Posted by *merlin-7* on January 6, 2007, 10:33 pm

This is a planning question so I have no real numbers here, so lets take a

few items for granted..

Lets say you have 5 hours of full sun per day and want 1 day of reserve

power.

(inverted) You want to run a full size fridge, a 1500 watt microwave for 30

minutes a day. A washing machine for 30 minutes a day.(washing machine would

only be used in daytime while panels are in full sun) plus a TV (no real

numbers here but lets say 100 watts, mostly when panels are in full sun for

4 hours a day)

(dc load 12 volts) ham gear...5 amps at night for 3 hours average ...

lights ....cfls 100 watts 4 hours....average...

other.....100 watts 5 hours......average...

What I think I need here is a bit of an unbalanced system. that is, more

panels vrs. battery storage.

Looking at the above. How many watts of solar panels and AH of battery bank

would you start with?

Rember, this home is on grid, I am just looking for a starting point for

off grid.

heating, cooking etc. will be done with gas.

I do not expect you all to do all the math but some of you will have a good

idea anyway...

Thanks

Joe ki4ilb

Posted by *Eric Sears* on January 7, 2007, 9:48 am

wrote:

*>This is a planning question so I have no real numbers here, so lets take a*

*>few items for granted..*

I can't say how realistic your guessed numbers are - but the fridge is

the bigger load - so it might make a lot of difference what you choose

(or already have!).

*> Lets say you have 5 hours of full sun per day and want 1 day of reserve*

*>power.*

(I would have said that in many places 3 - 4 hours was a better

average over a year, for a fixed panel - but YMMV). Certainly 5 hours

is possible for summer time.

*>You want to run a full size fridge, a 1500 watt microwave for 30*

*>minutes a day. A washing machine for 30 minutes a day.(washing machine would*

*>only be used in daytime while panels are in full sun) plus a TV (no real*

*>numbers here but lets say 100 watts, mostly when panels are in full sun for*

*>4 hours a day)*

*>(dc load 12 volts) ham gear...5 amps at night for 3 hours average ...*

*> lights ....cfls 100 watts 4 hours....average...*

*> other.....100 watts 5 hours......average...*

In very round terms, the power usage EXCEPT the fridge are around

2kwh+. The fridge (of the size you describe) might very well add

another 2kwh (or at least 1.5kwh).

Unless you have some way of determining which appliances operate

"when", you might need a pretty big inverter (ie if the microwave were

running and the fridge switched on, the peak load could be excessive -

which would call also for a larger battery).

Assuming you limit your max load, and only need one day of reserve,

probably about 1kw of panels might do it (very rough guess).

As far as battery bank is concerned, you highest load is likely to be

created by the microwave (except for a brief switch-on surge for the

fridge). A 10kwh battery might be sufficient - but running a 1.5kw

load on that for any length of time will probably drop its voltage a

bit! I think most would opt for something like 15kwh battery.

You should be able to work out the amp-hr - it will depend on the

voltage of your system.

Having grid power as a secondary source (when solar insolation is

low), is a definite plus (its much what I do here with a lot less

solar at present). My "less than 450 watts" of panels provide a max of

about 1.8kwh on a good day in summer. An MPPT would add to that.

Eric Sears ZL2BMI.

Posted by *Ron Rosenfeld* on January 7, 2007, 3:56 pm

*>This is a planning question so I have no real numbers here, so lets take a*

*>few items for granted..*

*> Lets say you have 5 hours of full sun per day and want 1 day of reserve*

*>power.*

*>(inverted) You want to run a full size fridge, a 1500 watt microwave for 30*

*>minutes a day. A washing machine for 30 minutes a day.(washing machine would*

*>only be used in daytime while panels are in full sun) plus a TV (no real*

*>numbers here but lets say 100 watts, mostly when panels are in full sun for*

*>4 hours a day)*

*>(dc load 12 volts) ham gear...5 amps at night for 3 hours average ...*

*> lights ....cfls 100 watts 4 hours....average...*

*> other.....100 watts 5 hours......average...*

*> What I think I need here is a bit of an unbalanced system. that is, more*

*>panels vrs. battery storage.*

*> Looking at the above. How many watts of solar panels and AH of battery bank*

*>would you start with?*

*> Rember, this home is on grid, I am just looking for a starting point for*

*>off grid.*

*> heating, cooking etc. will be done with gas.*

*> I do not expect you all to do all the math but some of you will have a good*

*>idea anyway...*

*> Thanks*

*> Joe ki4ilb*

In order to come up with reasonable numbers, you need to provide accurate

values for your electrical usage. You have not provided any numbers for

the fridge or washing machine.

I will assume 1.6kWh/day and 300W while running for the washing machine.

Since you are on grid, I will assume that the 5ESH is an average for the

year, and that this is how you want to size.

If you were off-grid, you'd want to size for the worst month.

But you state that you are "looking for a starting point for off=grid", so

I'm not sure what you want to do here. But we'll use the 5 hours/day, and

also assume you will be using high-quality batteries that you can discharge

80% DOD.

Because of your 12VDC load, I also assumed a 12V system. But a 24V system

might be more appropriate with a DC-DC voltage converter, depending on your

eventual total size.

Your wire sizing will be less, and your inverter choices wider.

That being the case:

DC Load: 1080 watt hours per day

AC Load: 2700 watt hours per day

Inverter losses 15%

Net AC req 3176 watt hours per day

Array Sizing Procedure

DC load in watt-hours/day 1080Wh

AC load expressed as DC 3176.47 Wh

Total load 4256.47 Wh

System nominal DC voltage 12 volts

Total load in ampere-hours 354.71 Ah

System losses/safety factor multiplier 1.2

Adjusted total daily load 425.65 Ah

daily average insolation 5 ESH

Required array current 85.13 amperes

Current at maximum power (Imp) of selected module 7.1

(These are BPSolar 3125 12V/125W modules)

Number of parallel modules required 11.99

Line 12 rounded up to next whole number 12

Number of series modules required 1.0

Total modules in array (Line 13 X Line 14) 12

Battery Sizing

Selected days of reserve 1

Adjustment for cycling capacity 0.8

Adjusted total daily load 425.65 ampere-hours

Recommended nominal battery capacity 532 Ah at 12 volts or

6.38 kWh

--ron

Posted by *BobG* on January 7, 2007, 4:50 pm

Great numbers Ron. I changed the title to something more interesting

for readers scanning titles.

Posted by *merlin-7* on January 7, 2007, 5:37 pm

*> ><snip>*

*> In order to come up with reasonable numbers, you need to provide accurate*

*> values for your electrical usage. You have not provided any numbers for*

*> the fridge or washing machine.*

*> I will assume 1.6kWh/day and 300W while running for the washing machine.*

*> Since you are on grid, I will assume that the 5ESH is an average for the*

*> year, and that this is how you want to size.*

*> If you were off-grid, you'd want to size for the worst month.*

*> But you state that you are "looking for a starting point for off=grid", so*

*> I'm not sure what you want to do here. But we'll use the 5 hours/day, and*

*> also assume you will be using high-quality batteries that you can*

discharge

*> 80% DOD.*

*> Because of your 12VDC load, I also assumed a 12V system. But a 24V system*

*> might be more appropriate with a DC-DC voltage converter, depending on*

your

*> eventual total size.*

*> Your wire sizing will be less, and your inverter choices wider.*

*> That being the case:*

*> DC Load: 1080 watt hours per day*

*> AC Load: 2700 watt hours per day*

*> Inverter losses 15%*

*> Net AC req 3176 watt hours per day*

*> Array Sizing Procedure*

*> DC load in watt-hours/day 1080Wh*

*> AC load expressed as DC 3176.47 Wh*

*> Total load 4256.47 Wh*

*> System nominal DC voltage 12 volts*

*> Total load in ampere-hours 354.71 Ah*

*> System losses/safety factor multiplier 1.2*

*> Adjusted total daily load 425.65 Ah*

*> daily average insolation 5 ESH*

*> Required array current 85.13 amperes*

*> Current at maximum power (Imp) of selected module 7.1*

*> (These are BPSolar 3125 12V/125W modules)*

*> Number of parallel modules required 11.99*

*> Line 12 rounded up to next whole number 12*

*> Number of series modules required 1.0*

*> Total modules in array (Line 13 X Line 14) 12*

*> Battery Sizing*

*> Selected days of reserve 1*

*> Adjustment for cycling capacity 0.8*

*> Adjusted total daily load 425.65 ampere-hours*

*> Recommended nominal battery capacity 532 Ah at 12 volts or*

*> 6.38 kWh*

*> --ron*

I do not have real numbers at this time, It is for my home when I move in 6

months or so...

However, you did confirm what I had expected...I will need to go to A 24

volt battery bank. Not so much as a current loss or wire size thing but the

problems and cost of charge controllers.

If I understand the numbers correctly, 1 mx-60 charge controller can handle

the system you outlined above at 24 volts but not at 12 volts. correct?

Now I will need to find a good 24 to 12 volt converter in the 30 amp range

for my ham gear.

I am also thinking about having a small array (400 watts or so) to run my

ham gear and lights.( I have that now) and building the new system from

scratch for the inverter/ac....

Or, I could, get a small 24 to 12 volt charge controller to go between the

large 24 volt battery bank and a small 12 volt battery bank (just something

to handle the load spike when I transmit)

Is this realistically possible?

I also have a mixed set of solar panels at this time that works ok for 12

volts but would be a problem adding them to a 48 volt string without loseing

some ouput.

Thanks joe KI4ILB

>This is a planning question so I have no real numbers here, so lets take a>few items for granted..