Work shops vary in power requirements making the solar selection more of a cautious purchase, of this can be a off set exercise to reduce the costs of your energy bill. lets take a look at solar power in the work shop and see how you may be able to do this and what options there are.
One of the first steps is working out the power you need, its not just a matter of watts but the induction current. Induction current is the power drawn from the hardware, such as a lathe spin up, compressor motor and these are not placed on them, so it wont be that easy to work out, you can get a tool for this.
You can wing it, and triple the wattage which would be about right.
Most modern inverters are HF ( high frequency) which tend to be problematic as they don’t have a “bulk” drop of power available, they have to “spin up” which can cause start up problems with many hardware even Fridges. The inductive loads really do require a LF ( low frequency) inverter to ensure that they have the power to perform the demands you make of your off grid system.
Victron multi plus II is a LF inverter that could solve issues with power delivery, there are a few others available, but it is a limited choice. If you have a lighter load, then a HF inverter will save you money and provide power for most “lighter” or “normal” users.
High demand or large inverters need to have a good power delivery from the battery without adding stress to the battery shortening its life. this can often mean having a “load share” with more than one battery pack. Small batteries, tend to offer a lower output current, to explain this better, lets look at the power delivery.
Battery @ 48v
3kw of AC power is 13 amps AC and 62.5 amps DC.
A small battery ( 5kwh) will be made of 16 105ah cells. and the C rate would be .5 or (51 amps) which is the “rated” test and maximum use testing the battery went thought to get its data.
so you may find the 62 amps drawn from the battery will result in 83ah cell rates in the real world.
To mitigate this we could use 200ah+ cells or we could buy two batteries. Having two batteries would halve the load going from 62.5 to 31.1amps. and while this seems great and simple when we add batteries we need to resistance balance them, typically this means keeping the cables the same length.
It also can mean having to have a bus bar, this battery bus bar allows you to connect more batteries together where the inverter may limit the space to fit more than one cable set. this is not essentially costly, but little things add up with solar.
Bonding – Protection
For off-grid systems, it’s necessary to include an earth rod, while most systems utilize a negative to earth bond. In the UK, we install earth rods and bond the inverter to the earth within the circuits. To protect both users and the system, grounding our solar hardware is essential. You may want to consider AFD (arc fault detection), SPD (surge protection), and ensure all necessary fuses (breakers) are in place.
For those with sensitive devices, consider UPS that are line-interactive or a power monitor that detects faults or abnormalities and shuts down to protect connected devices. The cost range for these varies. It’s also crucial to use the correct type of fuse, such as DC for DC circuits.
In our installations, we err on the side of caution and fault protection, especially if the system deviates from standard norms.
Taking a battery as an example, if we have a load of 62 amps, we would use a bus to inverter fuse rated for 62 amps. However, if we have two batteries, we would fuse each at 31 amps.
This protection strategy addresses several faults, which I will list:
- Battery issues: If one battery fails and goes offline, the other’s breaker will trip, indicating a problem. This also protects against internal battery faults, as a 200 amp fault in one battery will cause the other to discharge as well, safeguarding both the battery and inverter.
- Unnoticed faults: A battery fault can remain undetected while the system is operational. This level of protection will disconnect the system once it reaches the trip threshold.
- Bus to inverter fuse: This should match the inverter’s expected current levels. Typically, a battery is fused between 150 – 250 amps, which corresponds to a significant fault of around 9600 watts; for two batteries, a 19kw fault.
Be aware that the market has numerous counterfeit protection devices, so ensure you’re purchasing the correct and authentic ones.
Grid Charging
Charging using the grid during winter is quite common in the UK. However, there can be issues with some inverters that have an earth bond, as your home also has one. While they may seem similar, an earth fault to neutral is part of your home’s protection system. Therefore, if the inverter has an earth to negative bond, it could be indicated as a fault. It is advisable to purchase an inverter with an earth bond relay, which disconnects the off-grid earth from negative when the grid is connected, allowing you to use the grid’s earthing system. This is why having your own earth rod is necessary, as I mentioned before. Typically, an off-grid inverter will trip your home’s protection system.
Solar array
The power to keep your system running, your battery charged all comes from the sun. while you can use the grid to charge, this is good if you have a lower tariff time where you can charge for less. You will tend to need to make adjustments around the seasons to make the most of solar or the grid, in summer you may not charge at all as the power used overnight may be restored during the day. as winter comes, this will lower and therefore you may add charge until you are making a full night charge with the battery supporting you through the day. you will want to off set to make the most of the solar or your off grid system shuts down production to just cover the load.
Our off grid system as I write this was off solar this morning as the battery starts charging at around 5 in the morning, by 8 when I had gone and checked the system, it was fully charged, I had cut down loads in the evening to allow for a charge to take place as the day wasn’t a full sunny day. Typically your solar will want to produce more than the load and over time ( the day) the solar should charge the battery.
If your battery use was 5kwh and your load is 500W, over 6 hours or longer depending on your system…
we do the math as 5000 / 6 = 884w per hour, + load, gives us a array power of 1.333kw
Solar arrays tend to NOT provide a constant source of power and go up and down. The angle of the panel will also make a seasonal difference and the direction will give a time difference to the actual power produced.
For a roof mount +35% right away. but its important to know that the load and charge differ than a broad concept.
This is what I mean.
if the sun provides the power for the load for 8 hours, you will NOT use 4kw of power in that time.~
if you work from 10 till 5 (7 hours). from 5 to 10 the next morning you may reduce power to say 150w. I will say this is the over night power, but our power use changes so for 17 hours we use 2.5kwh.
From 5 – 8 in the evening we produce 400w of solar power, then in the morning 6-10 we also make 400w, that’s 1.2kwh and 1.6kwh giving us 2.8kwh of power. What will the state of charge be for the battery?
How much you need to cover from the grid changes on the basis of season and therefore the amount of power that you want from the solar will vary, spring and autumn use 4 hours, summer 8 hours and winter 1.5 hours. This is sun hours which are used to measure the power, simply multiply the array size by the hours to work out how much solar you will get at the given time of year.
this is a average, and you can adjust this with the array tilt and angles. follow the link for more information.
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