Is it better to have an inverter?

medic29 · December 2008

I am new to this. I have a cabin that is located several miles off the main road, typically we have to walk back to it and carry everything; during periods of dry weather if we have access to a 4 wheeled truck we can haul some equipment there, but that is only on rare occasions.

Anyway, there is no electricity close. I do have a 5k generator there that we use on occaision, but getting gasoline there is a challenge. I'm looking at setting up a solar type setup down there for some pretty basic energy needs. We use propane for the stove/oven; I can also use it for some wall mounted lamps. I'd like to have electrical power for a few lights (say maybe 4), for a small radio/CD player, and a fan or two for use during the summer. I could use the generator, but I believe it would be overkill for what I am wanting to run, plus we would want the fans mainly during the night while we are tying to sleep and the generator is far from quiet and I would rather not have it running; this is all besides the fact that it is difficult to get gasoline there.

A friend suggested I look at getting a 50-watt solar panel, 100Ah AGM deep cycle battery, 10 Amp charge controller, and a 500 watt inverter.

So to my initial question, is it better to have an inverter and run everything on 120v AC, or would I make better use of the energy captured by running 12v DC equipment such as lights and fans that would run on the 12v DC? How much energy is lost or utilized by the inverter?

My thoughts at this moment is that we could get the battery and possibly the inverter (or not), the appropriate lights and fans initially and charge the battery initially with the generator, then a short time later get the solar panel and charge controller, but we could go ahead and get all of it at the same time.

Thoughts or comments/suggestions??

Thanks!!!

icarus · December 2008

Re: Is it better to have an inverter?

You situation is not unlike my own.

First off, I think that 50 watts of panel is too small. Spend some time looking at and calculating your reasonable loads. Keep in mind that they WILL grow with time. We live in a 3 room cabin, and run a hybrid system. We have ~200 watts of panel, 400 ah (12vdc) of battery.

The small Morningstar 300 watt sine wave inverter is very efficient, especially at idle. If memory serves it is better than 90% efficient. We use 12 volts for a ceiling paddle fan, fridge ignitor, radio/cd player and a couple of 12vdc lights for when the inverter is not on, and the water pump.

We use 120vac for general lighting, computer charging, small tools, modem and router.

The advantage of 120vac is the availability of a myriad of light bulbs, as well as conventional 120vac hardware. Of course 12vdc has lots of options for fans, radios, light bulbs, pumps etc. Another advantage is running bigger amperage load on smaller wire size.

For the record, we burn ~ 30-40 amp hours a day, ~400 watt hours. That is for lighting, water pumping, computer charging/modem, radio, paddle fan, fridge fan etc. ( We have no TV) Propane fridge.

I use a number of 50% to calculate the size of the panel required for a give amount of load. (Bill uses a bit more scientific calc,,, but mine is faster) For example, taking in all the charging loses, inverter loses etc it works out close to 50%. So a 50 watt panel, would give somewhere around 25 watts of useable power, multiplied by the number of hours of sun you can expect. (I use 4 hours for that). Your efficiency is better if you can use the power directly rather than putting it into, and then out of the battery. I try to pump water, and use the internet and charge the laptops during the day for that reason. So in the case of your 50 watt panel you might expect to get ~100 watt/hours day. You will need to build in a reserve based on how many days you wish to go without needing to run a generator without over drawing your batteries. There are some suggestions that you can draw your battery down to 50% routinely, but I think that only drawing them down 20% leads to better longevity. So in your situation, if your loads are 1/2 of mine,,, (pretty small) say 20 ah/day you would be drawing or battery down 20% each day, so if you wanted to go to 50% you could go ~2.5 days without recharging. (I suggest that you read the following links about batteries:http://www.rpc.com.au/products/batteries/car-deepcycle/carfaq4.htm#charge http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries http://www.batteryfaq.org/

There are formulas here for the size of panel arrays compared to battery banks. Batteries need to be charged at a minimum rate, but not to high a rate.

The long and short of it is spend a bunch of time reading and calculating what our loads are likely to be,, and as I suggest, they will grow with time. Then research here and other places the best system for those needs. I would always suggest that you buy the biggest controller and the biggest battery bank you can afford (and keep charged!) As I wrote in a recent post, systems are hard to "grow into" but you can plan ahead as best you can. As I suggested in that post, the other thing I would do is buy a "good" 4 stage battery charger. Too often people try to get by with an automotive type charger.

Good luck,
Tony

PS Here is a link to a very similar thread: http://forum.solar-electric.com/showthread.php?t=4038 especially posts 8 and 9.

BB. · December 2008

Re: Is it better to have an inverter?

I would also suggest you understand your loads better... Unless your cabin is a three bedroom home with an entrainment center, microwave, and central heat--A 5kW generator is probably way bigger than you need--and not very fuel efficient.

A Honda eu1000i or eu2000i would probably be a much better match to your daily loads (and probably much quieter too)... (I would suggest the eu2000i as it is more versatile and perhaps a bit more fuel efficient at moderate loads). The eu2000i is pretty fuel efficient even down to 400 watt load (~15 hours per 1.1 gallons at 400 watts)--unlike many other portable generator sets.

And, in any case, you should look at getting a kill-a-watt meter so you can see how much power you need so you can plan your generator or solar needs better and avoid an oversized (waste of money) or undersized (disappointing output, possible battery damage).

Kill-A-Watt AC Power Monitor Meter
P4400 Cumulative Killowatt-Hour Monitor
Regular price: $39.00
Sale price: $27.50, 2/$54.00, 1

Kill A Watt P4320 Power Strip
Kill A Watt power monitor strip with surge protection
Regular price: $99.95
Sale price: $78.85

In general solar (and generator) power is expensive (on the order of $1.00 per kWhr)--and you will want to conserve as much as possible to keep your total costs down too.

-Bill

medic29 · December 2008

Re: Is it better to have an inverter?

BB. wrote: »

I would also suggest you understand your loads better... Unless your cabin is a three bedroom home with an entrainment center, microwave, and central heat--A 5kW generator is probably way bigger than you need--and not very fuel efficient.

Hi Bill,

Thanks for the info. The 5k generator is just a portable generator I picked up. It cost me $20, so it seems pretty affordable for me. I have looked at picking up a smaller Honda generator, but the cost for that is pretty significant. Now I've needed this size generator down there for building the cabin; actually I could have used a generator with a little more power, but I'm pretty sure we are over the hump of power usage in the building process.

The cabin is not very big, 400 sq ft, single room. It is twice the size of the one we are replacing. For the most part it will only be used on the weekends, but not every weekend. Occaisionally we may spend a few days to a week down there, but that would only be maybe 5-6 times a year.

With the above in mind where there will not be everyday use on the batteries, say on average 48 hrs out of a week; how much does this change things?

Again, we have been doing without electric power down there in the past and have supplied all of our lighting with kerosene lamps and the wall lamps that run off propane. Since I am re-building the cabin and I would like to spend more time there during the summer, this is why I'm looking at trying to add an electrical power source for the fans and maybe some lighting and radio. There will not be any TV, no computer, no phone, no cell phone, we hand pump out water out of the well, there is an outhouse, so no toilet or running water or anything like that. This is a basic cabin. I'm thinking our energy needs are pretty small.

I do have a Kill-a-watt meter. I haven't taken it to the cabin to see what our energy use is because we have none right now except when we are running the saws, air compressor, etc. when we are building the building, but once it is finished we won't be running those items and they will be brought back home here.

Again, I'm just looking at running a few lights, a radio, and a couple of fans (probably in the windows - I won't be running a ceiling fan I don't believe).

In the future, things may grow and we may want to add more, but it has been this way for the past 50 yrs. The area is pretty isolated and we use it as an escape.

icarus · December 2008

Re: Is it better to have an inverter?

If you are only going to use it 5-6 times a year it is a no brainer.. Buy a Honda EU 1000 for ~$500 It will run for something like 8 hours with a 250 watt load on .6 gallons of fuel. It is so quiet that if you put it away from the house 100' you won't even hear it. If you want to run fans all night, go to NAPA or sears and buy a cheap 12 vdc deep cycle battery, run the fans at night,, even with a small inverter you could run a few lights for a bit at night. You could charge that battery with a Xantrex TC 10 charger. You could then use your 50 watt panel to keep the battery charged while you are not there.

As I said before,, using such a small PV solar system would not be terribly efficient.

Keep the 5 kw to run the shop/tools.

Tony

BB. · December 2008

Re: Is it better to have an inverter?

Regarding your loads... Everything adds up, and motors tend to take a lot more power than a few CFL/LED lamps and a small boom box or old car radio.

A couple 9 watt CFLs and 10 watts for a radio--20-50 watts... A box fan maybe 50-300 watts.

Icarus' / Tony's calculations are just fine... And remember that when we talk about 4 hours of sun per day... That is really 6-12 hours of full sun (no shade) that "averages" out to 4-5 hours of "noon time" sun worth of power a day...

Your location, amount of shading, will all affect your true output.

But, using Tony's numbers--4 hours * 50% * 50 watt panel = 100 Watt*Hours

100 Watt*hours / 50 watt avg load = 2 hours per day
100 watt*hours / (300+40 watt loads) = 0.29 hours or 17 minutes per day

Since a cabin sized solar system costs, very roughly, around $10 to $20 per watt--the difference between a 100 watt system and a 1,000 watt system is huge. You really need to know your loads.

Regarding the 5kW generator vs a Honda eux000i--looking at the amount of fuel you will use--say you save 2 gallons per day... The generator would pay for itself with about 200-400 (actually--probably 100-200 days--given the cost of gas) days of run time (assuming $800-$900 for a eu2000i--don't know the current price for the eu, and don't know the price of gas in your area--also don't know what it is worth to you to pack in less fuel).

In the end, run the math on everything (spread sheet can be handy for what if calculations).

-Bill

mike95490 · December 2008

Re: Is it better to have an inverter?

Just price it out, for a 12V system, 100W of PV, a 300W sine inverter, charge controller and a battery or 2 (2, 6V batteries in series works better than 2, 12V in parallel - same capacity, but better load sharing because of series wiring)

$
430 85W PV http://store.solar-electric.com/so85wa12voso.html
88 12A PWM Charge controller http://store.solar-electric.com/c12.html
or a slightly more efficient
230 15A MPPT Charge controller http://store.solar-electric.com/mosumpsochco.html

250 300W PureSine inverter http://store.solar-electric.com/mosu300wasiw.html
(600W for up to 10 min)

200 pair of 6V, 100AH batteries

50 connection cables & a 40A battery fuse
http://store.solar-electric.com/maxfusholinw.html

So solar would START at $1,018

Cariboocoot · December 2008

Re: Is it better to have an inverter?

Some words from a "been there, done that" guy.

First, it is better to go with the 120VAC (inverter) system than to try and find 12v equipment, which tends to be more expensive. Also, the AC allows longer wire runs to the load with less voltage loss.

My second suggestion would be determine your usage, get an inverter/battery that can handle that, and use your existing generator to re-charge the battery when needed. Then you can add panels/controller when you can afford to: they are expensive and so is a Honda genset (up here in Canada it's much, much worse).

The trick is to plan ahead so you're not buying solar equipment that won't be viable as you expand your usage. That said, you can always put in an entirely separate system later to handle the larger loads (i.e. start out with a inexpensive set-up dedicated to the lights).

The other caution I would add is that certain devices do not like the less-expensive modified sine wave inverters. This would include AC induction motors and some (but not all) audio/video equipment.

Otherwise I'd just say that a 50 watt panel won't produce enough current to properly charge a 100 A/hr battery - you'd need 2 of them at least, depending on how much sun they'll actually get. The 500 watt inverter would definitely handle the few loads you mention, but the 100 A/hr battery might be too little - depending on how long the loads are used - as you don't want to draw it down below 50% discharge at most. If you add more battery capacity, you have to add more recharging capacity. And make sure that battery is a true deep cycle type, unless you can get it cheap and don't mind it failing in a year or two.

icarus · December 2008

Re: Is it better to have an inverter?

I wouldn't use a MSW inverter for anything except light bulbs long term. Some small tool chargers will fry right now when plugged into MSW. The price of one fried tool or component is saved by the purchase of a good SW inverter up front. Someone, sometime who doesn't know will plug something in and poof!.

Also, most small inverters cannot be wired to a building breaker grounded neutral, because they have floating neutrals. The Morningstar PS300 is one exception. (I believe there is a small exeltech that can be wired in as well but I don't remember).

In the long term, a properly designed hybrid/hybrid system works pretty well. (Hybrid/hybrid= genny-inverter/ 120vac-12vdc system). An Iota automatic transfer switch, feeding the main breakers will run the house from the genny. Plug in a battery and an inverter to the transfer switch and they will feed the house. All seamlessly and safely. Add solar panels and a charge controller in the future and they will feed the battery and the inverter,,seamlessly and safely.

If I was doing it again, here is how I would proceed. I would wire the house with 120vac circuits as needed, fed from a conventional breaker panel. I would feed that breaker panel from a transfer switch. I would live with a genny until I could afford/get a battery bank big enough to serve my needs with a bit of room to grow. (remember, adding batteries to an existing string is not a good idea). I would wire 1 dedicated outlet circuit that is fed only from the genny side of the transfer switch, to feed a Xantrex TC charger (Sized for the battery bank). I would then buy a Morningstar 300 sinewave inverter, wire it to the transfer switch. Finally, I would buy the biggest mppt controller I could afford (within reason) and couple it to as many panels as I could afford,,all the while keeping the batteries FULLY charged with the genny and the TC charger.

I would also wire as many 12vdc circuits as I need, radio, fridge ignitor, night lights, pumps, fans etc using a second breaker box (Square D QO breakers)

That is the way that I would build a system that I could "grow into". come to think of it, it describes my system exactly! I built my system originally to get away from the noise, but evolved into not wanting to pay and haul for fuel. Before the days of EU type gennies noise (and fuel use) was a considerable issue. As I stated earlier, the sound issue is SO much better with an EU as is the fuel consumption.

Good luck,

Tony

As has been state here ad infinitum,,,do the math regarding your loads, and then the rest of the equation comes clear pretty easily. I will warn you once again, that no matter what you think, your loads WILL increase with time. Once you have 24/7 lights after having nothing but lamps, the desire for "just one more light" or "just a few more minutes" will take over. We even have a light in fridge! Something unheard of 15 years ago!

Is it better to have an inverter?

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