Standby Inverters PDF Print E-mail
Written by Graham Gillett   
Friday, 23 April 2010 17:35

There has been a lot of hype recently about alternative energy sources, especially with the Eskom load shedding (long since forgotten but about to start again), but most people do not know the basics behind this subject and what I need to do here briefly, and without too much technical jargon (that most suppliers use but no-one understands), is to explain these basics, and the pros and cons. The subject of inverters is wide and perhaps beyond the scope of this introductory article.

 

The power that we are used to forms the heart of daily life, these the funny little electrons that are supplied by the power utility company, and that you can't see, but you can as sure feel the presence of them :) The utility supply is normally 230Vac in single phase.

 

Energy Sources:

There are many ways of generating your own power - solar panels (photovoltaic or PV), diesel generators, wind turbines, water based generators etc. but if you have a suitable supply of batteries and wish to generate 230Vac from this source then you need an inverter or UPS (uninterruptible power supply).

 

Inverters take a standard DC input from a battery or bank of batteries and convert this low voltage DC into AC power that you can use to run your application. There are however many pitfalls into thinking that this is an easy source of energy, not least that inverters (decent ones anyway) are expensive, as are batteries, but do at least offer an alternative to the utility power when it is interrupted.

 

As with anything else in life "you get what you pay for" and the subject of inverters is no different. Should you decide to buy a cheap inverter you will get a cheap, or non-performing, product. Spending that extra money on the product will benefit you in the long run. My workshop makes lots of money fixing poor quality and badly designed units, not sold by ourselves I might add, so beware of the "after sales service", or the lack of it. There are far too many companies that sell inverters, when load shedding hits, to make a quick buck but have no idea of what they have sold you or have any form of workshop facilities to repair the unit when it fails

 

The technology of inverters is basically split into two types of technology - that of "stepped or modified sine wave" or "pure sinewave". The choice is up to you but a pure sinewave machine will cost at least twice as much as the modified sinewave unit, but in 95% of cases spending that extra cash on a sinewave machine is not required. There are however many derivatives of the modified sinewave solution - from the cheap product from the Far East to the expensive, but ultra reliable, product from Europe.

 

Power ratings are normally given in WATTS and this is a measure of exactly what can be used on the output from your product. It seems that a product marked 2200 watts will run a load of 2200 watts but this is not always the case. Cheapies will probably get very hot if you run at 100% load and so, in general anyway, it is never a good idea to run any product at 100% load. Use a “rule of thumb” factor of 80-900-90% for all inverter products, thus is you have a 2000 watt load try and go for a 2500 watt inverter.

 

To get an idea of your load requirement simply add up all the current rating of the equipment loads, but remember that the rating plate on the equipment is usually a conservative value. Try NOT to run heaters and any other energy-inefficient equipment as this will add major money to your budget requirement. Keep it to lights and controllers, even small pumps.

 

Now we know the load requirement we have to choose the inverter size, using the 80-90% rule.

 

Battery current:

The key issue here is that ALL inverters run at a low efficiency of typically 85% and this will affect your battery sizing dramatically. For all power out there must be power in and in this case the best case is:

 

Power In = Power Out / 0.85

 

 

i.e. 2000 watts of power out equates to 2352 watts of power in

 

 

As an indication a 2000 watt AC load (8.7 amp AC) will draw 196 amps from the battery supply, at a nominal 12Vdc, and 217 amps when the batteries are going flat (10.8 Vdc). If one looks at the typical discharge characteristics of say a Delco 105A/hr battery you will see that this battery will provide a discharge current of 200amps for 6-minutes, and this is probably not what you were expecting eh ? The solution to this low back-up time (sometimes referred to by the purists as “autonomy time”) is to parallel this battery with others of the same type and manufacturer. Adding say 3 more Delco 105A/hr batteries to make up a bank of four will have the effect of this nominal 200 amp load being shared across each battery thus the discharge current per battery will be 200/4 = 50 amps. If you look at the battery discharge tables now it will state that a load of 50 amps will give you 60-minutes ! Quite a difference, 10 times the back-up time for 4 times the batteries.

 

The next problem:

Whilst you now have a system that will provide your 2000 watt load with power for 45-minutes what happens when the utility power returns to normal, you now have to recharge the batteries, and this is where everyone falls into the bigger pit.

 

ALL batteries need to be recharged correctly, at the right level, else they will fail sooner or later. Here too small a recharge rate is almost as bad as too high a recharge rate. The “rule of thumb” here is that the charge current is the A/Hr capacity / 10 or, in the case of the Delco 105A/hr – 10amps. Remember here that we now have, in this example anyway, 4 batteries in parallel and thus EACH battery requires 10 amps of recharge current – a total of 40 amps. Now find a charger that will handle 12Vdc @ 40 amps and you will find that they are equally as expensive as the inverter was in the first place. Life is never easy is it ?

 

There are suppliers around that will supply you with a high current charger but you must still remember that the A/hr / 10 recharge rate (or C/10 for short) is to be applied for 8-10 hours in order that the batteries are charged to at least a 90% level. If this time is not adhered to the next time the mains utility fails you will probably only get 30-minutes instead of your original 45.

 

The last pitfall:

Please remember that most inverters do not like being turned on when a load is connected and as such turn on the inverter first before applying the load. Severe, and expensive, damage will occur if this procedure is not adhered to.

 

Calculations:

An automatic battery calculator can be found at www.cellcomm.co.za/BatteryCalcs.php that takes into account inverter efficiencies and charging current.

Last Updated on Sunday, 25 April 2010 07:54