# Why are off grid solar setups only 12, 24, 48 VDC?

I am looking to produce 50kW for an off grid solar project. Ideally, I'd like to have a high voltage DC battery system with a high power battery inverter and charge controller.

I have only found a couple high VDC (384V) inverters and charge controllers, and they are from Chinese manufacturers. I'm a little hesitant to buy from them.

All of the big companies only use 12, 24, and 48 VDC. I understand that it's the most common and you don't need much power for a home, but if I want to produce 50kW at 48 VDC that's over 1000A! If I run that in parallel to 48V inverters I would need 10 or more inverters. That is a lot of wire and work.

Is there an electrical reason why they would cap these products at 48 VDC?

• Less electrical and regulatory (legal) I'd think. Below 48V, it generally counts as low voltage and a different set of regulations applies (laxer standards, don't have to be an electrician, etc.) But, that's just a guess. – JRE Jul 24 '19 at 21:17
• The pv panels I installed supply 380V DC to the inverter, kept the losses down... the system I am designing for my son will have a 24V battery bank. DC can be considered as more dangerous by some as it locks the muscles unlike AC. Others have already pointed out selv... – Solar Mike Jul 24 '19 at 22:11
• 12V, 24V, and 48V are nice round numbers if you're trying to build a battery system (usually 12V lead-acid based) to go with the solar array. This is why even multiples of 12V are typically offered, specifically. – J... Jul 25 '19 at 13:11
• 50kW is a HUGE off grid system! – pjc50 Jul 26 '19 at 10:07
• This is really a very huge installation. What is this for? You're planning enough solar here for several households worth of electricity consumption. Is this for a commercial installation? I mean, you're looking at $40-50k just for panels and an inverter. Yes, you need a lot of wire and cable to hook that all up. The numbers are big, yes, but so is your project. You can't just cobble together a 50kW system with leftover speaker wire and a few car batteries. – J... Jul 26 '19 at 11:54 ## 5 Answers 60VDC is the cut-off for Safety Extra Low Voltage, or SELV, as spelled out in UL 60950-1. Besides being lower voltage, SELV circuits are also isolated from the mains by reiniforced insulation, which has specific spacing and materials requirements. In general terms, SELV voltages are ‘touch safe’, meaning that they don’t present a shock hazard with direct contact. 48V falls below this SELV threshold with some margin. It’s also conveniently four ‘12V’ lead-acid batteries connected in series (really up to about 58.8V at full float charge.) Voltages above the SELV level are considered in the same class as line voltage, and typically require an electrician to install. Reason? Electricians are familiar with codes and techniques to protect against inadvertent contact with potentially lethal voltages, including use of proper materials, fusing, fault protection, enclosures and cable routing. • Digital phones in the USA use -48vdc. PoE (Power over Ethernet) is limited to 48 DC. This SELV standard and older versions have been around for many decades. Exposed terminals in cars and trucks cannot exceed SELV values. – user105652 Jul 24 '19 at 21:42 • @Sparky256 Wrong, POE 802.3af and bt is limited to 57VDC. – Jack Creasey Jul 25 '19 at 0:16 • @JackCreasey Ok then, I stand corrected because I did not add it along with a gazillion other specific standards. – user105652 Jul 25 '19 at 2:41 • Nice research. Note: DC cutoffs seem a bit silly to me. 120V @ 100kHZ is a lot safer to the touch than 50VDC. The former will run a lot of standard equipment, though manufacturers may not certify it unless you put a (very inexpensive) rectifier on the plug. – personal_cloud Jul 26 '19 at 17:31 • It's worth noting that in Australia, you still need to be an electrician to legally install these sorts of set-ups - you need to be an electrician to do basically anything with wiring that goes through a wall; there was even a brief period where you needed to be an electrician to change a light bulb. – nick012000 Jul 27 '19 at 13:31 ## Field-assembled battery hardware can't go outside of SELV without help The primary limitation on the DC bus voltage of most off-grid systems is indeed due to touch safety limits (60VDC/42.4VAC SELV limit), but that's not due to who's installing it. Instead, the issue is parts availability: lead-acid single cells and monoblocs of the sizes used in off-grid systems generally are not available with touch-safe terminals due to manufacturability and application diversity issues. This can be somewhat overcome, depending on the environment, by using a battery cabinet or battery room as the touch safety boundary, or by using a factory-assembled and listed energy storage system, but that leads us to our next issue. ## DC switchgear is hard Light-duty LV AC mains switchgear (MCBs and associated mounting/bussing systems, as well as fusible switches/disconnectors, garden-variety mains fuses, and so forth) is, of course, readily available for typical utilization voltages. However, only a limited subset of this gear is rated for DC service at all, and if so, its ratings will be limited to around 48-60VDC. Furthermore, disconnectors and such intended for solar service, while rated for high DC voltages, have very low short-circuit/interrupting ratings in the grand scheme of things. This is because solar panels are inherently current limited sources: no matter how long your string is, it won't put out much more than its rated Isc no matter what you do to it, and solar panel Isc values are on the order of amps, not kiloamps. This means that you need much heavier-duty switchgear for DC service at mains-voltage-equivalent DC voltages, as battery strings are capable of kiloamp-class fault currents (easily equivalent to a mains source in this regard), and DC arcs are indefinitely self-sustaining once struck vs. AC arcs which will self-quench at zero crossings. Furthermore, even heavier-duty gear such as DC rated industrial-type MCCBs and heavy-duty fusible switches is often limited to 125VDC for single pole breakers and 250VDC for all-pole switching in multiple pole devices. While there are a few fusible switches out there that have ratings up to 600VDC, these are also limited by the inability to get fuses rated for mains OCPD service at voltages above 300VDC with only a few exceptions. Interrupting ratings are another issue; achieving DC system short-circuit ratings over 10kA requires careful component selection, and even the most robust breaker and fuse designs available only achieve 100kA or less of DC interrupting rating. (For comparison purposes, most MCBs are available with either a 10/22kAIC series rating or a 10, 14, or 22 kAIC straight rating for AC service, depending on product line, voltage and where you are on the planet, and industrial MCCBs often have ratings up to 200 or 300kAIC. AC mains fuses are similar to MCCBs in this regard, as well, with modern current-limiting HRC fusing achieving 200 to 300kAIC, even in its consumer-accessible incarnations.) TLDR: Industries use industrial gear, and homes don't have 50kw demand. Really. This cannot be emphasized enough: ## DC is one nasty customer. It is easy to get complacent after spending a youth and a career working with docile, harmless 5-24 volts DC, or well-behaved 100 - 240 V AC voltages because of its frequent zero crossings. DC in that same range is a mean drunk. You may have been in very old houses and felt switches that had a definitive SNAP when switched on or off. Those are throwbacks to when house power was DC, and they snap the contacts quite wide, to assure an arc is snuffed. Above that, you need magnetic or pneumatic "blowouts" designed to pull the arc up into an arc chute to blow it out. , Because if DC arcing gets underway, it will burn through almost anything. (note in this tram-fire video how the arc takes awhile to start, and then tears the tram apart. Keep watching, it relights a couple of times.) Look at the DC ratings for contactors and relays. You will see very different voltage ratings for DC than AC. As a result, the various regulations treat higher voltage DC differently from low voltage, and allowable maximums are typically in the 30-50 volt range. ## Rapid Shutdown Two thresholds of particular interest to solar panel installers (who work on roofs) are the 2017 Rapid Shutdown rules. You must now leave "aisles" between groups of solar panels for firemen to access the roof. This means there are groups of panels that are still adjacent. For roof installations, there must be a switch accessible to firemen that will do two things. a) Reduce voltage within a group of panels to 80 volts DC or less. b) Reduce voltage between groups to 30 volts are less. If system voltage is less than 30 volts, then no special provisions are needed. This is a direct reflection of the hazards of higher voltages. For a non-roof installation this is not an issue; food for thought. ## Tall battery stacks are less reliable Batteries are a series string of cells of 1.2 to 3 volts per cell. As such, a high voltage stack has a lot of cells. The more cells, the bigger the risk of a cell failure limiting or downing the entire stack. EVs have largely conquered this with exotica batteries, but you won't have the same luck with plain old lead-acids. ## Battery voltage needn't match solar voltage There is nothing wrong with high voltage on the solar panels (noting it is inherently current limited) and low voltage on the battery pack (which has literally no current limit, and will cheerfully explode). The solar charge controller would need to buck-convert, but it's doing that anyway. ## Do you really need 50 kW? Yes, if you were doing this commercially, e.g. to run a server farm or grow lights, then yes - you would subdivide the project into smaller chunks that fit the available inverter hardware. Keep in mind multiple inverters can't have their outputs paralleled because they can't sync. If they fall out of phase with each other, that amounts to a dead short between them. If you are expecting inverters to automagically sync with each other or with the grid, reset that expectation. If this is a commercial venture, e.g. running ventilators and pumps at a remote mine, then you ante up into industrial tier equipment, and pay the nosebleed prices for same. The very fact that you're contemplating cheap Cheese seems to cast this as a homeowner issue. ## Homes don't do this One of the great fallacies in the "eco" movement is unit substitution so you can keep your processes exactly the same. Remove nitrate fertilizer and insert bat guano, remove Roundup and replace with brand X, but still farm in exactly the same way. Raise a building with brand X parts instead of brand Y and claim LEED certification (because the parts are LEED), while completely ignoring earth-sheltering, passive solar design and other real winners in the field. That approach is wrong-headed. Tragically common, but wrong-headed. And honestly we get this more from naysayers: looking at their 200A main panel breaker, multiplying by 240 and declaring they need a 48KW solar system, and the naysayer immediately tosses out some numbers to prove a 48kw solar system is totally impractical. No poop Sherlock. In this case the naysayer is maliciously swerving into unit substitution specifically to assassinate the idea of off-grid living even being practical. Of course, now, a 50kw solar system actually is imaginable, but it's still just as ridiculous as when the naysayer "proposed" it. A random house with no conservation measures draws 1000 watts on average. That's according to the power companies, who rate power plants in terms of number of houses served. That's a fairly big system (100KWH of battery to ride through 100 hours of storm), so you think about loads and how to minimize them, particularly vampire loads which are 24x7. The biggest vampire load in your whole house is the inverter. Even with no load, it burns 1% of its rating simply by being "spun up". So at 50kw, that's 500 watts of power -- remember what an average unoptimized house takes? So you're already there - how wasteful is that!? So keeping inverters sanely sized is important. And you do that by thinking loads through carefully, and not having one giant customer that controls the inverter sizing. ## Careful load design Here's what you don't do: Avoid a$600 new fridge by keeping your old one, causing you to need to provision an extra \$3000 of PV capacity to power the inefficient thing.

So for instance, electric baseboard heat is Right Out. Electric oven, no; use gas. Building heat should be passive solar design then an active solar-thermal system. Hot water, solar thermal with storage tanks.

There's a fair exception: Heat pumping is perfectly reasonable, because the 20ish % efficiency of the solar panel x the 300-1000% efficiency of the heat pump > what you could possibly get from solar-thermal.

The upshot is the only really large electric load you'd expect from an off-grid home is the air conditioner. The inverter would be sized accordingly.

As such, 48V is plenty of battery.

## On-demand hot water

I've been racking my brain on what load exactly requires 50kw for a short time, and my conclusion is "on-demand hot water heat". I love 'em, but yeah. No... Just no.

That violates two tenets: making heat with PV, and running an inverter for a load that doesn't care about AC.

At the very least, you'd hotshot it off DC directly, perhaps using simple boost converters per channel that only spin up when the heater is calling for heat on that channel. But since you'd be doing 3-4 channels anyway on a large heater, you'd just do 3 smaller heaters - located right at the spigot, so you zero out that long, very expensive wait time for hot water to reach the spigot (with the now-filled pipe's hot water being abandoned).

The right way to do this is to use solar-thermal with a large storage tank, possibly augmented with a heat pump. The storage tank gets warmed as hot as you can possibly get it (90C if possible), then it's either heat-exchanged to make domestic hot water, or used as the heat sink for a heat pump (tanked) water heater. Heat pumps are more efficient when pumping "downhill", so that big solar-thermal tank of hot water saves a lot of PV energy even if it's not used directly.

• 50kWp is enormous - this cannot be overstated enough. It's big enough to service 3-4 off-grid households, even with significant electricity use. This is bordering on a commercial sized installation. – J... Jul 26 '19 at 11:50
• @J... 3-4 houses that aren't even trying to be efficient, or that are quite vast and luxurious. – Harper - Reinstate Monica Jul 26 '19 at 15:20
• I'd agree with not really trying to be efficient, but not necessarily vast. 50kWp should produce an annual average energy of something around 50kWh (using my standard UK yield of ~1kWh/kWp/annum - conservative), assuming a battery system that can spread that around to the times it is needed. The average US household consumes about 10kWh/year, so you could say it's maybe closer to 5 average households not doing anything atypical compared to a grid connected home. Anywhere in the sunny US would probably add 50-70% to the yield, so yes, in that case closer to 7-8 households worth of electricity. – J... Jul 26 '19 at 17:27
• @J... Don't forget that the inverter needs to be spec'd to peak power, which is often about 10X the average power. Look at the circuit ratings on your large appliances. One 50kW inverter won't service 4 large households. Possibly not even one air conditioner. (Last time I looked into natural-gas--powered air conditioning, it was not economical, even though I pay 35 cents per kWH. Anyways nobody wants an engine running near their house all the time). – personal_cloud Jul 26 '19 at 17:50
• @personal_cloud Obviously, the point is that a 50kW solar installation will generate WAY more energy than a single household can use. If OP is considering this for a residential application off grid then they will end up generating about 80% more electricity than they will need. And since they're off grid they won't be able to grid-connect and sell the surplus to help finance the whole project... which makes it either kind of dumb or intended for some kind of off-grid commercial enterprise. – J... Jul 26 '19 at 18:17

The optimal DC voltage also depends on the type of batteries you are going to install. Lead-acid are more affordable but wear out after ~1000 charge-discharge cycles, and for a high-voltage system you would need to series-wire a large number of them. Li-Ion batteries exist in high-voltage configurations (ca 500 V DC) which greatly reduce wiring losses since the currents are then proportionately lower, and have a much longer life time.

Last year I installed a "blueplanet hybrid 10.0 TL3" inverter, 10KW, with 200 to 900 V DC range on the photovoltaic side and a 500 V DC Li-Ion battery. The battery is indeed from China (Byd-HV), the inverter is made in Germany. The downside is that the firmware for enabling island mode is only coming at the end of September 2019 due to certification delays ... Other than that is has been working flawlessly since a year in grid-interactive mode, and the wiring can be done with 10mm2 solar cables (for my 10 KW installation).

• Nice research! I agree, get an inverter that supports both on-grid and off-grid mode. You're paying for it anyway. Good job on your system. – personal_cloud Jul 26 '19 at 17:55

You can definitely get larger off-grid AC inverters. For example, SMA Sunny Island goes up to 100kW.

https://www.sma-america.com/products/battery-inverters/sunny-island-4548-us-6048-us.html

And yes, I would also be hesitant about the Chinese-made inverters, if you have alternatives. Fortunately, in this case, I think you do.

• Thanks for the reply! But I would need 12 Sunny Island units to reach 100kW. I'm trying to avoid a large amount of inverters because there are more points of failure and a lot more wiring and effort involved. – Lost Puppy Jul 31 '19 at 0:12