Dear Forumites

I have grid power at my house, but would like to go solar anyway, in order to become as independent as possible, and have cleaner (read more stable) power. I don’t want to disconnect from the grid completely, not yet. At first I'm going for a minimal system, and will later fill up the holes in my calculations and naivety. I need advice please, as will become apparent, my electrical knowledge, never good to start with, is now rusty to boot.

I’m designing for 4.2kWh during daylight hours, 3.3kWh during the night and no autonomy beyond a single night. These are worst-case measurements over two weeks, not an average. I’ve settled on the following equipment, mostly trying to stay within my financial limits:
• Axpert inverter MKS 5K
• 6x Canadian Solar CS3K-300P, in two strings
• 4x OmniPower 240Ah 12V sealed batteries in series.

Solar panel output:
At our location, north of Windhoek, Namibia, we have about 6.17kW/m²/day at a panel angle of 67°. At a panel efficiency of 18.05%, 156x78mm cells times 120, I can expect 9.76kWh per clear day, which is 30% more that my total daily need. Then again, much of this will probably go towards re-charging inefficiencies.

MPPT range:
The MPPT range is 60-115VDC, max PV panel VOC is 145V. With two strings, optimal operating voltage (Vmp) 32.7V, Voc 39.3V, I have an optimum power point voltage of 98.1V and a VOC of 117.9V, so I’m on the safe side? Voc @ -5°C = 128.4V

Batteries:
I’d like to stay within a 25% discharge window, if possible. The batteries can provide 240 x 48 x 0.25 x 0.9 (inverter efficiency) = 2.6kWh. Which is 80%of what I need, so can I set the low DC cut-off voltage to some as yet unknown value and use grid power for the rest? I could also switch off the fridge and evaporative cooler at 01h00, and restart at 08h00. In which case I should probably be covered 100%.

Feed back into the grid
Incidentally, what is the cheapest, but still good, 48V 5kW inverter that can be used to charge and run off batteries and also feed excess power into the grid? I believe that would be called a bi-directional hybrid inverter? We are quite a few houses and offices that run off a single transformer, and the power that I can feed into the grid behind the utility's transformer will never be anywhere close to covering our consumption.

2 hours ago, Arthur Götz said:

At our location, north of Windhoek, Namibia

Hi there fellow Namibian :-) I'm Namibian by Birth, not by present residence though.

What we do over here, pretty much, is take the Wp rating of the array and multiply it by 5, which is roughly how much good sunshine we get on average per day. But your calculations look fine to me.

2 hours ago, Arthur Götz said:

The MPPT range is 60-115VDC, max PV panel VOC is 145V. With two strings, optimal operating voltage (Vmp) 32.7V, Voc 39.3V, I have an optimum power point voltage of 98.1V and a VOC of 117.9V, so I’m on the safe side? Voc @ -5°C = 128.4V

That seems just fine. Even if you live in Seëis, Namibia's own little Sutherland, that looks just fine.

2 hours ago, Arthur Götz said:

can I set the low DC cut-off voltage to some as yet unknown value and use grid power for the rest?

Voltage is a bad indicator of state of charge on a lead acid battery. There are ways that you can sort of make it work. You can experimentally determine, through successive observations each evening, what your average load is and how low the voltage sags, and then you can look at that same voltage again a few hours after you switched back to the grid and use that (almost open circuit) voltage to determine a state of charge, and then you can tweak it over time to get roughly where you want to be... but most people here will tell you to get a Victron BMV battery monitor and rig it up via some kind of software solution.

2 hours ago, Arthur Götz said:

Incidentally, what is the cheapest, but still good, 48V 5kW inverter that can be used to charge and run off batteries and also feed excess power into the grid? I believe that would be called a bi-directional hybrid inverter?

Goodwe. I want to sell you a Quattro... I really do... but you asked for "cheapest but still good" and that is the honest answer.

5 hours ago, plonkster said:

Goodwe. I want to sell you a Quattro... I really do... but you asked for "cheapest but still good" and that is the honest answer.

I so appreciate honesty! I spoke to a supplier this morning who reported that he didn't have any problems with Goodwe, but one, which was replaced promptly. "Upgrading" to a bi-directional Goodwe 5048D-ES , keeping to 5kW and  a 240Ah 48V battery bank, but maxing out on the number of panels, will see a payback period of just 31 months for the price difference of the upgrade. This is a rough estimate only, but it definitely warrants a closer look.

Yes, the Victron BMV is definitely the way to go. A tad expensive, but still cheaper than a single battery, never mind 4.

Thank you for your honest advice. 

11 minutes ago, Arthur Götz said:

A tad expensive

There aren't really any alternatives on the market. I've seen some, but none that are of this quality and accuracy. It has a 50mV 500A shunt, that's 1mV for every 10 amps, and this device measures down to 100mA. That's a precision measurement so low that your average oscilloscope can't see it. Considered in that light, 2.5k is cheap for decent measuring equipment :-)