solar power

Rays and Recreation: How Much Solar Power Do I Need for My RV?

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You may have seen them resting on roofs at a campground, offered as an option at a dealership or even propped up in a sunny parking lot with wires running back to an RV. Solar panels are now ubiquitous among RV enthusiasts, providing observers of the off-grid lifestyle with an extra boost of power while forgoing toxic emissions and unsustainable fuel sources.

In decades past, solar power was thought of as an incredible but idealistic solution to our planet’s climate woes: a wonderful concept but simply too expensive and underpowered to meet the average household’s energy needs. Hefty and cumbersome, they were considered impractical even for many stationary installations, let alone for use on a moving vehicle. But times have changed, and thanks to the invention of lighter materials and highly efficient energy conversion technologies, it’s never been a better time to power your RV with the sun.

Lower costs and higher efficiency aren’t the only factors that play into the rise of solar power – in recent years, we’ve experienced a massive shift in public consciousness and concern for the planet’s future. As we’ve become increasingly aware of environmental issues such as fossil fuel usage and pollution, the popularity of solar power and other alternative energy sources has skyrocketed.

Solar power allows us to reduce our environmental impact while still maintaining all the comforts of modern life, so what’s stopping everyone from ditching their generators and harnessing the sun? Well, despite their wide availability and recent optimizations, solar power systems can still be a bit intimidating to those without an electrical background. Many people, upon seeing the calculations and wiring that are required for installation, feel daunted and decide against using solar panels for their RVs.

If you’re hesitant to explore RV solar power because of the math, science, and engineering involved, put your fears to rest: it’s not as complicated as it may seem! Read on to learn more about how solar power systems work, how to calculate your personal energy requirements and how to optimize your RV for solar. You’ll be soaking up the sun in no time!

Sun 101: The Basics of Solar Power

Before we can address the issue of RV solar power, we first need to understand the different components of a solar power system and the energy they produce. Let’s take a look beneath the surface of a solar panel and take an electric journey through the wires and waves.

#1 – Solar Panels

Many people think that a solar panel itself is all there is to a solar power system, but it’s simply the most visible of the components. A solar panel is an assembly of solar cells, usually made of silicon and protected by glass, that charges your batteries by use of a phenomenon known as the photovoltaic effect.

The photovoltaic effect occurs on a subatomic level. A solar cell is two sandwiched layers of silicon with impurities added to give the layer a positive or negative charge. This creates an electric field. When a light particle known as a photon hits the cell, it knocks electrons free from the silicon sandwich’s atoms. The electric field pushes the electrons out into wires creating an electrical flow.

Stronger light generates more electrical activity, so solar panels work best when they receive direct sunlight for long periods of time. Some solar panels have motorized mounts that can tilt the panels to follow the sun as the day progresses. Others are thin and flexible, allowing them to keep a low profile on curved roofs and absorb the sun from many angles.

A solar panel’s power output is usually given in watts (W). The higher the wattage, the more power the panel can provide. However, the stated wattage represents the maximum power the panel can generate; the actual output will vary depending on factors like temperature, light intensity, cloud cover, equipment quality, age, and cleanness.

#2 – Batteries

Solar panels don’t directly power your electrical devices. Rather, the electricity they generate is stored in your RV’s batteries. From there, power is drawn as needed to operate electrical devices.

The majority of RV batteries are 12 volt (V) lead-acid batteries. Scientifically speaking, voltage is the measure of electrical potential between the positive and negative terminals. Practically speaking, voltage can be used to monitor the charge status and overall health of the battery.

A battery’s voltage rating is just an average – the actual voltage at any given time will vary depending on the battery’s current state. For example, when fully charged, a 12V battery’s voltage could go up to 12.9V, and it could drop down to 11.4V when fully discharged. While it’s receiving a charge from the solar panels, its voltage could measure as high as 14V!

#3 – Charge Controllers

Most solar panels for RVs are rated at 12V, so it’s reasonable to assume that they’ll put out 12V of power. But the panel’s voltage rating, like a battery’s, is just an average, and in practice, the voltage can be a lot higher – up to 20V when the panel is in very bright sunlight. If your 12V RV battery received a 20V power surge from your solar panel, it could be damaged and even start a fire – not good!

Thankfully, most RV solar power systems include a charge controller to prevent situations like this. After the electricity leaves the solar panel it passes through the charge controller, which monitors and regulates the flow to keep everything running optimally. By limiting the power the battery receives at any given time, the charge controller ensures the safety and longevity of your solar power system.

Adding Up the Amperage: How to Calculate Your RV’s Power Consumption

To find out how much solar power you’ll need for your RV, you’ll need to add up the power consumption of all your devices and appliances. You can then compare this number to your battery capacity to determine how long your batteries can power your RV, and from there you can calculate how much solar power you’ll need to generate per day to maintain your energy requirements.

#1 – Power Conversions

When calculating your power usage and the capabilities of your solar power system, you’ll come across three units of measurement: volts, amps, and watts. Volts measure electrical potential. Think of it as how hard the electricity is being pushed through the wire. Amps measure current and watts measure power. The three are interrelated; if you know two, you can find the third by plugging the values into the equation Watts = Volts * Amps.

Your battery’s capacity is measured in amp-hours (Ah), typically between 70 and 90Ah per battery. Amp-hours are calculated by multiplying amps and hours. A battery with 80Ah of charge could provide 80A for one hour, 1A for 80 hours, or 5A for 16 hours – any combination so long as the product of the two values equals 80. Keep in mind that this figure was decided under optimal conditions. Outside factors such as temperature and battery condition can affect this number.

This makes it easy to figure out how long your battery could run your RV’s DC equipment, since DC power consumption is often measured in amps. Take a look at your built-in lighting, water pump or exhaust fans and you’ll see an input rating given in amps. A 2A fan run for one hour will deplete your battery 2Ah; run it for two hours and it’ll use up 4Ah.

But what about AC equipment? Many of the more powerful devices that you plug into your RV’s wall outlets, such as microwaves and hair dryers, run only on 120V AC power and state their power consumption in watts, not amps. To find out how long your battery can run these devices, you’ll need to use the equation from before: Watts = Volts * Amps.

Let’s use an example of a 1000W microwave designed to run at 120V. We can rearrange the equation to calculate for amps: Watts / Volts =Amps. 

You may be tempted to write out the equation like 1000W / 120V = 8.33 amps. However, your solar system draws power from your 12V batteries.

Your solar system powers your 120V AC appliances by using an inverter that converts the batteries’ 12V DC to 120V AC power. The source of the power is still around 12 volts, so that is what must be plugged into the equation.

We will use 12.6 volts to represent a full battery. So 1000W / 12.6V = 79.4 amps.

#2 – Accounting for Losses

As mentioned, built-in DC equipment runs directly off the battery, but anything you plug into the wall outlets receives AC power through your RV’s inverter. Even devices like phone chargers that state input requirements in amps still rely on AC power because they’re plugged into your wall outlets; the DC conversion occurs in the device itself.

The inverter draws 12V DC power from the batteries, converts it to 120V AC, and passes it along to the outlets. During this conversion, some power is lost to heat; the exact amount depends on the inverter’s efficiency but is generally around 10-15%. The device you are running still requires the same amount of power to run regardless of inefficiencies in the conversion. In other words, the amp draw must include the power to run the device plus the power loss, so you’ll need to figure this into the above equation. You can do this by figuring out the 12V amps your 120V device will use and dividing that number by 0.85 to account for a 15% loss.

The new equation to find the amp draw of a 120V appliance on a solar system is (Watts / Volts) / 0.85 = Amps. Let’s plug in the numbers.

(1000W / 12.6V) / .85 = 93.4 Amps

So our aforementioned microwave will actually draw about 93.4 amps from the battery due to the inverter. The easiest way to figure this out for each device is to set up a spreadsheet.

If you don’t want to go through the math for each device, you can also use an online calculator. BatteryStuff.com has a handy calculator to calculate AC to DC amperage through an inverter. You input your AC voltage, the AC amps of the device, and select your battery voltage.

#3 – Tallying the Total

Now that you know how to convert the various units of electricity, you can calculate approximately how many amp-hours you drain from your batteries each day. Just take each AC device’s amp rating in 12V DC and multiply it by the number of hours you use it per day. 

Next, calculate how many amp-hours your built-in DC equipment draws each day. Five 1A lights run for six hours a day will draw 30Ah; your 2A exhaust fan run for three hours a day will draw 6Ah. When you’re done, add everything up. There is no need to account for the 15% energy loss of the inverter since DC devices run directly off the battery.

Finally, add your AC (converted to 12V DC amps) and DC amp-hour totals and you’ll get the total amount of power consumption per day. Very conservative electricity usage could result in a total as low as 15Ah per day, meaning that one 90Ah battery could power your RV for six days. On the other hand, a large RV with many lights, screens and appliances running all day could easily surpass 150Ah a day – you’d need multiple batteries just to make it through a 24 hour period!

screenshot of a spreadsheet showing how tally up and calculate amp-hour usage.
This is a bare-bones example of the method to figure out power usage above.

An alternative way to find out how many amp-hours you use is to go camping. Make sure your batteries are fully charged by the time evening comes, and turn off any sources of battery charging. Use your RV like normal, including running 120V appliances as needed with your inverter.

Regularly check your battery level. If it is lead-acid, you don’t want to drain it past 50%, which is about when it reads 12.2 volts. Once you have discharged your battery to 50%, turn your generator on or plug your rig in to charge the battery. At this point, you have the numbers you need to calculate you Ah.

Let’s say you have a 100Ah battery, and it took you 24 hours to drain it 50%. That’s 50Ah per day. If it took 42 hours, you could divide 50 by 42 to get your hourly Ah usage and multiply that by 24 to get about 28.5Ah per day.

#4 – Battery Matters

As you can see, your battery capacity must be adequate to supply your power requirements – if it’s not, no quantity of solar panels will suffice to power your RV! And it’s not enough to simply have enough capacity to meet your daily usage needs. Lead-acid RV batteries can be damaged if they fall below 50% capacity; even if they’re not physically damaged, letting them discharge past 50% repeatedly will significantly shorten their lifespan.

You’ll want a total battery capacity of at least twice your daily amp-hour usage, but ideally more. Should you get caught in a rainstorm or otherwise not receive enough sunlight one day, you’ll be powerless unless you’ve got extra batteries all charged up and ready to power your RV. To be on the safe side, aim for four times your daily amp-hour usage in battery capacity.

Judging the Panel: How Many Solar Panels Do You Need for Your RV?

The final step in calculating our solar power requirements is picking the panels themselves. Here’s where things get a little tricky, because a solar panel’s power output is heavily dependent on external factors.

#1 – Panel Perfection?

A 100W solar panel only puts out 100W when conditions are absolutely perfect: it’s receiving direct noon sun on the entire panel, the glass is clean, the air is clear, the temperature inside the solar cell is around 77 degrees Fahrenheit and the panel is free from any damage or deterioration. In this scenario, the panel will generate its full stated power of 100W.

But in the real world, things rarely work out so perfectly. Maybe you’re up north in autumn and the daylight is dwindling; what little there is gets obscured by clouds for half the day. Or you’re exploring the southwest and while the sun’s strong, temperatures are high and dust seems to permanently coat the solar panels.

So your 100W solar panel won’t actually put out 100W, at least not for extended periods of time. Because the actual output is so dependent on the environment, it’s impossible to give a universal figure.

However, there’s an easy way to estimate how much solar power you’ll need to run your RV. Remember the amp-hours per day that you calculated? Get that figure ready – you’ll need it for these next steps.

#2 – The Final Countdown

Since solar panels are rated in watts, we could convert your amp-hours to watt-hours to figure out the number of panels you need, but there is an easier way. The general rule is that on average a 100W solar panel will generate about 30Ah a day. Suppose you figured you use 80Ah a day. You would need three 100W solar panels to fill that requirement.

Lithium-ion batteries can be safely drained further than lead-acid and maintain their voltage down to about 20% rather than 50%. In that case, 200Ah worth of lithium will get you more like 160Ah or more of usable charge, which will provide enough battery storage with reserve for the 80Ah per day scenario. If you are using lead-acid on the other hand, you will probably want at least 320Ah worth of battery storage if you wanted to set up the above system.

If you have a large RV with plenty of roof space, you can install multiple solar panels to meet your power requirements without issue. Smaller RVs present more of a challenge; you’ll need to use fewer solar panels with higher efficiency. You may want to invest in a tilting panel mount so your panels can move with the sun throughout the day, maximizing the rays they catch and thus the power they generate.

Living Solar-Fully: Optimizing Your RV for Solar Power

Solar power may not produce as much energy as other sources like generators or grid power, but you can make the most of it by cutting back on your energy usage. No, this doesn’t mean you have to give up your TV or live by candlelight! There are many easy ways to reduce your energy consumption without sacrificing your comfort or enjoyment of your RV.

#1 – A Bright Idea

If you use compact fluorescent (CFL) light bulbs in your RV, consider switching them to LED bulbs – you could cut your lighting power usage in half! A 12W CFL running on your 12V battery uses 1A, but a 6W LED uses just 0.5A. It doesn’t seem like much at first, but switching all your lights to LEDs can really add up when you consider how many lights you have and how long you run them for each day.

#2 – Screen Time

Looking for an excuse to get a new TV? Well, here’s a good one: modern 120V LED TVs use a third or less of the power of CRT TVs. When running off an inverter A 24” CRT TV uses around 120W, or 11.2 amps, while a 24” LED TV uses just 40W, or 3.7 amps – that’s a big difference in battery drain when you’re binge-watching for hours!

#3 – Alternative Appliances

Kitchen appliances are some of the most power-hungry devices out there. Blenders, microwaves, coffee machines and electric kettles are notorious battery drainers. Thankfully, there are plenty of ways to keep your RV kitchen running while still saving energy!

Many microwaves, especially older and larger units, use huge amounts of power: up to 1,500W in some cases. Consider replacing your microwave with a newer, smaller one. Recent models are engineered with efficiency in mind, so it’s easy to find one that uses less than 1,000W.

Coffee machines, though convenient, are just as consumptive as microwaves if not more so! Topping out at 1,500W, your coffee maker could be putting a surprising amount of stress on your power supply. Many RVers swear by percolators, French presses and other non-electric coffee brewing methods – it’s not quite as quick as pushing a button, but it only takes a little extra effort to save a lot of power.