Home Inverter Comparison: Off Grid and Grid Tied, by L.K.O.

Off-Grid Origins

Residential power systems – particularly the inverters that provide more popular Alternating Current (AC) voltages, standards and connections – are a far cry from their primitive ancestors of only a few decades ago, when hobbyists and off-grid home or cabin owners needed a fair amount of electrical expertise, as well as tolerance for not-quite-ready-for-mainstream technology and performance. Increased world-wide demand, dramatic improvements in the semiconductor and microprocessor industries, economies of scale, improved safety standards, regulations, plus diligent and competitive engineering have all contributed to the superb home inverter offerings available today. From it’s infancy as an inferior, pioneering substitute to grid power systems, usually chosen only out of necessity for off-grid installations, the technology has matured to the point where pure sine inverters can typically offer cleaner, better regulated, and more stable power solutions than utility grid power companies can offer. An added benefit of the precise sinusoidal waveforms is the extra longevity that most computers, consumer electronics, motors and other electrical devices with inductive loads gain as a result of lowered internal friction from surges, spikes, blackouts, brownouts and other voltage irregularities in utility-supplied power.

On-grid Evolution

The lure of a potential market many orders of magnitude larger than strictly off-grid customers encouraged inverter manufacturers to address the technical hurdles of allowing inverters to use both local – e.g. photovoltaic (PV) solar, wind, small hydro, etc. – sources and imported grid-supplied AC to power both consumer loads and backup batteries. An on-grid inverter must synchronize the AC output of the inverter with the incoming AC power from the grid, be able to immediately supplement any outages or drops in grid power with power from the batteries, solar panels, wind generator, etc., and adjust its phase instantaneously when outside utility power is restored. Today’s class of pure sine wave, synchronous inverters do all this and more, while meeting and/or exceeding all the needed safety and regulatory requirements such as Underwriters Laboratories (UL) and the National Electric Code (NEC). These ‘best of both worlds’ inverters can often dramatically reduce the need for backup generators, fuel and having to oversize collection (such as PV panels, wind turbines) and storage (battery) components. The caveat with this approach is that it presumes that extended utility outages lasting many days or weeks will be very rare. However, if one wishes to build a self-sufficient home energy system in stages, this is often a good compromise. Backup generators and fuel can be added as budgets allow, with grid-tied systems still providing immediate benefits for both new construction and retrofitted homes. Since the vast majority of inverter applications have access to grid power, this article will focus on these modern grid-tie pure-sine inverters.

Power Buy-back

Because grid-tie inverter systems can frequently generate more electricity than is being used, utility meters will actually run backwards or sometimes a second meter is installed to measure the power delivered back to the utility company. The home becomes (at least in those moments when household supply exceeds demand) a net energy producer rather than a consumer. Some more progressive states and municipalities allow home-generated power to be sold back to the utility company at their retail power rates; Ashland, Oregon, for example, even pays a 25% premium (1.25 times the highest residential rate) for home-generated power for the first 1,000 kiloWatt-hour (kWh). Here is a net metering map for USA locations which shows how 42 states, at the time this article was written, support some form of net metering. Check with your local utility. In some cases, power is bought back at wholesale rather than retail rates, reducing the cost-effectiveness of an alternative energy system for those locations. In either case, there are self-sufficiency and ecological gains, and often economical gains, with effective break-even strategies.

Self-sufficient ideals for any home

One important benefit of looking objectively at home energy consumption, in addition to reducing ongoing monthly utility costs and the corresponding environmental benefits, is the potential for scaling down the size, cost and complexity of an inverter-based power system. Typically, the largest energy ‘gluttons’ include space heating (and cooling), water heating, cooking, clothes drying, and refrigeration. If you can, find non-electric or high-efficiency options for these needs, such as wood-fired cookstoves, gravity-fed water supplies (since well pumps often draw significant current) ceiling and exhaust fans, solar water heating, clotheslines and drying racks. Judicious use of these technologies can reduce ongoing power needs and system design costs to a fraction of what they might be otherwise. Plus, these strategies work equally well for both grid-tied and non-grid homes. This is most easily done with new home construction, taking advantage of microclimate factors, daylighting, prevailing breezes, site location for PV panels, wind generators, small hydro stream/penstock siting, etc. However, even retrofits can gain considerable benefits by careful planning and appliance selection. It behooves one not to overlook the benefits of a conservation-oriented lifestyle. Unplugging not-in-use phantom loads like battery chargers, and turning off unused lights, computer peripherals, etc. can make a significant difference. Energy Star appliances, high-efficiency LED and/or occupancy-sensed lighting, timers and a vast assortment of other energy-saving devices can simplify the effort for this lifestyle. Another ‘elephant in the room’ – specifically the garage – is the enormous potential (fuel) energy savings of a home-based business instead of a commute-intensive and fossil-fuel dependent livelihood and community. Here’s a list of some energy conserving ideas and resources that might be helpful in scaling down your inverter, battery and power source needs. The Department of Energy (DOE) tip web site for Money and Energy Savings is another useful resource.

Older inverters paired with a UPS (off-grid only)

If you have access to an Uninterruptible Power Supply (UPS) that meets your power needs and can handle less than pure sine wave inputs you might be able to economize by using an older, second-hand non-sine-wave inverter with modified sine wave or other coarsely stepped output waveforms. Just make sure to carefully check the manufacturers specifications and then make an explicit inquiry to both vendors about the specific combination to avoid any safety or device/system longevity issues.

Small Inverters

There are numerous small wattage inverters for automotive or small load applications with outputs of 1 kW (kiloWatt) or less. When selecting inverters of this type, make sure both the nominal (rated) and peak or surge wattage ratings are a good fit for both the intended load and the inverter being considered. Keep in mind that these less expensive inverters often use a modified sine wave output that is a poorer approximation to ‘pure sine wave’ inverters. This may work fine for incandescent bulbs and other purely resistive loads (although an audible buzz is a classic artifact), but efficiency, performance and device lifespan may suffer with computers and home electronics that require cleaner power. Consider using a UPS as noted above. Anything with reactive (capacitive or inductive loads) such as transformers and motors tend to ‘fight’ dirtier power and waste more energy in heat with correspondingly compromised life spans and reduced efficiency.

Vulnerabilities of the On-grid Only Approach

Aside from the smaller (typically for mobile or portable application) inverters, there are three main inverter configurations: On-grid only, off-grid only, or systems designed to work either way. The ‘on-grid only’ option, while becoming the most common, is the most vulnerable, due to complete dependency on the grid. To be fair, there are a few advantages to this approach, but these don’t do much for a preparedness-oriented home. Most of the long term cost pay back calculations are based on grid-tied systems without batteries. Most tax credit and tax rebate plans apply only to grid-tied systems. However, after two years, the owner can usually reconfigure their systems legally, to make them truly off-grid, but only if the inverter is designed to work off-grid also. This is a must to keep in mind when choosing an inverter, which is one of the most expensive system components. An “Achilles Heel” design flaw of many grid-only systems prevents them from operating in the absence of grid power. There are plenty of mechanisms for grid failure. You have probably experienced your share of blackouts and brownouts. There are also probabilistic mechanisms that threaten the grid as well as the more common situations that trigger these events. The utility grid – in some respects analogous to a giant antenna – could be knocked out by an Electromagnetic Pulse (EMP) from massive solar flares or high-altitude nuclear detonations. The resonant wavelengths needed to disable power systems are minimized by relatively tiny wiring runs from PV panels to inverters and batteries in typical home power systems, compared with miles or thousands of miles of grid wiring. The longer the cable runs, the longer the unintentional antennas for EMP resonance. Rather than wait for the next power failure, try (with advance preparation) living without utility power for a day – or a week – and make careful note of what you will provision yourself with if/when this becomes a permanent (or even semi-permanent) situation.

The ‘Total Off-grid’ or ‘Best of Both Worlds’ Decision

The other two inverter topologies that mitigate grid frailties are the ‘total off-grid’ approach and the ‘best of both worlds’ configuration that allows for grid-tie benefits and complete functionality when the grid is down. Both approaches use batteries or some form of energy storage. The cost of off-grid systems are substantially higher, and the pay-back period is much longer. Despite some encouraging developments in battery technology, sulfation and other intrinsic longevity issues with lead-acid batteries (the most commonly chosen type) require purchase of new battery banks at roughly 6 to 8 year intervals. Other battery types tend to be more expensive, which outweighs typical lifetime advantages.

Some inverters are designed to work strictly on-grid, which ties the system to the grid’s vulnerabilities; for the ‘both’ approach, make sure explicitly that the inverter you select keeps on running regardless of whether the grid is up or down. The automatic grid power detection circuitry should disconnect the inverter from the grid and switch over to batteries within a few milliseconds, and then reverse that automatically when (if) grid power is restored. Caveats and cons for this “both” approach include the extra expense for a system that handles both grid and home-generated power; the synchronous part of the inverter and the switching logic and circuitry. Advantages of the ‘both’ approach include the greatest flexibility and source versatility, and possibly lower initial cost, since batteries (and additional panels and/or turbines) could be added later after budgets allow. Check with your inverter/PV consultant to make sure a staged approach like this is designed optimally for future expansion.

The advantages of the total off-grid approach include lower inverter costs, lack of expense and regulatory involvement needed for the synchronous circuitry and disconnect switching. Disadvantages include the considerably larger system size, complexity and expense of a system that must rely on strictly on-site power, which usually must be purchased at installation, rather than added later in stages. If the local supply fails (no wind or sun for extended periods or component failure), often equally unsustainable fossil-fuel based backups require additional expense and design considerations. The psychological benefits in terms of self-sufficiency may outweigh these issues.

Sizing, Options and Selection

Regardless of the type of system selected, proper sizing is always important. Buying more wattage (and complexity) than you need is often a result of not being thorough in a realistic, yet vigilant review of conservation lifestyle and appliance changes noted above. If you have the luxury of designing a new home, carefully plan to include primary non-electric (preferably on-site generated) alternatives for space heating/cooling, water heating, cooking, clothes drying, and refrigeration (such as a SunFrost brand refrigerator). This might make the difference between a system that uses 4 dozen pricey PV panels or half that. With a very frugal lifestyle, design and carefully solar orientation, etc. it’s sometimes possible to cut the needed system size – source, storage and conversion components (e.g. PV collectors, batteries, and inverter) in half again. While retrofits are usually more challenging to realize savings of this magnitude, there are still many opportunities to explore and an abundance of energy conservation resources online. Keep in mind that the idle current draw (a.k.a. wasted ‘phantom load’ power) is proportional to the size of the inverter. This is yet another reason to think through the big picture, all major power loads and size the inverter (and panels, batteries, etc.) for an optimal match between sources and loads.

Despite the tremendous advances in inverter technology, simplifying installation tremendously, there are still a number of choices to be made for a given power system installation. These often include (but aren’t limited to):

Rated output power in Volt-Amps (VA) which is related to Watts (W); here’s an article on the difference between VA and W ratings. Rated output power is often different for different output voltages, such as 240VAC or 208VAC output.
Output voltage(s); typically 240VAC.
Input voltages; AC (grid) and DC (PV panels, wind generator, etc.) input voltages.
Peak efficiency; typically 90% or higher. The lost efficiency is converted to heat.
California Energy Commission (CEC) weighted efficiency; a measure of average efficiency.
Maximum input current
Maximum output current

Online Comparison Chart

Once you have defined your power needs and selected the parameters above, here is a handy interactive comparison chart tool that allows comparison of these vendors (at the time this article was written): Advanced Energy, APS, Blue Frog Solar, Carbon Management, Chint Power, Delta Energy, Diehl AKO, Emerson Network Power, Enasolar, Enecsys, Enphase, Eversolar, ExelTech, Fronius, Growatt New Energy, KACO, KLNE, Kostal, Motech, Power-One, PVPowered, Refusol, Samil Power, Samlex America, Satcon, Siemens, SMA, Solar Bridge Tech, Solar Edge, Solar Energy Australia, Solectria, Sunpower, and Xantrex. You can group inverter comparisons by size (Wattage ranges in kW brackets) to make selection easier. This chart tool has a wide range of inverters for both off-grid and on-grid applications.

Installation

Both off-line and grid-tied inverter systems generally require licensed electrical contractors as well as applicable inspectors from your local jurisdiction(s). Always check all pertinent requirements, net metering regulations, and use UL, CSA and NEC certified components to pass safety, inspection, insurance, and other requirements before beginning an inverter-based power system project. When in doubt, consult a professional solar/inverter installer. It’s also a good idea when you’re not in doubt, too! Electrical equipment has safety as well as economic considerations, so always play it safe. Often solar/wind/inverter/alternative energy professionals can eliminate significant research time investment and quickly guide you to a suitable system tailored to your location, budget and specific needs.

Vendor Contact Info

Here are some of the more popular grid-tie inverter (GTI) manufacturers (click on the links to visit their web sites):