[RE-wrenches] Tiny load problem

Wed May 12 10:37:09 PDT 2010

Bob,

The power draw of a system with a 4kW inverter powering a 26W load will just be 26W plus the tare loss of the inverter. The inverter will be damn close to 100% efficient at that load.

Inverter efficiency charts are a bit confusing at the low end. All of them will show efficiencies of under 50% at the low end, but that is just the effect of the inverter's tare losses. For instance, if the inverter draws 20W at full voltage idle (no load, but not sleeping) and you run a 20W load, then you are operating at 40W draw from the battery. This shows up as 50% efficiency on the chart, but in fact, you are only using 20W to power that 20W load (alright...maybe 21W)

The Magnum MS4024-AE has a no load draw of 25W (according to the spec sheet), so with the 26W load, you will draw about 51W, or about 2A from a 24V battery sitting at 25VDC. If you figure a 12hr night, that's 612 Whrs, or about 25AH from the battery. This is quite a bit of power, and if you were in a sunnier area, it would certainly make sense to just add PV. Where you are, it might be more iffy, but I still think that you are better off just adding to the PV array.

Brian Teitelbaum
AEE Solar

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of bob
Sent: Wednesday, May 12, 2010 3:29 AM
To: Allan at positiveenergysolar.com; 'RE-wrenches'
Subject: Re: [RE-wrenches] Tiny load problem

48 volts is a consideration and I have done many, but the problem is still the 26 AC watt load and the inverters efficiency at that light a load. Just hate to burn that much DC to generate that little AC. I am unsure if it is better at 48 volts or not.

Maybe I should not sweat it but it just bugs me. It probably comes from years of looking at "E meters" and playing with loads.

Bob

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan Sindelar
Sent: Wednesday, May 12, 2010 2:09 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Tiny load problem

Bob,
If I step back and look at your question, another bigger question comes up: if you are designing a system with two inverters in order to double charging current, you must have a pretty good sized battery bank. In general, I would suggest that you look at a 48V system, rather than 24V, once it gets to that size. Equivalent charging rate, but smaller wire, lower component amperage requirements and fewer battery strings to get the capacity you're apparently looking for.
Allan Sindelar
Allan at positiveenergysolar.com<mailto:Allan at positiveenergysolar.com>
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com<http://www.positiveenergysolar.com/>

Phil Undercuffler wrote:
If the normal connected loads are within the capacity of a single inverter (which may actually be possible -- I run my whole house off one Outback FX2024, microwave, dishwasher, washing machine and all) then you can just leave one of the inverters in sleep mode or disable output but leave input/charging enabled.

Which inverters are you planning on using for the generator charging?  Depending upon which brand or how sophisticated you want to be, there are a couple ways to skin this cat.  Easiest and cheapest is to simply not connect up any AC output wiring or stacking cables to the second inverter, and program it to stay asleep and charge only when it sees AC input power.

I used to be religious about making sure my inverter was asleep, and even had a second baby inverter to power a handful of full time or sensitive loads.  However, it got difficult trying to explain why you couldn't just plug the vacuum cleaner into just-any-old-outlet, and as my electronic widget count expanded it became less and less advantageous to have that baby inverter.

Phil Undercuffler
Conergy

On Tue, May 11, 2010 at 9:58 AM, bob <reellison at gmail.com<mailto:reellison at gmail.com>> wrote:
I am sizing a system that will have dual inverters to shorten generator charging in the winter and enough solar to charge the rest of the time.
Here's the problem, the inverter will not be able to get into sleep mode at night.

There are 2 cell chargers and a "cpap" machine having to run all night.
It only totals 26 watts, has anyone found a better way to do this without having to keep an inverter awake to produce 26 watts?
I would like to dedicate a Morningstar 300 watt inverter for the job but the system is 24 volts and Morningstar has no plans to make one in 24 volts.

Anyone got any other ideas, or am I best to just deal with it?

Thanks,
Bob

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