Battery sizing

powersurge

In order to run a 800 watt toaster 800 watt coffee maker 800 watt washing machine and a 1200 watt microwave not all at the same  time. 
What size 24 volt system  do I need?
Batteries inverter panel watts etc

PNW_Steve

It depends... 

What is the duty cycle of each of those loads? 

mike95490

I've been there !!

Toaster 800w  0.1 hr  (6 min)

Coffee  800w  0.2 hr  ( Then off, put the brew into an insulated carafe )

Washer  800w 1 hr

uWave 1200w  0.2 hr

Toaster and coffee happen before 10am , when sun is on panels, so you are running on batteries that were tired from all night, and a bit soft.

 Washer, you have to load shift that to sunny hours, on sunny days.

 uWave, likely after 3 pm, when sun is too low to be of any help.

Washer, is it a modern one with variable speed & reversing motor, or old one motor, belt and gears ?  

The 1 motor belt & gears has a large starting surge, likely needing a 2Kw inverter.  That sucks because it will be a lot of inverter running all the time, just for 1 appliance 2x a week.

uWave, that's another annoying load, going to need a 1500w inverter for that too.   Still a lot of inverter to be idle 24/7

What about lights, computer, TV, heater fan,   any other electrical loads ?

For the uwave and washer, I'd really start to lean to a 48V system.   1200W @ 24V is 60A, and will need a large battery to not sag under that load.  At 48V, it's only 30A, much easier to manage the DC current.

mcgivor

The coffee maker, although a seemingly large load, is in fact not so due to the duty cycle. The main concern is voltage drop, the battery needs to be of sufficient capacity so as not to cause the inverter to fault due to low voltage. Being that it is a resistive load the inverter can be sized according to the rated capacity as power factor is not an issue.

Microwaves on the other hand are inductive loads, the 800W unit I have draws  ~1500W , so the inverter needs to be of greater capacity to handle the power factor corrected wattage. The current drawn from the batteries however will be calculated according to the rated wattage because power factor is strictly an AC phenomenon which affects the generation sources electrical limitations. This means a 1500W microwave  may need an inverter of 2500W or greater, cheaper inverters may have problems even with a significantly higher rating, case in point, I have an inexpensive  2000W inverter which cannot support the 800W microwave for more than a few minutes. So it's difficult to predict with certainly the inverter capacity because there are too many variables, the battery capacity needed is, as in the case of the coffee maker needs to survive the voltage drop.

Washing machines are largely dependent on type, I've measured my Samsung for a regular 48 minute cycle with a power recording device, the total used was 0.143kwh, this was a surprise to me as I expected it to be significantly higher, the maximum running wattage is 320W,

Battery capacity needed really depends on the type used, lithium can discharge at much higher current without significant voltage drop, lead acid would need to be of greater capacity than lithium as a general rule and depending if flooded or sealed will determine that. The PV needed will also depend on battery type or chemistry, lithium can accept much higher current than lead acid without the long absorption so an array could be smaller, however both would be sized according to available sun. 

To even provide a PV  guestimate one would need to know the intended battery type, loads and cycle duration, maximum loads and geographic location 

Estragon

I understand the PF is an AC issue, but does the microwave draw 800w ÷24v = ~33adc, or 1500w ÷24v = ~62adc (ignoring other losses)?  It seems to me it's the latter, and the battery has to supply the higher current because of the poor PF?

mike95490

regarding Power Factor (PF)

The inverter must supply the extra out of phase component, but the total drain on the battery is the original load, not the PF loading the inverter sees.   (it's a clumsy way to explain it, but the inverter has to manage to produce the 1200w, while it only draws 800w from the battery bank).  It's an artifact of how out of phase voltage and current get handled by the inverter, not the battery.

mcgivor

Estragon said:

I understand the PF is an AC issue, but does the microwave draw 800w ÷24v = ~33adc, or 1500w ÷24v = ~62adc (ignoring other losses)?  It seems to me it's the latter, and the battery has to supply the higher current because of the poor PF?

The difference is between apparent and real power the generation source, the inverter in this case  must be able to support, the inverter needs to support apparent power, but the battery only needs to support the real power. The attached file explains in more detail than I choose to explain, i hope others learn something from this.

Estragon

Thanks, I think I understand it now.  The confusion (at least in my KISS brain) boils down to to the (mis)specification of inverter capacity in watts. 

I guess they really should be rated in VA, with the watt rating depending on the PF of connected loads. Eg a 2kva inverter is a 2kw inverter with a heating element load, but a 1kw inverter with 2kva .5pf load.  I vaguely recall server room UPS systems being kva rated (for capacity planning with lousy pf gear). 

softdown

powersurge said:

In order to run a 800 watt toaster 800 watt coffee maker 800 watt washing machine and a 1200 watt microwave not all at the same  time. 
What size 24 volt system  do I need?
Batteries inverter panel watts etc

I see your line of reasoning but a better question would reveal:
1) Peak load requirement. That is often the surge of a well pump for off gridders. For you it appears to be the surge of the washing machine.
2) Daily kWh usage - which one is doing their best guess on. 
3) Worst case scenario. is your area prone to extended cloudy weather with snow - for example. 
4) General scenario. Cloudy environ? Sunny environ? Northern latitude? Equatorial latitude?

Washing machine without a fridge? Propane fridge? Fine if you have it but electric fridges make a lot of sense these days. 

I'm a newb compared to many here but I try not to overlook the basics while focusing on the more esoteric. 

706jim

With a bit of power management you can scale the inverter size down somewhat. I'd suggest a 2500 watt pure sine if you can find one. How likely is it that all of those things are going to be running at the same time?

powersurge

Never at the same time. I have a 2000 non inverter generator that will run each one separately. If I understand it correctly a 450 ah 24 volt bank at c/8 gives me ~56 amps at 24 volts 1350 watts. At c/5 SURGE 90 amps or 2200 watts. Seems the microwave is the biggest load? Or would it be the washer surge. What if it was a 700 watt microwave?

powersurge

Propane fridge.  Using 8 t 105 225 ah la batteries. 37° latt. 2kw per day usage. Usually sunny 3 months now since July 4 and only 1 rainy day. No well pumping. Washer 2 loads 30 mins each 1 hour.

BB.

Here is a quick example of sizing your system using our nominal off grid system design. Assume that your total loads are 3.3 kWH per day (3,300 WH per day or 100 kWH per month). That is generally enough power to run a full size energy star refrigerator (1,000 - 1,500 WH per day  or 365 kWH per year yellow tag at 1 kWH per day), LED lighting, Laptop, LED TV, solar friendly well pump, run a washing machine a few times a week... Basically a very conservation minded small home/cabin.

• 3,300 WH per day * 1/0.85 AC inverter eff * 1/24 volt battery bus * 2 days storage * 1/0.50 maximum discharge (longer battery life) = 647 AH @ 24 volt battery bank

The 2 days of storage seems to be a good starting point (1-3 days typical system design). Discharging your battery bank to 50% maximum during normal operations gives you longer battery life. For RVs, 1 day of storage and 50% discharge or a bit more can be a better fit for the space available (panels, batteries, etc.).

Then there are two calculations for sizing the solar array. First, based on the size of the battery bank (larger battery bank, needs more solar panels for minimum charging). And hours of sun you get per day (summer only use vs 9-12 months a year).

Based on battery bank size... 5% for summer/weekend/emergency backup use (minimum rate of charge). 10% - 13% to 20% rate of charge. 10% is usually the minimum recommended by flooded cell battery mfg., 13% is a good maximum "cost effective" for Flooded cell batteries, and 20% is usually the maximum.

• 647 AH * 29.0 volts charging * 1/0.77 solar panel+charge controller eff * 0.10 rate of charge (nominal) = 2,437 Watt array "nominal"

And then there is sizing the array based on your hours of sun per day (location), mounting angle, and seasons when you plan on using the system. For example, a fixed array facing south in El Paso Texas:
http://www.solarelectricityhandbook.com/solar-irradiance.html

El Paso
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 58° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.01	5.54	6.45	6.67	6.65	6.47
Jul	Aug	Sep	Oct	Nov	Dec
6.03	5.83	5.65	5.56	5.17	4.68

El Paso, has lots of sun (>3 hours of sun per day, especially, during winter, is not bad). For winter in northern systems, I typically suggest tossing the bottom 3 months and using the next lowest month as the "break even month"--Assuming you will use a genset to make up for bad weather/poor solar harvest).

• 3,300 WH per day * 1/0.52 off grid system eff * 1/4.68 hours of sun per day (Dec) = 1,356 Watt array for December "break even"

So, if full time offgrid, using a 2,437 Watt array (10% rate of charge), it will produce an average during December average day:

• 2,437 Watt array * 0.52 off grid system eff * 4.68 hours of sun per day = 5,931 Hours of sun per average December day

Note--You should not count on using 100% of predicted energy every day... using 50% to 65% of predicted power every day (fridge, LED lighting, water pumping) and on sunny/clear days run the other loads (vacuum cleaner, washing machine, pumping to pond/cistern).

And battery bank size does also give you rule of thumbs limits... For Flooded Cell Lead Acid deep cycle storage batteries, on a 24 volt battery bus with 647 AH of capacity, the maximum AC inverter (and more or less the largest suggested solar array) would be:

• 647 AH * 500 Watts per 100 AH @ 24 volts) = ~3.235 Watt AC inverter and Array
• And ~1/2 that size AC inverter -- 3,235/2 = 1,618 Watt inverter nice minimum size

For any battery bank, I suggest roughly an 800 AH battery bank maximum... I.e., 800 AH @ 24 volts = 400 AH @ 24 volts for various reasons (charging sources, thickness of copper wire, etc.).

No right or wrong answer here... But the math can help you figure out where to spend your money (conservation, larger system, generator fuel).

-Bill

Estragon

For the microwave, I found a cheaper MSW inverter doesn't work well.  A pure sine wave type may also be better for the washing machine.  A decent PSW inverter should handle ~2x rated power surge for a few seconds, so my guess is the microwave is the load for sizing the inverter ~2000-2500w.  If you want AC for lighting or other small but long duratiin loads, you may want to consider a second, smaller inverter to run them instead of keeping a bigger one lit all the time.

With 450ah@24v bank, that's ~10kwh total storage.  Taking it down to no less than 50% gives you 5kwh usable, so 2-3 days at 2kwh. 

If you got say 1500w (STC rated) panels, they might produce ~1200w in full sun for a few hours around noon, about 12% of bank capacity.  To get from 50% to ~85% with few other loads would take ~3 hours, and another 2-3hrs with dimishing current in absorb to get full.

Do you use the place in winter?  If so, at 37° latitude, available sun will be considerably reduced and it might make sense to add a bit more pv.  If just summer and/or you don't mind charging with the genny if needed, you could get by with less.

Using STC rating and 22v charging to be conservative, 1500w/22v = 68a.  You'd probably never see that though, so ~60a min. charge controller should do.

How long would the wire run be from the pv mount to the batteries?  For a short run, you might be able to put say 6 x 250w panels in parallel and use a cheaper pwm controller.  If more than ~50-75' it likely makes more sense to put in series strings of 2 or 3 into mppt controller.

Anyway, a few numbers to think about FWIW.