First solar experience: offgrid + generator

mi_cin · July 2012

Hi,

To start with solar I'm planing to organize a little pv system on my summer cottage which is used only Jun-Aug. Right now there and no electricity. I counted that I will use 6,5 kWh daily. I found that in my region during the summer 1,84kWp (eight 230Wp PV modules) should cover it. The modules are 24V.

I found a SunnyBoy Island off-grid inverter which I can use but I still have two questions:

1. What kind of battery should I use? What capacity and voltage?
2. How can I tie a diesel generator to that system? I would like to use a generator only in case of emergency and lack of sun. Generator will be started manually.

Cariboocoot · July 2012

Re: First solar experience: offgrid + generator

Welcome to the forum.

First problem: 6500 Watt hours a day? What are you planning to run?
Supplying that much power will be very expensive, particularly for a Summer only cabin.

A Sunny Island is going to be expensive too; these things are like $5,000 new.

It is more likely that 1840 Watts of panels will produce about 4.5 kW hours AC daily, not 6.5.

I think we should go back to the beginning and re-examine your loads before you get the cheque book out.

ggunn · July 2012

Re: First solar experience: offgrid + generator

mi_cin wrote: »

Hi,

To start with solar I'm planing to organize a little pv system on my summer cottage which is used only Jun-Aug. Right now there and no electricity. I counted that I will use 6,5 kWh daily. I found that in my region during the summer 1,84kWp (eight 230Wp PV modules) should cover it. The modules are 24V.

I found a SunnyBoy Island off-grid inverter which I can use but I still have two questions:

1. What kind of battery should I use? What capacity and voltage?
2. How can I tie a diesel generator to that system? I would like to use a generator only in case of emergency and lack of sun. Generator will be started manually.

You cannot connect PV modules directly to a Sunny Island; it is a battery only inverter. If you want PV, you'll need to add a PV inverter; to play nice with the Sunny Island it should be a Sunny Boy. Alternatively, you could get a charge controller to charge the batteries from PV. It depends on whether you'd rather be DC or AC coupled.

A Sunny Island requires a 48V battery bank. Also, it is 120V only; to produce 240V and/or interact with a 240V PV inverter you'll either have to get two SI's or incorporate an autotransformer.

You'll need to be careful with having a PV inverter and a generator on the same system; if the inverter backfeeds the generator it's bad news for the genny.

You're going to need a lot of battery to supply 6.5kWh/day and a lot of PV to keep it charged up.

Cariboocoot · July 2012

Re: First solar experience: offgrid + generator

ggunn;

You can connect PV's to a Sunny Island. Through charge controller connected to the battery like any other off-grid inverter. Kind of expensive way to go off-grid, though. It relies on AC coupling to recharge otherwise, and really is not designed to interface with a generator. You'd have to have the gen run a stand-alone charger to the battery. Really not a good off-grid choice, is it? :roll:

ggunn · July 2012

Re: First solar experience: offgrid + generator

Cariboocoot wrote: »

ggunn;

You can connect PV's to a Sunny Island. Through charge controller connected to the battery like any other off-grid inverter. Kind of expensive way to go off-grid, though. It relies on AC coupling to recharge otherwise, and really is not designed to interface with a generator. You'd have to have the gen run a stand-alone charger to the battery. Really not a good off-grid choice, is it? :roll:

Man, you are fast! I edited my post within about 30 seconds to add the bit about the charge controller option but you beat me to it!

BB. · July 2012

Re: First solar experience: offgrid + generator

Using PV Watts for Warsaw Poland, fixed array tilted to 52 degrees above horizontal, Hours of Sun per day by month is:

Month    Solar Radiation (kWh/m2/day)
1      0.90     
2      1.97     
3      2.96     
4      4.09     
5      4.81     
6      4.14     
7      4.68     
8      4.65     
9      3.65     
10      2.20     
11      1.43     
12      0.84     
Year      3.03

Call ii around 4.0 hours of sun per day... 6.5 kWH per day for an off grid system with typical end to end efficiency of 0.52 :

6,500 WH per day * 1/0.52 system efficiency * 1/4 hours of sun per day = 3,125 Watt Solar Array Minimum

Battery bank size, assuming 48 volts, 2 days of "no sun" (1-3 days rule of thumb) and 50% maximum discharge (longer battery life):

6,500 WH per day * 1/48 volt battery bank * 1/0.85 inverter eff * 2 days no sun * 1/0.50 maximum discharge = 637 Amp*Hour 48 volt battery bank

That will support about a 6kW inverter (~100 AH @ 48 volts per 1kW of inverter rating).

For an off-grid home, I suggest a maximum of ~3.3 kWH per day as a starting point. If you just do lights and fridge, you can probably get down to ~1.5 kWH per day.

Another issue with solar--It works best in a sunny climate--If I have guessed your area correctly--You do not have great sun, and building such a large system for use only during summer will be pretty expensive... Many times, for weekend/summer cabins, a genset+fuel costs can be less than the cost of a solar system (initial costs for equipment and installation, plus battery replacement every ~6-8 years, new electronics every 10+ years, etc.).

-Bill

mi_cin · July 2012

Re: First solar experience: offgrid + generator

Thanks for all replies!

If we assume 3,25kWh daily all results counted by BB. should be divided by two, which is still a lot. Electrical company wants to charge around $2200 for connecting me to the grid. For that money I can buy here 1800Wp panels. With PV I can forget about the bills with electrical company I have to pay them every month, even if I'm not using the electricity.

What other inverter would you recommend instead of SunnyIsland? I found that in catalog and I know it's a good brand as many installations in Germany are based on SMA. But maybe there is something cheaper but also good quality?

Cariboocoot · July 2012

Re: First solar experience: offgrid + generator

The cheaper option is connecting to the grid. Honestly. $2,200 is nothing compared to the cost of a 3kW solar install. The inverter price alone will exceed the grid hook-up.

If you insist on off-grid solar you also need the number for your peak Watt demand (to size the inverter).

Let us assume you can use a reasonably inexpensive inverter like a Magnum 4kw. That's about $2000 right there: http://www.solar-electric.com/maenms4444wa.html

Next you'll need enough battery to supply that 3kW hours. On 48 Volts that would be about 250 Amp hours. You could trim that to the least expensive battery type, the 220 Amp hour 6 Volt "golf cart" batteries. You need eight at around $140 each http://www.solar-electric.com/cr225am6vode.html for $1120. You might find some "warehouse" brand version locally for less. Shipping on batteries is very expensive.

Now those batteries will need panels and a charge controller to keep them working. At 25 Amps @ 48 Volts the controller is easy. You could even use a PWM type if there's nothing difficult about the install. Something like this Morningstar 45 http://www.solar-electric.com/trts12vochco.html That's $150. An MPPT type might be needed if you have long wiring runs or unusual panel Voltages. That would add $250 or more.

As for the panels, basing the recharge on my usual rule-of-thumb you get 1371 Watts of array. Of course you have to round up to the nearest available panel size. Six KD 235's perhaps, at $329 each http://www.solar-electric.com/kykd235wamus1.html for $1974.

The bill so far: just over $5,000. That does not include wires, fuses, mounting hardware, shipping, or taxes.

Now if you're still thinking solar is right for you we can work on the details some more. I just wanted to give you a 'ballpark' idea of what you'd be in for.

BB. · July 2012

Re: First solar experience: offgrid + generator

Since you are in Europe--What is "reasonable" for us in the US may be expensive for you...

SMA makes very good inverters and they will give you good service. Victron is another European brand that has good reviews too.

Before picking brands/models, understanding your needs will be a big help.

If this is for lighting, TV, computer, household water from a cistern, the peak loads may be low and you can use a smaller inverter.

If you have a deep well and need lots of peak power, you may be pushed into getting a very large/expensive inverter.

Also there is the whole question of True Sine Wave (TSW) and Modified Square/Sine Wave (MSW) inverters. TSW are less trouble for some loads and MSW is much cheaper to purchase.

All About Inverters
Choosing an inverter for water pumping

Some folks choose a small TSW inverter (computer, cell phone charger, lighting, radio/TV) and a larger MSW inverter (in-well pump, shop tools). And there are 12 or 24 VDC water pumps that work well for pressurizing your cabin domestic water system.

A bit if this is--Do you like to play around and experiment/tinker will appliances/electricity--Or do you want to install and "forget"? (note: you never forget solar/off grid power--It will be a constant battle of power management, monitoring your battery bank, and trying to keep family members/guests from killing your battery bank).

There a lots of inverters out there... Remember that even an AC inverter turned on but driving no loads will pull around 6-20 watts (larger inverter, more idle power, TSW inverter has more idle power vs MSW, etc.)--So you either need to decide what needs 24x7 electricity, or install an Inverter On/Off switch, or size your system to power the inverter 24x7.

More expensive inverters will have remote on/off switches and "sleep mode" where the inverter turns on for 1 cycle ever 1-2 seconds looking for >6-8 watts of AC load--If load turns on, then the inverter "wakes up" to power the load(s) until they turn off again.

Another suggestion will be a Battery Monitor (Victron is also good)... Simple 0-100% State of Charge display--You can educate spouse/kids/guests about what it means (below 75% SOC, reduce loads; below 50% start generator/call you, below 20% buying new battery bank).

Everyone wants to buy the solar PV system parts iimmediatly because they just ran across a great deal--But that, many times, ends up not working out well. Understanding/defining your loads is usually first priority. Much of the hardware/battery requirements will fall out of that analysis.

And, for AC loads, get a Watt*Hour or KWH meter (like one of these that matches your voltage/outlet needs). Measure your planned loads and see what you find out (peak watts, kWH per day, etc.).

-Bill

icarus · July 2012

Re: First solar experience: offgrid + generator

$2200 to connect to th e grid? DO IT! And do your. Home work, and you will quickly discover that the grid is cheap by a factor of at least 4 compared to battery based solar!

But the way, 6 kwh/day is a lot for a cabin/vacation house unless you use A/C. Conservqtions is far and away ychepaest energy dollar. We use less than 1 kwh/day but we re pretty exceptional.

Tony

techntrek · July 2012

Re: First solar experience: offgrid + generator

I'll play the other side of the coin for a moment. If this cabin is out 20 miles from the substation, through woods, etc., the grid may be cheaper but far less reliable. Having your own battery-based backup (with or without a genset) can have non-monetary advantages. Not even counting future grid instability, which many believe is in our futures.

To the OP, a note about diesel. It can be very hard to size a diesel genset for home-based loads. If it spends most of its time below a 50% load it will have problems, and most of the time a house may only need a few hundred watts (overnight and when you are away). A small cabin would need even less. A modern gasoline inverter-genset is probably a better choice. An exception is if you will always power the same thing - such as your battery charger for a bulk charge - where the load is known and steady.

stephendv · July 2012

Re: First solar experience: offgrid + generator

Cariboocoot wrote: »

You can connect PV's to a Sunny Island. Through charge controller connected to the battery like any other off-grid inverter. Kind of expensive way to go off-grid, though. It relies on AC coupling to recharge otherwise, and really is not designed to interface with a generator. You'd have to have the gen run a stand-alone charger to the battery.

No you don't. It's designed to work with a normal AC generator and has a dedicated "AC in" connection where you can connect either the grid or a generator.

Cariboocoot · July 2012

Re: First solar experience: offgrid + generator

stephendv wrote: »

No you don't. It's designed to work with a normal AC generator and has a dedicated "AC in" connection where you can connect either the grid or a generator.

Really? Usually GT inverters will not accept normal gen output as a grid substitute, claiming the power is off spec.

The quality of SMA equipment never ceases to amaze me. It really is top-notch stuff. But ... you pay for it.

Cariboocoot · July 2012

Re: First solar experience: offgrid + generator

Right. Sunny Island. Island. Right.

Only reason I'm still on the computer is I'm too tired to get up and walk to the bedroom and lay down.

stephendv · July 2012

Re: First solar experience: offgrid + generator

Cariboocoot wrote: »

Right. Sunny Island. Island. Right.

Only reason I'm still on the computer is I'm too tired to get up and walk to the bedroom and lay down.

Hehe, it doesn't help that every second product is some kind of "sunny".

To the OP, 2000 usd is very cheap for a grid connection. And I you're in any of the proper European countries (without crippling beuracracy) then youll likely have attractive grid feeding options via solar too.
At a guess you're looking at 6000 euro+ for pv and battery and inverter

ggunn · July 2012

Re: First solar experience: offgrid + generator

Cariboocoot wrote: »

Really? Usually GT inverters will not accept normal gen output as a grid substitute, claiming the power is off spec.

The quality of SMA equipment never ceases to amaze me. It really is top-notch stuff. But ... you pay for it.

The Sunny Island is not really a GT inverter per se; it never feeds the grid. If you use one AC coupled with a Sunny Boy, the SB still may have probs with the generator waveform. But yes, SMA gear is great stuff.

ggunn · July 2012

Re: First solar experience: offgrid + generator

Cariboocoot wrote: »

ggunn;

You can connect PV's to a Sunny Island. Through charge controller connected to the battery like any other off-grid inverter. Kind of expensive way to go off-grid, though. It relies on AC coupling to recharge otherwise, and really is not designed to interface with a generator. You'd have to have the gen run a stand-alone charger to the battery. Really not a good off-grid choice, is it? :roll:

One other thing about that setup; you can't use it to export power to the grid. Once the batteries are fully charged and if the loads are off, the CC will shut off the PV. It's not what the OP is asking, I know, but it's a difference between AC and DC coupling with a Sunny Island.

First solar experience: offgrid + generator

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