camper has:
| RV Appliance | Power | Usage | Daily Energy |
|---|---|---|---|
| LED lights | 30 W | 5 h | 150 Wh |
| Refrigerator | 60 W average | 10 h | 600 Wh |
| Water pump | 60 W | 1 h | 60 Wh |
| TV | 100 W | 3 h | 300 Wh |
| Laptop | 60 W | 4 h | 240 Wh |
| Phones | 20 W | 3 h | 60 Wh |
| Fans | 50 W | 5 h | 250 Wh |
| Total | — | — | 1,660 Wh/day |
That’s:
1.66 kWh/day
Add a 20% planning margin:
1.66 × 1.20 = 1.99 kWh/day
So your planning target becomes roughly:
2 kWh/day
RV Solar Panel Calculation
Assume:
4.5 peak sun hours
and:
80% system efficiency
Calculation:
2,000 ÷ 4.5 ÷ 0.80 = 556 W
So approximately:
600 W solar
would be a reasonable starting point for this example.
But if you travel during winter or spend time in cloudy regions, you may want more.
RV Battery Calculation
Suppose you want:
2 days of autonomy
Daily energy:
2 kWh
Battery requirement:
2 × 2 = 4 kWh usable
If your battery design allows approximately 80% usable capacity:
4 ÷ 0.80 = 5 kWh nominal
So a practical target would be approximately:
5 kWh battery capacity
Again, use the manufacturer’s actual usable-energy rating when choosing the battery.
What About Running an RV Air Conditioner?
This changes everything.
Air conditioning is one of the biggest challenges for solar-powered RVs.
Suppose an RV air conditioner draws approximately:
1,500 W
for several hours.
At three hours:
1,500 × 3 = 4,500 Wh
That’s:
4.5 kWh
just for the air conditioner.
Your previous daily load might have been:
2 kWh
Now you’ve reached:
6.5 kWh/day
before considering other loads.
Suddenly, your required solar and battery system becomes much larger.
This is why RV air conditioning is often difficult to run entirely from rooftop solar.
RV Solar: Roof Panels vs Portable Panels
Portable panels can be extremely useful.
Roof-Mounted Panels
Pros
- Always available
- No setup each day
- Good for travel
- Automatic charging while parked
Cons
- Limited roof area
- Fixed orientation
- Can be shaded by roof equipment
- Harder to expand
Portable Solar Panels
Pros
- Can be positioned toward the sun
- Useful when RV is parked under partial shade
- Easier to expand
- Can supplement rooftop solar
Cons
- Must be deployed
- Security concerns
- Requires storage
- Setup takes time
For serious off-grid RV users, combining roof-mounted and portable solar can be a smart approach.
The 1,000W RV Solar Example
Let’s build a stronger RV system.
Solar
1,000 W
Battery
5–10 kWh
Inverter
2,000–3,000 W pure sine wave
Battery voltage
24 V or 48 V, depending on the architecture and equipment
This could be a powerful setup for many RV loads.
However, it doesn’t automatically mean you can run everything.
A system like this may handle:
- Refrigerator
- Lights
- TV
- Laptop
- Internet equipment
- Fans
- Small kitchen appliances
- Chargers
- Some power tools
But long-duration electric heating, large air conditioners and electric water heating can quickly consume the available energy.
Solar Panel Calculator Cheat Sheet
Here’s a simple starting table.
| Daily Energy Use | Approx. Solar at 5 Sun Hours & 80% Efficiency |
|---|---|
| 1 kWh/day | ~250 W |
| 2 kWh/day | ~500 W |
| 3 kWh/day | ~750 W |
| 4 kWh/day | ~1,000 W |
| 5 kWh/day | ~1,250 W |
| 6 kWh/day | ~1,500 W |
| 8 kWh/day | ~2,000 W |
| 10 kWh/day | ~2,500 W |
These are planning estimates, not guaranteed production figures.
Your actual solar requirement depends on location, season, shading, panel orientation, temperature, system losses and desired reliability.
NREL’s PVWatts approach exists precisely because location and system characteristics materially affect estimated production.
Battery Calculator Cheat Sheet
Assuming two days of autonomy and an 80% usable fraction:
| Daily Consumption | Approx. Nominal Battery |
|---|---|
| 1 kWh/day | 2.5 kWh |
| 2 kWh/day | 5 kWh |
| 3 kWh/day | 7.5 kWh |
| 4 kWh/day | 10 kWh |
| 5 kWh/day | 12.5 kWh |
| 6 kWh/day | 15 kWh |
| 8 kWh/day | 20 kWh |
| 10 kWh/day | 25 kWh |
Again, use the manufacturer’s actual usable capacity rather than assuming every battery can be discharged to the same percentage.
Three Example Off-Grid Systems
Small RV
Typical use: Weekend trips
- Solar: 300–600 W
- Battery: 2–5 kWh
- Inverter: 1,000–2,000 W
- System voltage: 12 V
- Backup: 1–2 days
Good for basic electronics, lighting, refrigeration and small appliances.
Medium RV
Typical use: Extended travel
- Solar: 600–1,200 W
- Battery: 4–8 kWh
- Inverter: 2,000–3,000 W
- System voltage: 12/24 V depending on design
- Backup: 1–3 days
A stronger setup for longer off-grid stays.
Small Off-Grid Cabin
Typical use: Part-time or light full-time occupancy
- Solar: 1–3 kW
- Battery: 8–15 kWh
- Inverter: 3,000–5,000 W
- System voltage: 24/48 V
- Backup: 2–3 days
This can support considerably more household equipment, assuming heating and cooling loads remain reasonable.
What About a Generator?
For serious off-grid systems, I wouldn’t automatically eliminate a generator.
A generator can provide an additional safety net during:
- Long cloudy periods
- Heavy winter storms
- Unexpected battery problems
- Extremely high loads
- Emergency situations
Think of it as insurance.
You might never use it.
But when the weather stays cloudy for four days and your battery hits 10%, you’ll be glad it’s there.
Off-Grid Solar System: Biggest Mistakes
Mistake #1: Buying Panels First
Don’t.
Calculate your energy requirements first.
Mistake #2: Ignoring Winter
A system that works perfectly in July might struggle in December.
Always size for the season that matters most to you.
Mistake #3: Forgetting Surge Loads
Motors and compressors can require significant startup power.
Your inverter needs to account for this.
Mistake #4: Undersizing the Battery
A giant solar array with a tiny battery isn’t necessarily useful after sunset.
Mistake #5: Oversizing Everything
The opposite problem is also real.
A massive battery and solar array can be expensive and unnecessary if your actual energy demand is tiny.
Good design is balanced design.
Mistake #6: Ignoring Shade
One tree can destroy your production assumptions.
Before installing panels, examine morning, midday and afternoon shading.
Expert Tip: Measure Your Actual Electricity Usage
If you’re replacing an existing grid-connected cabin or RV electrical system, don’t guess.
Measure it.
A plug-in energy monitor can help you understand the consumption of individual appliances.
For a cabin, smart-meter or utility data can also provide useful historical information where available.
Then design the solar system around real consumption.
This is much better than saying:
“I think I use about 3 kWh.”
Maybe you do.
Maybe you use 6 kWh.
The battery doesn’t care what you guessed.
Expert Tip: Separate Essential and Non-Essential Loads
For an off-grid cabin, divide appliances into two categories.
Essential
- Refrigerator
- Lighting
- Water pump
- Internet
- Medical equipment
- Security equipment
Optional
- Microwave
- TV
- Power tools
- Coffee maker
- Hair dryer
- Space heater
This lets you preserve battery power during bad weather.
An energy-management system can prioritize critical loads when battery state of charge falls.
Expert Tip: Consider Energy-Efficient Appliances
The easiest watt to generate is the watt you don’t need.
Replace:
Old refrigerator → Efficient refrigerator
Incandescent bulbs → LED
Electric resistance heating → Efficient heat pump where practical
Always-on electronics → Switched or scheduled loads
Reducing consumption can allow you to purchase a smaller solar array and battery.
The savings can be substantial.
Off-Grid Solar vs Generator
| Feature | Solar + Battery | Generator |
|---|---|---|
| Fuel required | No fuel during normal operation | Yes |
| Noise | Very low | High |
| Maintenance | Low | Higher |
| Night operation | Battery | Yes |
| Cloudy-weather backup | Limited by storage | Excellent |
| Renewable | Yes | Usually no |
| Automatic operation | Possible | Possible |
| Long-duration backup | Requires adequate storage | Excellent |
For the best reliability, many remote cabins use a hybrid approach.
Solar handles normal energy production.
Battery handles night and short outages.
Generator handles prolonged bad weather or unusually high demand.
Can an Off-Grid Solar System Run a Whole Cabin?
Yes—but only if the system is sized for the cabin’s actual loads.
A small cabin using 2–4 kWh per day is dramatically easier to power than a cabin using 15–20 kWh.
The biggest energy killers are often:
- Electric heating
- Air conditioning
- Electric water heating
- Electric cooking
- Large refrigeration systems
- Well pumps
- Workshop equipment
If your cabin uses these heavily, solar can still work, but the system becomes much larger.
Final Off-Grid Solar Sizing Formula
If you want to remember only a few formulas from this article, save these.
Daily Energy
Watts × Hours = Wh/day
Solar Array
Solar Watts ≈ Daily Wh ÷ Peak Sun Hours ÷ System Efficiency
Battery
Battery kWh ≈ Daily kWh × Backup Days ÷ Usable Battery Fraction
Inverter
Inverter Size ≥ Maximum Simultaneous Load + Appropriate Surge Capacity
Charge Controller
Choose a controller based on:
PV voltage + PV current + battery voltage + manufacturer limits
These formulas won’t replace professional engineering, but they’re excellent for creating a first-pass system design.
Final Verdict
An off-grid solar system for a cabin or RV doesn’t need to be complicated.
But it does need to be properly sized.
Start with your appliances.
Calculate daily kWh.
Then determine your local solar resource.
Size the panels.
Next, calculate the battery around your desired backup period.
Finally, select an inverter that can handle both your normal loads and the startup requirements of demanding equipment.
For a small RV, 300–600 W of solar and 2–5 kWh of battery storage can be a reasonable starting range for basic energy needs.
For a medium RV with heavier usage, 600–1,200 W of solar and 4–8 kWh of storage may make more sense.
A small off-grid cabin could easily require 1–3 kW of solar and 8–15 kWh of storage, depending heavily on appliances and climate.
But don’t treat those ranges as universal answers.
Your electricity consumption decides the system.
And if you’re building a serious off-grid installation, use a location-specific solar-production model such as NREL’s PVWatts/SAM tools and have the final electrical design checked against applicable electrical, fire and installation requirements. NREL specifically provides tools for estimating PV production and modeling system and battery characteristics.
The smartest off-grid system isn’t necessarily the biggest.
It’s the one that produces enough energy, stores enough energy and wastes as little energy as possible.
