I've got access to 12 flexible Solar Panels - 320VDC each

Main voltage is 220VAC (here in the UK) and also might buy an electric engine running at 48VDC.

Would a 220VAC Uninterruptable Power Supply (UPS) be able to step down the 320VDC to either 220VAC or 48VDC for use on a boat?

I'm thinking of connecting 12 panels in parallel and use either:

  • Double-conversion UPS to deliver 48V by bypassing the conversion from the battery back to 220V
  • Line-interactive UPS to deliver 220V directly from the 320VDC.

The question is would the UPS's surge protection be able to deal with the high voltages? (Keeping in mind the sun isn't real bright in the UK).

Also with regard efficiency will there be a lot of power loss in the system?

I don't really know how surge protection from overvoltage works in UPS's and assume it uses some kind of buck conversion...is it true?

I'm guessing that the surge protection circuits might struggle with continued voltage overload. Does anyone know if this would be the case?

The question is: Will the UPS's be able to reliably deliver either 220VAC or 48VDC?

Here are the specs for each individual panel:

Open circuit voltage(Voc): 429V

Optimum power voltage(VMP):319 V

Short circuit current(Isc): 0.39A

Max operating current(IMP): 0.288A


Maximum system voltage: 1000V


Temperature coefficient ɑIsc +0.08%/ ° C

Temperature coefficient ɑVoc -0.35%/ ° C.

Temperature coefficient ɑPmax -0.15% / ° C.

Over-current Protection 30mA

  • \$\begingroup\$ Prior to EU harmonisation, UK voltage was 240, it is now nominally 230 with tolerance of +10%/–6%, (253 to 216). My outlet currently measures 240. \$\endgroup\$ – RedGrittyBrick Aug 2 '13 at 12:41
  • \$\begingroup\$ Seems like you are asking about the internal workings of a UPS which you have not specified. I peeked into my little 110V UPS, it seems to have an H bridge across a high voltage section for the final output. No idea if it regulates the high voltage to control the output, seems more likely they would regulate the H bridge. BUT, that configuration connects your high voltage to the mains AC. If you feed your panels into that, they, too, would be connected to mains AC. \$\endgroup\$ – Bobbi Bennett Aug 2 '13 at 12:48
  • \$\begingroup\$ You should really get a proper inverter, the UPS may simply not operate at all from DC. \$\endgroup\$ – pjc50 Aug 2 '13 at 13:46

Solar energy from solar panels, when not being able to be consumed at the rate at which it's being produced (almost always the case), has to be either of the following: - stored (usually in a battery) or - fed back to the grid (e.g. a grid-tied system), or - dumped into a dummy load (usually just beefy resistor packs) to convert the energy to heat the energy has to go somewhere.

So it sounds like what you're interested in using the UPS as the battery bank, but your UPS system probably is designed internally to only charge its batteries from A/C power, where as your solar panels will give you D/C.

You could theoretically (but maybe not very practically) use an inverter to convert your D/C to A/C (if one such inverter existed with such voltage levels input/output specs, wouldn't really surprise me though), and then feed that A/C into the UPS (since the inverter would essentially just give you a normal wall outlet to use), but that would probably just be rather wasteful (e.g. not efficient due to the D/C->A/C in the inverter and then the A/C back to D/C within the UPS (and eventually you might want to go back to A/C for your appliances running off the UPS).

What you might try instead (although this is hardly trivial) is to disassemble (carefully) and try to tap into the battery banks of the UPS directly and figure out what voltage it works at (likely 12, 24, or maybe 48 volts DC). You could then theoretically step down your solar panel's output using some kind of custom (or off the shelf, it anyone knows of one) DC-DC converter, then you can feed that into a battery charger, preferably an MPPT type one. Those kinds would adapt the incoming power to what would be most suitable to charge the UPS batteries to do maximum power point tracking (what MPPT stands for) and reach upper-90% efficiency.

Or if possible (and i don't know if it is) re-arrange the solar panels such that they don't give you 320 volts (are you sure you don't mean watts?) but give you a lower voltage at a higher current rating. Otherwise you'll likely be hard-pressed to find a battery charger that accepts 320vdc to charge the batteries of your UPS.

  • \$\begingroup\$ I would bypass the UPS entirely if I could find a reasonable way to step down the 320vdc to 48vdc. The only reason for the UPS was to avoid the high cost of a solar charge controller capable of high voltage inputs. I don't know where to begin to look for an DC to DC convertor that would accept 320V. Any ideas? \$\endgroup\$ – Meggy Aug 3 '13 at 11:47

Internally a lot of UPSs (and other high power mains devices with non-resistive loads) use 315Vdc internally, this is because of the rules on power factor correction (PFC) for mains connected kit. Often the simplest and cheapest means of achieving PFC is to convert the AC to DC at a high voltage using a DCDC (this is called Active PFC). The output voltage is this high voltage bus passed through a second DCDC or DCAC converter. Also, even non-PFC 'off-line' power supples often simply rectify the mains AC into DC at the front end, so you could just find an off line UPS that uses such a circuit.

So, if you find a UPS that uses a Active PFC front end, theoretically you could just connect the DC output of the panels direct to the bus, bypassing the PFC. However, I would seriously not recommend doing this because without a circuit diagram it would be impossible to know what might go wrong and cause a fire or risk of electric shock. Also, the output of the panels, though nominally 320Vdc would rise and fall, this might put stress on the circuit that was not designed to operate over such a large range compared to what normally comes out of a mains socket.

Bottom line: If you need to get AC from solar panels I would recommend using kit designed for the purpose.

  • \$\begingroup\$ What do think about cutting it up in 6 pieces and resoldering at 48V? It may be a bit easier to find a compatible MPPT charge controller that way. \$\endgroup\$ – Meggy Aug 6 '13 at 10:14

A 230VAC, that's "AC", input is quoted as 230V "RMS". That means a (sine wave) peak value of 230 x 1.414 Volts which is approx 325Volts.

AC enters the SMPS through a filter and is rectified (turned to DC) and that voltage (325-ish minus a couple of volts across the rectifier) is stored on a "reservoir" capacitor.

I suggest that an input of 325V "DC" would amount to the same thing. The only caveat I can see is there is a 100% duty cycle on 2 of the rectifier diodes which may be problematic. (You could upgrade the diodes?).

The off load output (or open circuit) of 429V cannot be ignored as SMPSs present a low/very low impedance when not supplying a load so you would have to use a power supply that takes an input of AT LEAST 305VAC (429 x 0.707) AND can operate below 225V though SMPSs usually have quite a range of operating voltage.

I don't envisage a problem with voltage spikes as the system wouldn't produce them. The maximum panel output is 429V and is a feature of the construction of the device. The supply could only "surge" up to that level if there was enough light and no load on the SMPS.

Of course you really have to look closely at the spec and graphs as there can be hidden pitfalls in the actual operation of any device.


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