Sizing an Inverter for Mobile Applications

boat-helm-inverter-mobile-application

Inverters are indeed handy for powering a wide range of AC loads from your DC power sources while “off the grid.” You can run everything from laptop chargers to microwave ovens to refrigerators and, just like sizing an inverter for stationary applications such as in an off-grid home, the loads will greatly determine the necessary inverter size and specifications. However, mobile applications may also warrant further analysis and consideration.

Particularly in mobile power systems, house batteries often power many DC loads such as phone chargers (through USB connections), LED lighting, and even some entertainment systems. While these loads must be considered when correctly sizing the battery bank and charging systems, they do not need to be factored into the total mobile AC load for inverter size calculations.

A mobile inverter’s major loads may, therefore, be limited to large appliances such as a microwave and a refrigerator. And while these are not trivial loads, they’re also not usually continuous. For example, a 1000 W microwave may actually be rated as an 1800 W load, but its power level setting will also vary the actual load. Similarly, a modern fridge may be specified as a 600 W load when starting the compressor, but the running load may be just 100 W or less the majority of the time.

In many cases, a 2500 W true sine wave inverter along with a healthy and well-sized battery bank should be able to comfortably handle such a load combination. The inverter’s typical specifications should indicate sufficient surge capacity for these dynamic loads, and, under normal operating conditions, it would also operate at relatively high efficiency.

Some people have considered another strategy using multiple inverters. In the example above, a large inverter could be turned on to power just the microwave, and then turned back off when finished cooking. A second smaller inverter – one with sufficient surge capacity – could then be used to continuously and efficiently power the fridge. While this approach may help reduce low load losses and conserve a bit of battery energy when the microwave is off, it would also require creating two separate AC wiring systems within the coach or vehicle. One side benefit of this approach is that the larger inverter could serve as a back up for the smaller one, or to handle unexpected loads.

Also, keep in mind that inverter specifications are typically based on limited temperature and other environmental ranges. Accordingly, a mobile inverter’s more dynamic operating environment may warrant further analysis and consideration. If the inverter is going to be installed in a confined space, then adequate ventilation may be a concern. Deployment in very cold or hot conditions may require environmental protection and/or power derating. Operation at high elevation may also require power derating, which could mean specifying a larger inverter.

Power requirements, load types, and dynamic operating conditions must be factored into sizing an inverter (or inverters) for an efficient and reliable mobile system. Give us a call at 615-285-0663, and we’ll be happy to discuss your application and help you identify a solution that fits your particular requirements.


About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.

Boondocking Made Easy With Solar Power

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Boondocking (aka dry camping, dispersed camping, wild camping) may mean slightly different things to different people. You can boondock in the wilderness, at a state park’s primitive site, on a farm, or even at a vineyard. The common denominator is no hookups for your RV; no shore power electricity; no pressurized water source; and no sewer connection.

Boondocking isn’t everyone’s cup of tea, as some want many of the comforts and appliances of home. But for those who are willing – or want – to avoid the crowds, noise, and tight quarters of an organized campground or RV resort and get away to quiet and spacious new vistas, boondocking may be just the ticket. It just takes a fresh perspective, some planning, and compromises, and managing your precious resources like fuel, water, and electrical energy.

A large “house” battery bank may be able to supply an RV with electrical power for several days, but such a battery bank will likely be quite heavy and take up a lot of space. A generator can recharge your battery bank and/or power large loads, but it requires fuel, space, and supervision; it also makes noise, and the exhaust smells. Your vehicle alternator may be able to recharge your house batteries but shares drawbacks similar to those of running a generator.

Alternatively, a PV (solar electric) system for your RV can quietly and reliably help sustain even a moderately-sized house battery bank while boondocking so you, your family and friends can relax and enjoy your site and surroundings. Your boondocked neighbors will also appreciate your quiet and emissions-free setup.

While there are exceptions to every rule, boondocking with a PV system typically involves addressing four application-specific variables: daily energy requirement, autonomy, environment, and space. See our blog post “How to Size a PV System for Your Boat or RV” for a detailed discussion.

Buying and installing an RV PV system isn’t exactly free. However, buying and installing a PV system for your RV instead of extra batteries and/or a generator may make for a compelling financial consideration that complements a PV system’s operational and environmental benefits. And, since the PV system will likely weigh less than extra batteries and a generator, you may be able to carry more water to help extend your boondocking adventures.

Give us a call at 615- 285-0611 and we’ll be happy to discuss how a PV system for your RV can help you extend and enjoy your boondocking adventures.


About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.

Meet Nathan Jones – Self-taught Solar Power Dude

Nathan-Jones-Solar-Dock-Power

Inventor. Wanderer. Off-grid Pioneer.

Nathan Jones is one unique individual. He’s been living and working off-grid for 21 years. The energy he uses to power his 50-acre ranch and manufacturing operation is run strictly on solar renewable energy. That’s why we call him The Solar Power Dude.

Like many self-made entrepreneurs, Nathan’s curiosity and self-reliance was a driving force in his life. He wondered, Why do we build all our homes and businesses that require an umbilical cord to be habitable? Why don’t we build structures that are self-sufficient?

While building his own home he didn’t want to be dependent on the local utility, so he started learning how the solar equipment worked and could be integrated into his home. He hooked everything up and it worked!

Nathan was off-grid.

Timing is Everything

Nathan freely admits he was in the right place at the right time. When Y2K became an event, word of mouth led to his first business venture, Missouri-based Power Source Solar, building solar power systems.

For those too young to remember, Y2K was a phenomenal event in the solar renewable energy world. At that time, renewable energy was still in its infancy, so a lot of the technicians and engineers at the factories were the ones who answered the phone when Nathan called.

Many of them took Nathan under their wing and spent time answering his questions, illustrating how things worked and explaining why they built systems the way they did.

“I was a sponge absorbing what I could from people who knew a lot more than me,” said Nathan.

Y2K Event Jumpstarts Solar Power Business

Y2K Time Magazine Cover

Here’s Nathan’s memory of Y2K:

“I live in the Missouri Ozarks which is a pretty rugged part of the country and a huge survivalist mecca. There was a huge influx of people with the ‘end of the world’ mentality or what I refer to as ‘the fanatic fringe.’

“We built a huge amount of solar/renewable systems using the technology and equipment we had at the time. Batteries were much cheaper than they are today and the cost of solar panels was horrifically expensive.

“So those types of systems tended to have very small solar arrays, massive battery banks and they always included a generator. People using these systems operated in a feast or famine mode. When a generator was running and doing a charge on the battery bank, you had a lot of electricity to pump water, do the laundry, run the dishwasher, take a shower, lounge in the Jacuzzi, run the equipment in the shop and do all the things you needed to get done. Then you shut the generator off and you went into survival mode.

“You used all the solar you had and then robbed the shortfall from the battery. Usually, we tried to size our systems out for about 72 hours. Then the cycle would begin again once the generator was fired up for another five or six hour charge time on the battery bank and that gave you five or six hours of a lot of spare capacity of electricity to allow you to take care of heavy loads again and then you go back into survival mode on the battery and solar.

“The Y2K folks weren’t concerned about energy efficiency. They were more interested in a survivalist lifestyle.”

Solar Renewable Industry Evolves

Nathan’s been in the renewable energy industry from the beginning, so he’s seen the industry evolve and change. Batteries have gone up in price tremendously. Solar has come down proportionately. So the battery banks have gotten smaller; the amount of solar has grown larger, and generators, by and large, have moved to an absolute backup scenario instead of being an integral part of the system like they were back in the day.

As the industry grew and evolved, so did Nathan’s business and influence within the industry.

The Personal Becomes Political

 

electricity wire power grid

Nathan harnessed his passion and knowledge to help create a renewable energy industry in Missouri. He and Derek West, his business partner and stepson, went to the voters and asked if they wanted to create a renewable energy industry in the state. In 2008, the voters overwhelmingly said “Yes” by a ⅔ margin. Net metering became law.

Net metering is a way for a consumer to put solar panels on their home and back feed it into the house’s electrical panel while at the same time the utility is back feeding it into that same panel.

Essentially, electricity from the solar panel and the power grid meet in your electrical panel. You use your solar production first and if you’re short, you buy the surplus you need from the utility. If you have surplus generation, then the solar just back feeds into the electrical grid and runs your meter backward. The consumer gets a credit.

When night falls, you run the meter forward until it gets back to where it started. The homeowner still doesn’t owe the utility any money. This cycle continues until at the end of the month, the utility bill is based on the difference between those two meter readings regardless of what happened throughout the month.

Net metering gives customers control over their electricity bills.

During this time, Nathan became a board member of the Missouri Solar Energy Industries Association. They worked with utility companies around the state as well as the Public Utility Commission to establish the rules, regulations, and requirements to address both the political and practical side of making this new law work.

The Association also established the Renewable Portfolio Standard (RPS) which required the investor-owned utilities to meet 15% of their generation with renewable energy. In order to meet that requirement, the utility companies created a rebate as an incentive.

“What that did was bring the industry out of its infancy and into maturity and it brought prices down” which sparked the renewable energy market in Missouri, Nathan said.

Solar Dock Power Spins Into Its Own Universe

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According to Nathan, after Y2K became a non-event, the solar industry pretty much “collapsed.” The industry had geared up for volume and “it was over in a day.”

“I really didn’t want to go back into what I’d been doing but neither did I want to be in a business that required a perceived end of the world crisis to motivate someone to do anything,” Nathan said.

He started looking for a business he could create that had a ready market. Two events converged that led him to Solar Dock Power. First, he read a report about an electrocution that happened on Lake of the Ozarks and realized how it had happened and, secondly, a friend of his moved a boat dock onto some property but had no way to get electric power to it.

“Why don’t you run the thing on solar power,” Nathan said.

And so an idea was born.

He realized that eliminating the connection to the earth would have prevented the man on Lake of the Ozarks from getting killed and started zeroing in on the benefits of what solar dock power entailed. His overpowering thought was “We need to do this.”

Nathan started prototyping some systems, doing development work, testing equipment, finding out what it took to make things work. He finally got a system put together, approached an engineer who produced a schematic, then went to the U.S. Army Corps of Engineers and spoke to one of their electrical engineers.

“After an eventful battle, they acquiesced and allowed me to proceed with what I was doing,” Nathan said.

Eventually, he started speaking at a number of local conferences on the solar dock power concept. From the local arena, he moved on to the regional stage and then was invited to a number of Army Corps of Engineers’ conferences.

Over time, “they [the Corps] have moved to mandating these systems on all new construction on the lakes, and they are pushing very hard to have all electrical power removed and powered only by renewable energy in our region in the Midwest,” said Nathan.

Derek served as the Project Manager, while Nathan kept up with the manufacturing end of the business. “Derek gave me the time to think and design and develop. I set the bar. Derek raised the bar,” Nathan said.

US Army Corps of Engineers Locations map

This all started in the Ozarks with the Army Corps of Engineers when they built a system for powering floating facilities on their lakes. Each lake in the U.S. has a governing jurisdictional authority over what happens on that waterway. The Tennessee Valley Authority (TVA) is the governing body in our neck of the woods.

Nathan and Derek knew that once the Corps signed on other authorities would follow. “As the Corps goes, so goes everybody else,” Nathan said.

This success allowed Nathan to transition to that work full time for various U.S. jurisdictions. Derek continues to manage and operate residential/commercial solar systems at Solar Energy Services LLC.

My “Go-To” Guys – Inverter Service Center

Nathan has been doing business with Inverter Service Center since 2000. Here’s what he had to say:

“PJ and his crew have worked really hard to find the equipment we’ve needed to stand behind. They supply equipment but they do more than that. They are able to go to the manufacturers and search out what we need. They’re our mouths to those ears. I don’t have access. So they’ve helped us a lot. When I’m in over my head, they always take the time to listen and think how to fix it and how to address the problem.”

What’s next for Nathan Jones?

“We’ve been doing additional R&D, harnessing current technology differently and evolving into a whole other aspect of what we do,” Nathan said.

We know one thing for sure: Nathan puts his money where his mouth is.

We look forward to following you on your journey. Good luck!

Pure Sine Wave Inverter vs. Modified Sine Wave Inverter

sine-wave-inverter

Power inverters are, well, powerful devices which help you live with independent mobile power systems and improve your comfort level whether you’re on a boat, RV or living off-grid.  With an inverter, you don’t have to rely on hard-to-find DC-powered appliances or a noisy generator when your home or business loses power due to a strong storm.

One of the big questions when choosing to buy an inverter is which do I buy — a pure sine wave inverter or a modified sine wave inverter?

This article points to the pros and cons of both types and will help you make the best choice for your application.

Choosing the Best Sine Wave Inverter for Your Mobile Application

Inverters are generally electronic devices that convert DC voltage to AC. For example, you can use an inverter in your RV to convert 12 Vdc power from the battery to 120 Vac power to charge a laptop computer.

There are two popular inverter types: Pure Sine Wave (aka True Sine Wave) and Modified Sine Wave (aka Modified Square Wave). PSW inverters generate a sinusoidal waveform similar to that of AC voltage from the grid. MSW inverters generate a chunky waveform resembling steps.

On a per watt basis, PSW inverters are generally more costly than MSW inverters. However, PSWs will power any device designed to operate from the grid at home or in the office.

Sensitive devices like microwave ovens, CPAP machines and other medical devices, clocks, some chargers for cordless tools or laptop computers, radios, audio equipment, and similar products may require a PSW inverter to operate properly.

High-quality PSW inverters often have the surge capability required to start a high-surge load like a fridge compressor. Sizes typically range from the 120 W range to several kW.

MSW inverters are less costly per watt than PSW versions. They are popular built-in or add-on accessories in vehicles and are often good enough for powering non-sensitive loads like fans and lights, and some may be compatible with battery chargers for cordless tools, laptops, and cell phones. However, some of these loads may buzz, and they may run hotter (less efficiently) than normal. Sizes range from 20 W to several kW.

Give us a call at 615-285-1734, and we’ll be happy to discuss the appropriate solution for your particular needs. Visit Inverter Service Center today!


About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.

How to Size a PV System for Your Boat or RV

pv-system-sizing

In general, sizing any PV system depends on many variables. You want to ask yourself the following questions:

  • What is your anticipated daily energy requirement?
  • Will the system be off-grid (with batteries) or grid-tied (with or without batteries?
  • For an off-grid system, how much autonomy (days without Sun) do you prefer?
  • Will there be a backup energy source?
  • How much sunlight is available at your location, and does it vary seasonally
  • Will the array tilt be adjustable, or will the array azimuth track the Sun?
  • How much maintenance will the system need?
  • Are there ways improve efficiency and reduce loads?

To size a system for a boat or RV, let’s focus on four application-specific variables: daily energy requirement, autonomy, environment, and space.

How Daily Energy Requirements Influence Other Variables

Your daily energy requirement (DER) greatly influences the other variables. The DER is often expressed as Ampere-hours (Ah). Although Ah is technically not a correct expression for electrical energy, the assumption is that the DC electrical system is operating at either 12V or 24V (nominal), so you can infer a valid energy requirement. For example, 83 Ah x 12 V = ~1 kWh. And, deep-cycle battery capacity is typically expressed as Ah.

There are two methods for determining your DER: a hard way and an easy way. The hard way requires conducting a detailed energy audit to determine the daily net energy requirement for each and every house-battery-powered electrical load, then adding up all the values. Don’t overlook “small” or hidden loads. For example, an LP gas detector that draws “only” 75 mA will consume 1.8 Ah per day, and the radio and fridge may have standby power requirements.

Any AC load calculations must include inverter losses. You may be able to leave out major AC loads such as an A/C or a residential fridge if they’re not to be operated from the batteries. And, the DER may vary seasonally. For example, the furnace fan runs in the winter and not during the summer, and lights operate longer during short winter days.

The easy way to determine your DER is to use your battery bank as a gauge. If your two fully charged Group 27 batteries (210 Ah total) last for two days in moderate temperatures before dropping to 50% capacity, then you’re consuming 52.5 Ah per day. Note that battery operating efficiency isn’t 100%. Losses during charging and discharging may be ~85%, so 62 Ah may need to be generated in order to recharge the battery bank after supplying only 52.5 Ah to the loads.

How Operational Factors Determine PV System Autonomy

Autonomy is the number of days your system will meet typical DER without requiring recharging from an external source (shore power, generator, and/or vehicle alternator). Do you want to live quietly “off-grid” for two days or a week, or are you okay with running a generator for some period of time every day?

The operational environment includes many factors. Will the PV array be exposed to the sun or be shaded? Is the array laying directly on the roof, or will there be an air gap between the modules and the roof to help cool the array? A tilt-able array is an important feature in the winter when the sun rests lower in the sky. Will it be tiled up while parked and down while traveling?

How Weather & Space Requirements Affect Sizing a PV System

Geography and weather are other important environmental factors. Will you be using the system in the sunny Southwest or in the rainy Northwest? Solar radiation data and models show how much sunlight is available in various locations at different times of the year for assorted PV module orientations. Ambient temperature affects module performance (cold ambient temperature equals higher module voltage), and temperature also affects battery performance.

Space is usually at a premium in mobile installations, but it is crucial to consider space for batteries and PV modules. Ask yourself, is there enough accessible space to safely store batteries? Is there room on the roof to install PV modules while working around the refrigerator vent, fan vents, and antennas?

3 Examples of Sizing a PV System

Example No 1: 52.5 Ah net DER for 5 days, with 2 consecutive days autonomy
• Gross DER = 52.5 Ah (net) / 85% battery charging efficiency = 62 Ah
• Total load for 2 days autonomy + 3 days to recharge: 62 (Gross DER) x 5 = 310 Ah
• Environment: Tilted array with 6 hours equivalent full sun x 3 days: 18 hours sun
310 ah / 18 hrs = 17.2 A x 14 V nominal voltage / 85% module efficiency = ~285 W STC PV array
• System size and space: 3x 95 W PV modules (285 W STC total) modules and 2x Group 27 batteries (105 Ah ea x 2 = 210 Ah; 52.5 Ah net DER x 2 days = 105 Ah; 105 Ah = 50% discharge)

Example No. 2: 52.5 Ah net DER for 5 days with average daily full sun
• Gross DER = 52.5 Ah (net) / 85% battery charging efficiency = 62 Ah
• Total load for 5 days: 62 (Gross DER) x 5 = 310 Ah
• Environment: tilted array with 6 hours equivalent full sun x 5 days: 30 hours sun
310 Ah / 30 hrs = 10.3 A x 14 V nominal voltage / 85% module efficiency = ~170 W STC PV array
• System size and Space: 2x 85 W PV modules (170 W STC total) modules and 1x Group 27 batteries (50% discharge per day) or 2x Group 27 batteries (25% discharge per day)

Example No. 3: 88 Ah net DER for 5 days with average daily full sun
• Gross DER = 87.5 Ah (net) / 85% battery charging efficiency = 103 Ah
• Total load for 5 days: 103 (Gross DER) x 5 = 515 Ah
• Environment: tilted array with 6 hours equivalent full sun x 5 days: 30 hours sun
516 Ah / 30 hrs = 17.2 A x 14 V nominal voltage / 85% module efficiency = ~285 W STC PV array
• System size and space: 3x 95 W PV modules (285 W STC total) modules and 2x Group 27 batteries (42% discharge per day)

PV System Design Experts

Despite similarities in battery bank and PV array sizes in the examples above, energy use and environmental factors will affect system performance.

Here are two handy sizing tools from MidNite Solar to get you started: the MidNite Classic Sizing Tool and the MidNite Kid Sizing Tool

Or give us a call at 615-285-1734, and we’ll be happy to help you review your PV system design. Visit www.inverterservicecenter.com.


About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.