Sizing 1600 watt-hr array: sanity check needed please!

Registered Users Posts: 10 ✭✭
Hello everyone!

Been lurking a few days and finally plucked up the courage to post. But first I want to say that this forum is fantastic: it’s rare to find one where the mods and members are so knowledgeable and so generous with their time. Thank you!

I am a solar newbie. I’ve been doing loads of research, but every time I think I have a handle on things I read something new and begin to second-guess myself.

I am trying to size a solar system for a tiny house. I have a ton of questions, but I’ll start with the basics here. Apologies in advance that this post is long; I've tried to keep it clear and easy to follow.

I’ve estimated our consumption at 1,600 watt-hours / day. (Couple laptops, wi-fi router, couple phones, iPad, LED lighting, DC energy-efficient fridge, small DC water pump, a few other bits and bobs.)

I began (like most noobs do, it seems) by using the 1,600 watt-hours to size the required solar array. The house will be in Sonoma. Triangulating different methods I found online (included for posterity at the end of the post) I arrived at an array of 450–650 watts.

I then calculated battery bank based on 12-volt system and 50% DoD: 1,600 / 12 / 0.5 = 266.6 amp-hours. I figured an extra day’s storage would be good: 266.6 x 2 = 533 amp-hours.

I’d like to leave the inverter out of this discussion for now, because it will throw up a whole bunch of other issues. (But basically we will be drawing little AC current, no more than 500 watts and perhaps a lot less, unless we decide to have a blender. Or we want to expand!)

We will use around 900 watt-hours during the day and 700 at night, so I thought this set-up should provide our needs throughout most of the year — with the batteries able to get us through a few cloudy days, and enough power available from the array to charge the batteries even if depleted to 50%. (We’d also likely have a small generator for charging the batteries during periods of bad weather, but that’s probably another discussion.)

However, having poked around this forum, I’m no longer so sure! It seems that what I should have done was to begin with the battery bank, and use that to determine the size of the array that would be needed to charge it. I want to check, please, that I’ve understood these calculations correctly. So:

1,600 watt-hours required. 12-volt battery bank. We’ll go with 25% DoD since that seems to be the rule of thumb here, and fits with my “it would be nice to have a bit extra but I’m not too fussed about it” thoughts. Assume we are using higher voltage “grid-tie” panels + MPPT charge controller.

1600 / 12 / 0.25 = 533 amp-hours.

So far, so good, but this is where I start to get confused. If we assume charging voltage of 14.5 and 77% efficiency (panels & charger losses), then to charge the batteries at 5% would require:

533 x 14.5 / 0.77 x 0.05 = 501 watt array.

And to charge at 10%, in order to have power available to run loads at the same time:

533 x 14.5 / 0.77 x 0.1 = 1003 watt array.

Is this correct?

Thing is, I don’t really understand what the 5% and 10% are! I’ve just seen these numbers used on the forum.

A 1,000-watt array is much larger than I had anticipated needing for a daily load of 1,600 watt-hours. And it seems as though bumping up the battery bank to 24 volts does not help with this (though I understand there are other advantages):

1600 / 24 / 0.25 = 267 amp-hour battery bank at 24 volts.

I assume we double the charging voltage for a 24-volt bank, and so the array size remains the same:

267 x 29 / 0.77 x 0.05 = 503 watt array (5% rate of charge)

267 x 29 / 0.77 x 0.1 = 1005 watt array (10% rate of charge)

I’ve also seen it said here that the batteries need to be charged with a current of 10% their total capacity. (Although I thought I’d read elsewhere that the rate of charge should not exceed 10% of the battery bank’s capacity, but I probably misunderstood.)

The 12-volt bank above would therefore need 53 amps to be provided by the panels, whereas the 24-volt bank would need 27 amps. But I’m unclear on how to calculate this.

I think 500 watts of panels would provide:

12-volt bank: 500 / 14.5 x 0.77 = 26 amps

24-volt bank: 500 / 29 x 0.77 = 13 amps

Either way, the 500 watts of panels would only be providing half the required number of amps, and so I don’t see how halving the amps required by going with a 24-volt system helps. (I'm guessing my methodology is wrong.)

So yeah, I’ve gone from feeling pretty confident to feeling thoroughly confused. I realize I may be obsessing over these formulae, but I want to get a better understanding of how this all works.

I guess my questions boil down to:

1. How would you size a system for a 1600 watt-hour load, using a 12-volt battery bank and using a 24-volt battery bank?
2. What do the 5% and 10% rates of charge used in the array sizing calculation represent?
3. How do they relate to the current needed to effectively charge the batteries, especially with regard to any difference between 12 and 24 volt systems?

Apologies again for the long post, and thanks so much in advance for any help.

Best wishes, Nick.

--

For reference, how I initially came up with the 450-650 watts for the array:

Method 1. Divide watt-hours by 5 (assumption for hours of sun) then add a 50% fudge-factor (inefficiency) and a 33% fudge-factor (location) = 640 watts.

Method 2. Divide watt-hours by 3.5 (hours of winter sun in location) then add a 33% fudge-factor (inefficiency) = 610 watts.

Method 3. Plug data into Alt-E website = 420 watts.

Method 4. Ask a solar supplies company for input = 700 watts (and 320 amp-hours for the battery for one day) but they won’t be specific in how they arrived at these numbers.

• Banned Posts: 17,615 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Welcome to the forum Nick.

Let's see: you need 1600 Watt hours AC per day. Well you're already ahead of many people because you have a number to work from!

Now the first thing that happens to that number is it gets converted to DC Watt hours, and that depends on the inverter efficiency. Not all inverters have the same efficiency though. A good one should manage 90% efficiency, so now you've got 1778 Watt hours DC.

The next thing that happens is the inverter's consumption has to be included. Again this varies by inverter and can include differences based on use of standby mode if available. Hmm. Knowing the maximum Watts here helps. Another factor is DC only loads, which you mention but may not be the best choices. It is a fallacy to think that a DC device is automatically more efficient than an AC device. They may or may not be.

Anyway such loads (including inverter) would be added in or subtracted from if they were included in the original 1600 Watt hours.

Then we hit the system Voltage choice. In general 12 VDC should be avoided unless there is a specific need for it. The reason being that a 12 Volt system is not very efficient: current demand for a given amount of Watts is higher than with higher Voltages which means more energy goes in to heating wires that would be better used for work.

So let's see what it would be like with an all AC system and an inverter consuming 480 Watt hours per day. You've just hit 2258 Watt hours DC. This is not unusual for an OG system (around 3kW hours is quite typical).

How do you get it: 2258 / 24 Volts = 94 Amp hours used. If you got with 25% DOD on the batteries that's a 376 Amp hour battery bank. You won't find batteries this size so you have to go up or down a bit.

Be aware three 'fudge factors' just worked their way in: use of nominal system Voltage and ignoring power available 'directly' from panels and the adjustment of battery capacity. With a little luck you will actually have more Watt hours available.

Finally the array size is based on being able to recharge the battery. Let's say you got 390 Amp hour batteries and want to use the 10% rule-of-thumb (because it works most of the time). So you want 39 Amps @ 24 Volts plus the array & controller efficiency factor (typically 77% with MPPT controller). This works out as a 1216 Watt array. Once again you need to adjust to what is actually available: larger is preferable to smaller most of the time.

Is that more confusing or somewhat clarifying?
• Solar Expert Posts: 3,739 ✭✭✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Welcome to the forum Nick,

Living off grid is all about the batteries. And the batteries may influence the decision to go 12 or 24 volts...

Suppose you decided that you needed a 450 ah battery at 12 volts (or the equivalent 225 ah at 24 volts). If you were using 6 volt golf cart batteries, you could do a single string of 4 of them to make 24 volts, but you would need two parallel strings (of 2 batteries per string) at 12 volts. In that case I would recommend the 24 volt approach because parallel batteries are not the best possible design.

On the other hand, if you wanted 390 ah at 12 volts you could do that with a pair of 6 volt L-16 batteries.

My point is that to some extent the choice of system voltage may depend upon the batteries you would like to use.

Next... no two battery cells are identical and approximately half of them are below average. It is unlikely that you can assemble 6 cells (12 volts) that are all above average, and it is even less likely that you can assemble 12 cells (24 volts, or two strings of 12 volts) that are all above average.

It turns out that the life of the entire bank is determined by the weakest of the cells. The more cells you have, the greater the chance that you will have an outlier weak cell that will shorten the life of the entire bank. On average, the longest lived bank will be a single 2 volt cell. Of course, a 2 volt system is not practical. Best design means a single string of cells at the lowest system voltage that is practical. If a 12 volt system is practical for your application, then that is better than 24 volts. A 12 volt system has fewer potential points of failure than a 24 volt system.

Any time you put two strings of batteries in parallel you are doubling your chance of getting an outlier weak cell, but actually there are many better reasons to avoid parallel batteries. Short discussion here: http://www.wind-sun.com/ForumVB/showthread.php?14674

Another decision that may influence the 12 vs 24 volt decision is the distance between the solar panels and the controller. If the distance is great, the higher voltage system has an advantage. The reason is that MPPT controllers are more efficient when they perform less down-conversion of voltage. If the distance between your panels and controller is great you will want to configure your panels in series to have a high string voltage, but that high string voltage will be easier to down convert to 24 volts than to 12 volts.

One more consideration... If there is any chance you will want to expand the system, go with 24 volts.

You mentioned a DC fridge. The general consensus is that a good 120 volt fridge can be just as efficient, but much cheaper. The money you save buying a 120 volt fridge can be used to buy more solar panels or battery or whatever. However, 120 volt fridges need a large inverter to deal with their startup surge, and a large inverter has a large tare consumption. If the only reason you have a large inverter is for the fridge, you may be better off with the DC fridge. If you have a large inverter for a deep well pump, or microwave, or washing machine or whatever, you would IMO be better off with a 120 volt fridge.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 218 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Welcome to the forums.
I would highly recommend using 24V system.
• Solar Expert Posts: 139 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Information pack post, I love it. Quick question, tiny houses are normally not "code" compliant, ranging from built on wheels to built in trees. Is being up to "code" a factor here? I ask because it may impact the advice you get from others.

You want to use 1,600 watts a day and you have no equipment yet from what I understand. If it were me, the very first thing I would do is go up to 24v panels, it makes more sense cost per watt, no matter how you look at it, that will get you more power either by having more panels or using the money saved for an MPPT charge controller which will be more efficient. To make things really simple and future proof, I would go with four 24v panels (800-1,200 watts total) in series/parallel for 48v, an MPPT charge controller and a 24v battery bank. This will ensure you have enough power and you get the lower cost per watt of 24v panels and the smaller wiring of a 24v battery bank. The inverters also tend to like converting 24v to 120v more.

The only other thing I would mention is that sometimes energy star appliances use far, far less power than DC things, so I wouldn't be scared of putting an inverter to use as it may more than make up for the 90% efficiency.
• Solar Expert Posts: 1,571 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Hi Nick,

I think you did well working out the system size using your first method, based on sizing the battery with Wh/day and replacing those Wh/day with the solar array. Personally I don't like the rate of charge method because it's a one size fits all answer that would give someone in texas and someone in alaska the same array size.

Some things to bare in mind though:

- 5 sun hours/day is too optimistic, try and consult one of the many tables out there that shows the full sun hours for your area

- You could adjust your calculations so that they're based on a bad solar day and include inefficiencies for charging/discharging. The battery bank size would have to increase to deal with the inverter inefficiency, and the solar array size would have to increase based on the roughly 20% extra charging inefficiency.

- Because of the variation in solar power throughout the year, there is no single correct answer for the array size. If it's a bit small, it means you'll be relying on the generator more and if it's a bit too big you'll have spent more money on panels that'll take a long time to pay for itself in saved generator fuel. E.g. if you size the array based on a bad winter day with full 1.6kWh/day consumption at night and flat batteries then you'll need to recharge batts AND provide 1.6kWh/day for the loads. But that type of situation might only occur 5 days of the year... so it's up to you whether you want to be able to ride those 5 days out using purely solar or turn on the generator.
• Registered Users Posts: 10 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Thanks for the swift replies, everyone: they are very helpful and much appreciated.

I'm encouraged that I'm on the right track, and think I was perhaps getting too caught up in the differences between the alternative ways to size a system rather than in their similarities. It seems that as long as the battery bank is sized appropriately and the solar array can provide enough power to charge it, then the details of how large the array needs to be comes down to personal preference, comfort with uncertainty/redundancy, cost, etc.

To answer TucsonAZ's tiny house question by way of providing some context: My wife and I are building a 20'x8' house on a travel trailer. (So yeah, definitely tiny!) We will not, therefore, be legally subject to the building code. However, it's only sensible to comply with the safety aspects of the code wherever possible. We are building the house near San Francisco, and will park it somewhere (yet to be determined) in Sonoma. Even though it's on wheels, we wouldn't anticipate moving it more frequently than once a year. It will, however, need to be moveable.

One of the great things is that since we are building from scratch, we can design everything however we like. That does also bring in a certain paralysis of choice, though. Which brings me to the next area on which I need advice, please.

A few people have commented on our consideration of DC appliances, and have brought up the energy cost of inverters. Without going into unnecessary detail, our needs are roughly:

"Must" be AC:
- Charging 2 laptops: measured (killawatt) @ 130 watts total draw
- Charging phones, iPad, bike lights, etc: measured @ 55 watts (though they are never going to be all charging at once)
- Wi-Fi router: 25 watts
- On-demand gas water heater ignition: 57 watts

- There are a few other appliances that will occasionally be used (curling irons, shaver, etc) but nothing like a toaster or vacuum cleaner or blender that has a large draw.

"Must" be DC:
- Small on-demand water pump: avg 50 watts draw (seems more usual to measure DC loads in amps, but in order to keep comparisons easier I've converted to watts). I say this must be DC because I think that is a lot more efficient than an AC version.
- Toilet fan: negligible (2 amp-hours over 24 hours)
- Motion-activated security light/s at night: negligible (not sure how much power)

Could be AC or DC:
- fridge
- lights
- kitchen/bathroom vent/fan

The fridge we are looking at is a Novakool R3800, which is extremely energy-efficient and just the right size for us. The price is high, but within budget. They have 12-volt, 24-volt, and AC/DC versions. I don't know that there is a significant difference in price or efficiency between these models.

So: It seems to make sense to me to go with a DC fridge, DC LED lighting, and a DC vent/fan. That way our AC draw is minimal, meaning we can get away with a small inverter and avoid large idling losses. (The Morningstar 300 watt inverter may be too close to the bone; I haven't looked yet but there must be some that are <1000 watts.) I know that larger inverters like the Magnum 2000 do have a search mode, but our router will be on for 12+ hours each day anyway . . .

We would wire lights, pump, fridge, vent/fan as DC, and everything else (basically charging for laptops and phones, plus the water-heater ignition) as AC.

My questions are:

1. By using a small inverter here for our AC, are we cutting things too fine?

2. If we want to expand this system, eg buy a blender or toaster or something, then I presume we can buy an inverter capable of running these loads and just turn it on when needed (leaving the smaller one to cover the constant wi-fi router, the common laptop/phone charging and water heater).

3. Am I making things too complicated? It's easy to decide which appliances should be powered by which inverter, but it seems like I would need to decide which outlets need to be part of the small-inverter circuits, and which part of the (yet to be determined) large-inverter circuit. Would it just be easier to just have a single, 2000-ish watt inverter and size the panels/batteries to incorporate its energy consumption. This seems inefficient and wasteful, to me, given how small our AC loads will be, but it does sound simpler.

4. Another argument for having the larger inverter (perhaps): We will want a small back-up generator (Honda 2000 or 3000, something like that) to bulk charge the batteries in the event of too many bad weather days. The smaller inverters do not have a battery charger built in, so that is something else we would need to size, buy, and wire.

5. I'm convinced by everyone's responses that a 24-volt system is the way forward. We can get the fridge and pump in 24-volt versions; the lights I think would have to be 12-volt. We would need a DC-DC inverter (not sure if that's the right word): are we just shifting inefficiency from one part of the system to another?

Thank you so much in advance for any help and advice. I appreciate your taking the time to read such a long post. Hopefully this will be the last super-long one . . . it just seemed to require a lot of explanation.

Best, Nick.
• Banned Posts: 17,615 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
nick wrote: »
My questions are:

1. By using a small inverter here for our AC, are we cutting things too fine?

No, but you are limiting expansion capability. Given that this application is much like an RV this may not be an issue. As long as the inverter can handle the loads it will be fine.
2. If we want to expand this system, eg buy a blender or toaster or something, then I presume we can buy an inverter capable of running these loads and just turn it on when needed (leaving the smaller one to cover the constant wi-fi router, the common laptop/phone charging and water heater).

This is where things get interesting. A toaster is usually 1200 Watts +/- and then you are suddenly out of the small inverter range. You can install a second, larger inverter and use it only as-needed. Is it worthwhile? That's a choice you have to make. For example:

Morningstar 300 Watt 12 Volt inverter uses 6 Watts running, costs \$240
Samlex 1500 Watt 12 Volt inverter uses 12 Watts running, costs \$481
Samlex 2000 Watt 24 Volt inverter uses 36 Watts running, costs \$803

How often would you need the greater power?
Add stand-alone battery charger at \$100-\$125.

Full size inverter-charger: Outback FX2012T \$1,633 uses 20 Watts running.
3. Am I making things too complicated? It's easy to decide which appliances should be powered by which inverter, but it seems like I would need to decide which outlets need to be part of the small-inverter circuits, and which part of the (yet to be determined) large-inverter circuit. Would it just be easier to just have a single, 2000-ish watt inverter and size the panels/batteries to incorporate its energy consumption. This seems inefficient and wasteful, to me, given how small our AC loads will be, but it does sound simpler.

The question would be are you making things too complicated for you? Can you manage the dual current types and possible two levels of AC available? Or would you be more at ease with everything running off 120 VAC and one inverter, at the expense of some power to keep that unit going all the time? Full-time 20 Watt inverter adds up to 480 Watt hours per day vs. the MS 300 running 24 hours using only 144 Watt hours. On a 12 Volt system that's a capacity difference of around 100 Amp hours. That can be a lot with limited space.
4. Another argument for having the larger inverter (perhaps): We will want a small back-up generator (Honda 2000 or 3000, something like that) to bulk charge the batteries in the event of too many bad weather days. The smaller inverters do not have a battery charger built in, so that is something else we would need to size, buy, and wire.

As per inverter pricing above the stand-alone charger can be much cheaper than a combined unit. You might also consider running the gen for a few minutes to make the toast if necessary, rather than invest in larger inverter and extra capacity to power it.
5. I'm convinced by everyone's responses that a 24-volt system is the way forward. We can get the fridge and pump in 24-volt versions; the lights I think would have to be 12-volt. We would need a DC-DC inverter (not sure if that's the right word): are we just shifting inefficiency from one part of the system to another?

Usually a 24 Volt system is a better choice over 12 Volt. But that may not be he case here. As I said this is like an RV installation, where some 12 Volt devices will be in place. The wire runs will be short so the V-drop issue is negligible, and your AC demands are (hopefully) small enough to not warrant significant investment in powering them. By the same token a DC to DC converter is not terrible inefficient at small Voltage shift and lower current.

I think I would go 12 Volt here. Work out the Watt hour demands first, to see if power storage would be better at higher Voltage. Once you have a number to size the battery bank with it will be easier to pick a system Voltage. A little info here: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power
• Solar Expert Posts: 3,739 ✭✭✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
nick wrote: »
the lights I think would have to be 12-volt. We would need a DC-DC inverter (not sure if that's the right word): are we just shifting inefficiency from one part of the system to another?

If you have a small inverter on 24/7 then why not use 120 volt lighting? You will have better choice and cheaper prices on 120 volt lights and appliances.

Another consideration is variable DC voltages... a 12 volt system may run between 11 and 15.5 volts depending on where you are in the charge/discharge cycle... not all lights and appliances appreciate the variation.

If you run a 24 volt system with 12 volts lights, you need a DC-DC converter. Solar Converters makes high quality ones. They solve the problem of voltage variation, and yes, they shift inefficiency from the AC inverter to the DC-DC converter. And they cost money that you don't really need to spend if you use 120 volt lighting with an inverter that you already have.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Registered Users Posts: 10 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Thank you vtmaps and Cariboocoot for the quick replies. This is really useful.
If you have a small inverter on 24/7 then why not use 120 volt lighting? You will have better choice and cheaper prices on 120 volt lights and appliances.
This makes perfect sense, and I'm not sure why I hadn't thought of this. I also didn't know about variable DC voltages.

This, too, was eye-opening:
You might also consider running the gen for a few minutes to make the toast if necessary, rather than invest in larger inverter and extra capacity to power it.
Is this a practical solution? (I know even less about generators than solar power . . . ) As in, say we wanted to run a higher-draw appliance for a couple of minutes a day, or just every few days, is there a disadvantage (other than fuel) in using a generator to do so? For example, frequent small runs wearing it out quicker or something like that?

I think I am erring towards a single, small inverter to run all the AC (including lights, then), and avoiding high draw appliances for now (knowing that we could run them directly via generator if necessary, or buy a dedicated, larger inverter if we would want to use them often). 300 watts is a touch too small; 1500 watts takes us into the higher running costs I'm trying to avoid.

Any recommendations for a quality inverter in the 500 to 1000 watts range, please? (No need for built-in battery charger; seems to make sense to buy that separately.) And I guess these small ones are only available for 12-volt battery banks, which might make my battery voltage decision for me.

Thanks again!
• Banned Posts: 17,615 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

You can get a small inverter-generator like a Honda EU2000i for roughly \$1,200. It will use a very small amount of fuel because it can adjust its throttle to suit the load; does not have to run at a fixed RPM to maintain Voltage and frequency. Compare that burning a litre of gasoline in a couple of hours every now and then to adding more battery, PV, and inverter power just for those occasions when you need >1kW of AC.

I've got two of these little Hondas, a 1000 and a 2000, and they have both now got over 6,000 hours on them with no failure. I call that dependable.

For small inverters the Samlex line is good quality for the price, although they have fairly high consumption. Xantrex's Prosine series is good too, if you can find one (not sure they are still made - could be a Canada distribution thing). Either of these come in 12 or 24 Volt. The little Morningstar is, alas, only 12 Volt.
• Solar Expert Posts: 140 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
nick wrote: »
Any recommendations for a quality inverter in the 500 to 1000 watts range, please? (No need for built-in battery charger; seems to make sense to buy that separately.) And I guess these small ones are only available for 12-volt battery banks, which might make my battery voltage decision for me.

Thanks again!

Besides the inverters Marc mentioned, another inverter line to look at is the Exeltech series, which includes both 12V and 24V models. One nice thing about them is that they include a front-mounted on/off switch, so that you can eliminate the overhead power consumption when you're sure that you don't need AC power. They also feature 2 15A 120VAC outlets on the front plate, which means that you aren't required to wire them into your existing AC system.
• Registered Users Posts: 10 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Thanks! Checking out the Samlex and Exeltech inverters at the minute.

Last question for now (need to fiddle with some numbers a bit and confer with the wife): Given that the Samlex 1000K-24 seems to use 0.75A (so 18 watts, right?) and the Outback FX2524T uses ~ 20 watts, would there be any disadvantage to going with the Outback rather than the Samlex (other than the huge difference in up front cost)? Is there any practical difference, say, in pulling a 500 watt load from a 1000-watt inverter vs from a 2500-watt inverter?

Thanks so much for all your time. I have lots to digest!

Best, Nick.
• Solar Expert Posts: 3,739 ✭✭✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
nick wrote: »
Any recommendations for a quality inverter in the 500 to 1000 watts range, please? (No need for built-in battery charger; seems to make sense to buy that separately.

I have my eye on the Victron Phoenix 24 volt inverter... they make a 700 watt and a 1000 watt pure sine inverter, both with an incredibly low 5 watt tare loss. They are sold by marine suppliers, have great reputations, and are listed for marine use.

The ONLY reason I haven't purchased one is because they are NOT listed for permanent hard wiring into a residential electrical system... they have built in outlets.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 3,739 ✭✭✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
nick wrote: »
Given that the Samlex 1000K-24 seems to use 0.75A (so 18 watts, right?) and the Outback FX2524T uses ~ 20 watts, would there be any disadvantage to going with the Outback rather than the Samlex (other than the huge difference in up front cost)? Is there any practical difference, say, in pulling a 500 watt load from a 1000-watt inverter vs from a 2500-watt inverter?

Don't forget that the Outback has a built in battery charger and transfer switch. When you turn on the generator (or plug into shore power) the Outback will automatically pass through the generator power to your AC loads and at the same time switch from inverter mode to charger mode.

If you buy the Outback, you will also need to buy its remote control, the 'Mate'. There is a very good reason why the outback controls are not built into the inverter... these inverters can be stacked for high power systems. If you were buying four of the inverters, you wouldn't want to pay for four sets of controls.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Banned Posts: 17,615 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
nick wrote: »
Thanks! Checking out the Samlex and Exeltech inverters at the minute.

Last question for now (need to fiddle with some numbers a bit and confer with the wife): Given that the Samlex 1000K-24 seems to use 0.75A (so 18 watts, right?) and the Outback FX2524T uses ~ 20 watts, would there be any disadvantage to going with the Outback rather than the Samlex (other than the huge difference in up front cost)? Is there any practical difference, say, in pulling a 500 watt load from a 1000-watt inverter vs from a 2500-watt inverter?

Thanks so much for all your time. I have lots to digest!

Best, Nick.

A 2500 Watt inverter will be 'loafing' with a 500 Watt load: running at 20% capacity. The 1000 Watt inverter will be running at 50% capacity. Working harder = more heat which is usually something to avoid. But will you really be pulling 500 Watts continuously? Probably not: my constant load is <200 Watts and only occasionally is it spiked to 1kW or more. The inverter is over-sized because not everyone around here understands "do not turn all these things on at once" and certainly wouldn't be able to reset if it faulted. :roll:
• Registered Users Posts: 10 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

500 watts was a number I pulled arbitrarily for the purposes of that question. Our AC load will normally be more like 150 watts (couple of lights, charging a couple of laptops).

And that, really, seems to be the heart of the game: Do we use a larger inverter, with the corresponding power suck, just so we can run a high-consumption appliance for a few minutes each day? Alternatively, do we stick 'em on a separate inverter / use the gen / do without them? Needs pondering. (And that's kind of what I meant by the paralysis of choice, before: If we already had legacy equipment in play it would make decisions a lot easier.)

So the inverter coasting is ok, then? Sounds like you are normally pulling less than 200 watts from something capable of providing 3,500 . . .
• Registered Users Posts: 10 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
vtmaps wrote: »
I have my eye on the Victron Phoenix 24 volt inverter... they make a 700 watt and a 1000 watt pure sine inverter, both with an incredibly low 5 watt tare loss.
Thanks — I'll check these out!
• Solar Expert Posts: 252 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Nick,
I saw that you mentioned heating water a few replies back.

You will NOT want to do this with your system. I should also mention they do make cheap AC to DC converters for your small DC loads.

At first I had the same plan you now do. I found out that going with 120AC is much better in the long run and any Fridge or Freezer with a Danfos compressor will fit into your plan nicely. We love our Danby fridge and also have the Danby washer and dryer. They work great.

We use a 12V DC On Demand propane heater for showers and such. The only other DC load is a 12V fan in the composting toilet. One has a dedicated battery the other is running on converted 120V ac.
• Registered Users Posts: 10 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Thanks for your response. I should clarify: We are not using an electric water heater. It's an on-demand propane system like yours. The electric ignition uses 50 watts and is AC only.

What is the energy consumption of your Danby fridge? I couldn't find it on their site though I only looked quickly. I'm still leaning towards a novakool; the 12-volt model pulls 2.2 amps (according to their specs, anyway) so I guess the 24-volt model is half that. And that way, we'd have no need to run an inverter 24-7.
• Solar Expert Posts: 252 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Is that 2.2 amps dc? if so that about 20 amps AC if I'm not mistaken.

The Danby https://www.danby.com/en/US/our_products/refrigeration/dff261bsldb we use pulls between 7 and 12 amps AC per cycle, according to the Trimetric. It also cycles about 1/3 of the time from the previous fridge. The old one pulled 30+ amps and cycled about every ten minutes. I think we get three cycles in two hours now. They make even smaller ones that fit under a counter. One of those may fit better in your travel trailer. The Danby can be bit pricey, but we love the performance and the energy savings.

I hope you keep us updated on your progress and wish you all the best in your endevours.
• Solar Expert Posts: 3,739 ✭✭✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!
Is that 2.2 amps dc? if so that about 20 amps AC if I'm not mistaken.

The Danby we use pulls between 7 and 12 amps AC per cycle,

2.2 amps DC at 12 volts is closer to 0.2 amps AC than 20 amps AC.

If your Danby draws 12 amps AC, it must be the model with a built in toaster... 12 amps AC at 120 volts is about 1440 watts.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 252 ✭✭
Re: Sizing 1600 watt-hr array: sanity check needed please!

Thanks for the clarification. I see I got it all backwards. The Trimetric shows 7-12amps DC per cycle.