trying to leave square one

elesaver

OK, I had posted an initial post about wanting off-grid solar. Like many newcomers to the process, I had underestimated all that is involved. I purchased a kill-a-watt meter to get some appliance readings and now need to get some advice and direction on how to figure panels and batteries. I had measured the refrigerator, TV, and freezer for 24 hours. I did not measure any lights, computer, etc. These are the biggies, of course, and may be too much for me to even consider. Average sun (SE area) is 7 hours per day. From my calculations, that results in the need for 333+ watts/hour. (Total watts/7 sun hours/0.75 efficiency) For 333 wh @ 12V, I would need 28 amps/hour. This does not seem to be right. I'm certain I couldn't do this on two 12V batteries. Perhaps some instruction on how to calculate? I'd like to be able to understand how the numbers work. Thanks for all the patience, by the way.

Freezer: 540 watts
Refrig: 1100 watts
TV: 120 watts

BB.

Re: trying to leave square one

elesaver wrote: »

I had measured the refrigerator, TV, and freezer for 24 hours. I did not measure any lights, computer, etc. These are the biggies, of course, and may be too much for me to even consider. Average sun (SE area) is 7 hours per day. From my calculations, that results in the need for 333+ watts/hour. (Total watts/7 sun hours/0.75 efficiency) For 333 wh @ 12V, I would need 28 amps/hour. This does not seem to be right.

I am not sure I understand your math... And note that Watts is a rate (like Miles Per Hour) and Watt*Hours is an amount (like Miles driven). There is no (useful to use) Watts/Hour. Working your math backwards would seem to indicate:

333 Watts * 7 hours of sun * 0.75 eff = 1,748 Watt*Hours per day?

That sounds low... It is possible but unless you have very efficient appliances (just the refrigerator and freezer would take around 1.2 kWH per day each or ~2.2 kWH per day for a typical N. American set--pretty efficient ones at that). If the TV is a pretty new LED set--Then, it may not be too much power.

I'm certain I couldn't do this on two 12V batteries. Perhaps some instruction on how to calculate? I'd like to be able to understand how the numbers work. Thanks for all the patience, by the way.

Don't worry about calculating your battery bank/solar array just yet... Lets get the load number right first.

Freezer: 540 watts
Refrig: 1100 watts
TV: 120 watts

Are those Watt*Hours (or kWH i.e. 0.540 kWH per 24 hours) from the kill-a-watt meter?

-Bill

elesaver

Re: trying to leave square one

The kill-a-watt meter (each measurement for 24 hours)read as follows:

TV .12 kwh
Freezer .54 kwh
Refrig 1.10 kwh

Cariboocoot

Re: trying to leave square one

elesaver wrote: »

The kill-a-watt meter (each measurement for 24 hours)read as follows:

TV .12 kwh
Freezer .54 kwh
Refrig 1.10 kwh

That makes more sense: kwh = kilowatt hours. Your total there is 1760 kW hours for the day. But don't discount all those lights and such as they can add up to a surprising amount. As it is you are looking at around 2kW hours a day, which is not unrealistic.

Also don't count on 7 hours of sun. For one thing that doesn't equate to 7 hours of sun directly on the panels. We have 16 hours of sun in Summer here, but the panels don't get more than 5 due to the change of angle. For another thing, planning on average sunlight will leave you short whenever the hours fall below that. Instead look at the minimum hours of sun you'll get. PV Watts works well for examining the amount of daylight you can expect, although it does not equate directly for off-grid systems: http://www.nrel.gov/rredc/pvwatts/

You are correct in disregarding 12 Volt systems. They should not be used unless there is a specific need for 12 VDC such as in an RV. People who think they will 'save' by using 12 Volt devices do not understand electricity properly: lower Voltage is inherently less efficient for doing the same job.

You're not too far away from having a basic design concept now!
AC Watt hours per day convert to DC Watt hours by the efficiency factor of the inverter. This is usually 90% for a good one, so you'd have something like (but not exactly) "1760 / 0.90 = 1956 Watt hours DC". Then you have to add in what the inverter itself draws in a day. This may be 480 Watt hours (20 Watts is not unusual for a large inverter). Now you're at 2436 Watt hours.

At that point you divide by nominal system Voltage: 2436/24 = 101.5 Amp hours used. Notice how close that is to 100? You'll want at least double that for 50% DOD, and 4X for 25% DOD which will usually give the best balance. So you're looking at 200 Amp hours @ 24 Volts minimum, and 400 Amp hours @ 24 Volts best-case.

Apply that to some readily available battery sizes. The obvious choice here is the least expensive GC2's at 220 Amp hours. You could have two strings of four each and get 440 Amp hours @ 24 Volts.

Once you have that battery bank sized it is possible to calculate the needed array. 10% of the Amp hour capacity as a peak charge current works most of the time. So we have 44 Amps * 24 Volts = 1056 Watts, which needs to be derated for the panel & controller average output, typically 77% making the array size (1056 / 0.77) 1371 Watts. Obviously you won't find panels that match that exactly, so you try to come closest with preference for slightly larger rather than slightly smaller array.

Then you can check it back like this: 1371 Watts * 4 hours (or whatever minimum) equivalent good sun * 0.52 over-all system efficiency = 2851 Watt hours per day. This number needs to be more than you AC Watt hours consumed, and it is.

Also certain adjustments may be needed to account for local conditions such as marine layer or high heat, et cetera. But you have to be in the ball park before you can play the game.

elesaver

Re: trying to leave square one

OK, now I'm rolling...and, even beginning to understand. One of the things that I had begun to understand at the beginning is that this is no cheap seat. It would take a long time to have the system pay for itself through the savings and by then, it would be time to buy more batteries! Nobody said this was going to be easy. Throw in an MPPT controller and a pure sine inverter, you've got a "nice little system, there." I must mull some more. Thank you so much for the lesson, Cariboocoot. With some practice repetition, I think I can begin to calculate on my own.

Here's another question...is there any savings realized in running a freezer and/or refrigerator during the day and cycling it off and on in intervals of two hours at night? Freezers, especially, tend to really hold the cold and if no one is opening the door, even better? Or, is it harder on the appliance and in the long run, it's going to use as much as it would have if left to work on its own since it cycles anyway? Just trying to find ways to save.

bill von novak

Re: trying to leave square one

elesaver wrote: »

Here's another question...is there any savings realized in running a freezer and/or refrigerator during the day and cycling it off and on in intervals of two hours at night? Freezers, especially, tend to really hold the cold and if no one is opening the door, even better? Or, is it harder on the appliance and in the long run, it's going to use as much as it would have if left to work on its own since it cycles anyway? Just trying to find ways to save.

Short answer - no. Trying to "trick" the refrigerator by extending on/off cycles is generally counterproductive, since they are designed for that cycle length to begin with.

However if power is cheaper for you during some period it may be possible to shut down your refrigerator during this time, provided you leave the door closed and have a lot of thermal mass (i.e. ice, water jugs.) This is the case in many off-grid solar power systems; power is effectively cheaper during the day when solar power is immediately available and does not need to be stored in batteries. In my tests you gain about 2F in the freezer per hour of non-operation and 1.2F per hour in the refrigerator. With better insulation that is decreased, of course.

Cariboocoot

Re: trying to leave square one

I tried that cycling the 'frige and tried using the 'standby' mode of the inverter. More trouble than it's worth. 'Saved' about 80 Watt hours with my equipment. The warmer the 'frige gets during 'off' time the longer it has to run to re-cool during 'on' time, and there's only about 8 hours the inverter would be off during a day using the standby mode due to the daily demands of everything else.

Never try to live on the margins of a system. Build in plenty of tolerance and flexibility, because not every day is the same for usage or production.

BB.

Re: trying to leave square one

Also, freezer burn is worse and food probably will not keep as long with extending cycle times.

A thread from a few years ago on why ~0F (-18C) is really optimum for long term food storage (l.e., if you don't keep your freezer as cold, things probably will not last as long either):

Question - Best Freezer Temperature

-Bill