Oversizing solar panels

[william hamilton]

Hi all,

I am currently scopingn out solar panels for my robotic dairy farm which is currently using an average of 110 kWh single phase electricity from the grid daily, with a usage pattern which is relatively even, filled with equipment turning on and off 24/7 (compressors, coolers, water heaters etc).

I am currently planning to have 2 x 5kw inverters which (when should cover most of the equipment, - the grid will have to step when lots of equipment is runnign at once, but 10kw will cover 90% of the time.

As almost 50% of my load is at night, i am hoping to include a potentially 20 kWh+ battery pack to try and supply for the night as well, in order to charge these, i am hoping to massively oversize the solar panels (40-50KW), however i am concerned that the inverters won't be able to take this.

Is there a way for the inverters to only draw max 10kw, but the batteries be charged direclty from the solar panels? as opposed to being charged through the inverters?

 

[n2aws]

Hi all,

I am currently scopingn out solar panels for my robotic dairy farm which is currently using an average of 110 kWh single phase electricity from the grid daily, with a usage pattern which is relatively even, filled with equipment turning on and off 24/7 (compressors, coolers, water heaters etc).

I am currently planning to have 2 x 5kw inverters which (when should cover most of the equipment, - the grid will have to step when lots of equipment is runnign at once, but 10kw will cover 90% of the time.

As almost 50% of my load is at night, i am hoping to include a potentially 20 kWh+ battery pack to try and supply for the night as well, in order to charge these, i am hoping to massively oversize the solar panels (40-50KW), however i am concerned that the inverters won't be able to take this.

Is there a way for the inverters to only draw max 10kw, but the batteries be charged direclty from the solar panels? as opposed to being charged through the inverters?

You can connect the solar panels to "Charge controllers" to charge batteries independently of inverters.

That said, I think you should set your expectations. 110kwh a day, with half of that being at night.. lets assume it's exactly 50%.. that'd be 55kwh at night. your 20kwh battery pack will obviously not suffice. you'd need at least 3x (probably 4x when you factor in losses, and the usable depth of discharge) in batteries if you only want the grid as a backup.

Another thing to consider is, most batteries don't like being charged at over a 1C rating. 40-50kw in panels would be at least 2C, on the proposed 20kwh in batteries.

Depending on where you are, your utility may not allow a net metering agreement with 50kw arrays. I know here, we're allowed up to 10kw, anything beyond that, and we have to have 1million dollars in liability insurance. 50kw puts us in "tier3" as an energy producer with a ton of regulatory control involved.

If I may:
Unless you have a specific need for the batteries, I'd consider just doing grid tied, selling back.. and maybe doing 20kw in the solar arrays. Then buy the energy you need at night when the sun isn't producing.

 

[12VoltInstalls]

there a way for the inverters to only draw max 10kw, but the batteries be charged direclty from the solar panels? as opposed to being charged through the inverters?

No. You are confused about the word ‘inverter.’

You set the programming/menu options of the system up to run solar-battery-utility and disallow grid (utility) charging of . The ‘inverter’ is the wrong work, it sounds like you are describing an All In One device.

You select settings that determine how the unit functions for your situation.

 

[ghostwriter66]

Look at the midnite solar Barcelona .... these will allow you to charge the batteries directly from the panels .... it is a SOLAR CHARGE CONTROLLER only

So technically a SCC charges the battery -- and the INVERTER takes the power from the batteries and turns it into power 120 or 240 ...

But the Barcelona will solve the issues you are having and can take up to 600V DC input ... HUGE ... it runs about 2K

 

[12VoltInstalls]

You can connect the solar panels to "Charge controllers" to charge batteries independently of inverters.

Nit-picking on definitions here, I know, but inverters - unless hybrid inverters- do not charge batteries. A hybrid inverter-charger or AIO can charge batteries if opted in, or as suggested use an independent SCC.

 

[william hamilton]

You can connect the solar panels to "Charge controllers" to charge batteries independently of inverters.

That said, I think you should set your expectations. 110kwh a day, with half of that being at night.. lets assume it's exactly 50%.. that'd be 55kwh at night. your 20kwh battery pack will obviously not suffice. you'd need at least 3x (probably 4x when you factor in losses, and the usable depth of discharge) in batteries if you only want the grid as a backup.

Another thing to consider is, most batteries don't like being charged at over a 1C rating. 40-50kw in panels would be at least 2C, on the proposed 20kwh in batteries.

Depending on where you are, your utility may not allow a net metering agreement with 50kw arrays. I know here, we're allowed up to 10kw, anything beyond that, and we have to have 1million dollars in liability insurance. 50kw puts us in "tier3" as an energy producer with a ton of regulatory control involved.

If I may:
Unless you have a specific need for the batteries, I'd consider just doing grid tied, selling back.. and maybe doing 20kw in the solar arrays. Then buy the energy you need at night when the sun isn't producing.

thanks for response, I’m currently in the process of figuring out whether or not oversized solar and batteries for night are worth it.

Noted on the charging rates however, I live in an area where my panels will only be operating at full capacity for a small portion of the year, panels are significantly cheaper than the batteries, so to me it makes sense to oversize the panels and if that means I can’t use 100% of the power in the sunniest parts of the year then so be it.

With regards to utility arrangements, I don’t intend to have this system exporting to grid. The rate for exporting is small, the max I can export under single phase is 5kw. I have capacity to use all the electricity I produce, and I intend to.

 

[william hamilton]

No. You are confused about the word ‘inverter.’

You set the programming/menu options of the system up to run solar-battery-utility and disallow grid (utility) charging of . The ‘inverter’ is the wrong work, it sounds like you are describing an All In One device.

You select settings that determine how the unit functions for your situation.

Apologies for confusion, I am assuming a hybrid inverter.

 

[12VoltInstalls]

Apologies for confusion, I am assuming a hybrid inverter.

Just set it to grid-charge batteries at a voltage lower than your low-voltage disconnect.

 

[Warpspeed]

Thinking about all this....

The way I would do it would be to start out with a relatively small system, and divert part of my load onto that, and see how it goes.
Your usage pattern, and local climate, will strongly dictate sizing of the individual components.
No amount of research, engineering, or a crystal ball can make up for some practical experience over a full year of actual operation.
You will learn an awful lot in that first year.

With that under your belt, the system can then be expanded in the most efficient and economical way to best match actual needs.
Start out with a decent sized inverter, then add batteries and solar to that, both of which can be upgraded incrementally.
Loads too can readily be transferred across in stages.

 

[HarryN]

One possible way to start is to look at your power supplier and see if they have times of day that have heavier use in general than others. As an example, in this area, our grid has the heaviest demand in the late afternoon (by far ) so electricity pricing varies fairly dramatically depending on the time of day.

Over time, most utilities will be charging more for power during peak demand periods than off peak no matter what the current pricing structure is.

It is also possible that wind might be a useful way to capture some energy more consistently the course of the day than sunlight - sort of don't put all of your eggs in one basket.

 

[william hamilton]

One possible way to start is to look at your power supplier and see if they have times of day that have heavier use in general than others. As an example, in this area, our grid has the heaviest demand in the late afternoon (by far ) so electricity pricing varies fairly dramatically depending on the time of day.

Over time, most utilities will be charging more for power during peak demand periods than off peak no matter what the current pricing structure is.

It is also possible that wind might be a useful way to capture some energy more consistently the course of the day than sunlight - sort of don't put all of your eggs in one basket.

Unfortunately our utility doesn’t vary prices throughout the day, the fees in tarried is a bit brutal “day and night” price!

I’ve spent some time looking at wind, and I’m rejecting it for two reasons

1. Planning - in our area we don’t need planning for rooftop solar whereas for any type of wind we need planning permission, which will end up cumbersome and expensive

2. Cost - whilst large scale wind is one of the cheapest forms of electricity, I can’t see any way that small wind makes financial sense. The cost of the turbine is simply going to be too high.

I’ve briefly looked at micro hydro as were beside a stream but we don’t have enough fall or flow to be useful

 

[HarryN]

It sounds like you have a very good understanding of your needs and how it all works.

In general what you will want to do is try to generate power throughout the day by having panels facing various directions from early morning and through the day to late in the afternoon to both cover the power needed at that time as well as surplus to be charging the battery pack. (s)

There isn't any need to be "perfect" in your estimate but as a rough starting point:
- In a location like you are in, the panels will produce roughly 50% of rating at ideal time of day, and perhaps 1/2 of that in the early morning / late afternoon.
- So one way to do an estimate is

For mid day, to generate 10 kW, plus store another 10 kW

( 20 kW needed ) x (2) ~ 40 kW facing the sun

For early in the morning

( 20 kW needed ) x ( 4 ) ~ 80 kW facing the morning sun

For an array this size, it might be worth considering mounting them on a tracking system.

If you can mount them so that there is at least 3 meters clear under neath the panels, then it might be possible to still use the space underneath for farming if that is interesting. Much lower and it isn't possible to get equipment underneath.


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There are a variety of ways to create a hybrid arrangement so that the grid and / or a generator backs up your setup.

One approach is to simply run the incoming AC power through a charger to convert it to DC and feed the battery pack.

My sense is that you have a very critical operation there and regardless of if you have a solar arrangement or not, in the US we would want to have a generator with at least 24 hours of fuel on site for "when" things go wrong.