Small Off Grid Solar System
There are three key parts to designing your off grid solar system and ensuring the system you select will be right for your needs.
1. The solar panels collect your energy so they should be sized to collect enough energy to cover all of your daily needs. They are rated in Watts (W) or Kilo Watts (kW).
2. The batteries store energy for when there is not enough sunlight. This will generally be over night so your batteries need to be sized to handle all of your energy needs for the whole night - including dusk and dawn. Battery capacity is rated in kWh (kilo Watts times hours)
3. The inverter needs to be sized to meet your peak energy use and to start inductive loads such as electric motors.
The amount of sunlight reaching the ground can vary significantly from 1100W/m2 on a bright, clear, sunny day down to 100W/m2 on very overcast days. The problem is how to manage on the days where there is not much daylight. There are several options:
1. Increase the size of the solar panel array so that it always collects enough solar energy.
2. Increase the battery bank size to provide back up storage.
3. Reduce your energy needs on low light days.
4. Use a generator to supplement your energy needs on low light days.
5. A combination of the above.
A good compromise is to install enough solar panels for the majority of winter and then for the 10 to 15 very overcast days, run a generator to supplement the solar panels.
General recommendations:
If you prefer to not use a generator or to scrimp on low light days then the most cost effective option is to increase the size of your solar array. Increasing the size of the battery bank has a similar price as increasing the solar panel array but has limited extra storage capacity.
Ideally, the battery bank is sized to meet your winter energy needs for the 16 hours between evening, when the panels stop generating power, and early morning when they start generating again. Having 40% capacity in reserve is a good idea for times that you use a little more and to help on days of low light. For example, if your winter night time use is 2kWh then a 2.8kWh battery or more would be suitable.
The biggest task for inverters is starting inductive loads like motors. The starting current for a direct on line (DOL) motor is seven (7) times the normal running current. Switching on a 500W motor will draw up to 3500W for a brief moment.
Most washing machines, pumps and some air-conditioners and vacuum cleaners have inverter motor drives that don't use any extra starting current. These are ideal for using with an off-grid system.
Power tools, vacuums, fridges, pumps and some air conditioners have direct on line starting and so they will consume up to 7 times their rating for a brief moment. Starting these loads is usually what trips an inverter on overload.
A general rule is to size the inverter to run your normal maximum load as well as start the biggest direct on-line motor. for example:
This results in a peek power requirement of 1500 + 500 + 1750 = 3750W. A 2kW (4kW peak) inverter may just be able to do this but you would be better with a 3kW inverter.
If you have one or two large motors that would cause the inverter sizing to be expensive then installing a "soft starter" for these motors may be an effective alternative. These will significantly reduce the starting current of larger motors and are ideal for workshop tools.
When purchasing equipment such as air conditioners or welders, there are inverter options that will have much lower starting current and so will reduce the load on the inverter compared to the non-inverter options.
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