Rule of thumb for panel wattage?

[Dzl]

Is there a rule of thumb or standard assumption for determining actual (average real world) watt/hrs supplied to the battery bank from nominal panel wattage (for horizontal mounted panels)??

1. I remember years ago, the conventional wisdom on an expedition vehicle forum I frequented was [nominal panel wattage] x 0.7 (70%) = expected panel output in average clear conditions.
2. I know I've heard Will mention he assumes a ballpark 50% efficiency from panel wattage to actual power delivered to devices (that means accounting for all losses, including inverters, converters, etc, and is not what I'm interested in)
3. I believe AM Solar uses [total array wattage] x 0.9% / V = charging current on output size of MPPT controller (but this also isn't what I'm looking for and assumes perfect panel efficiency I believe).

Obviously any formula would just give a rough estimate of efficiency, and vary depending on temperature, and other factors. But I'm sure there must be a formula that people use to determine how many watts of panels are needed to supply a certain amount of watt/hours, and I would imagine there is a formula that specifically pertains to horizontally mounted panels.

Can anyone enlighten me or point me towards what I'm looking for?

 

[Ped]

5a/100w flat mounted is what I go by.

 

[svetz]

Is there a [way to] determining actual (average real world) watt/hrs supplied to the battery bank from nominal panel wattage...

I use SAM.

 

[Dzl]

I use SAM.

Hey svetz, thanks for the link. What exactly is SAM? From the website it looks like it is an renewable energy modelling software of some sort, is that about right? It may be overkill/over my head, for something as small scale as a vehicle based system.

Most of the modelling systems/calculators I've come across require a lot of specifics before they output useful data. Being vehicle based, its hard to provide a lot of information on latitude, compass direction, temperature, etc. At this early stage in my planning I'm looking for a simpler model to get a ballpark idea. Though when the time comes to deal with specifics I'm sure a more sophisticated system like SAM will prove valuable.

 

[svetz]

SAM is heavy duty, it even takes local weather into account, but it's free and there's no such thing as overkill when you're having fun ?
Once you have the file setup it's pretty easy to change location, panel orientation, etc. There's lot's of ways to ballpark it, SAM should get you close to the "actual (average real world) watt/hrs".

 

[Dzl]

SAM is heavy duty, it even takes local weather into account, but it's free and there's no such thing as overkill when you're having fun ?
Once you have the file setup it's pretty easy to change location, panel orientation, etc. There's lot's of ways to ballpark it, SAM should get you close to the "actual (average real world) watt/hrs".

Thanks Svetz, I'll definitely give it a whirl once I get some of the details dialed in. That sounds very useful for modeling a setup. And I love that its free and open source.

I did just give the PVwatts calculator a try (made by the same lab that makes SAM I believe), but I mustve done something wrong as the calculations seemed way off. A lot of the fields it asks for are pretty particular to residential/commercial installs, not smaller mobile setups.

 

[ghostwriter66]

5a/100w flat mounted is what I go by.

hoping your talking 24V

 

[Dzl]

hoping your talking 24V

5a x 24v = 120 watts
5a x 12v = 60 watts

It seems unlikely that 100w panels are outperforming their rated wattage by 20% after accounting for heat, horizontal mounting, roof temp, wiring, and mppt controller efficiency losses.

 

[Ped]

hoping your talking 24V

12v panels are generally about 18v - ohms law in wire runs - some dirt on panels - inefficiencies - actual charge voltage ~14.6v + mppt boost

Real world 12v flat mounted system 100w = ~5A

 

[Kitemanks]

All ratings are for a specific time considered High noon, and are done in a lab with artificial light. A real world ratio is to bank on getting 3/4 of a panels rating.

 

[Dzl]

All ratings are for a specific time considered High noon, and are done in a lab with artificial light. A real world ratio is to bank on getting 3/4 of a panels rating.

Thanks, this is the type of info I'm looking for. Is this 3/4 assumption based on raw output of the panels (i.e. pre charge controller) or is 3/4 what you generally assume makes it to the battery bank?

 

[Supervstech]

Series vs parallel changes the amount of useful watts you can get during the day as well...

 

[Dzl]

Series vs parallel changes the amount of useful watts you can get during the day as well...

Interesting, which of the two is more efficient and what magnitude of a difference are we talking about?

 

[Supervstech]

Interesting, which of the two is more efficient and what magnitude of a difference are we talking about?

Well, in low light conditions, a 24v panel might only put out 13v so would not send any watts into a 24v battery... however, 3 panels in series all putting out 13v, and you’ve got 39v feeding the mppt... so the batteries begin drinking much earlier, and continue sipping well into twilight.

 

[grizzzman]

77%

 

[Dzl]

Well, in low light conditions, a 24v panel might only put out 13v so would not send any watts into a 24v battery... however, 3 panels in series all putting out 13v, and you’ve got 39v feeding the mppt... so the batteries begin drinking much earlier, and continue sipping well into twilight.

And this is why I love this forum, I'm always learning new things.

 

[Wellydex]

I'm in New Zealand and travel in latitude ranges from 34s to 42s. I work on getting about 5x panel size in Kw/hr per day over summer and 3 x panel size in winter. This has proven to be a pretty reliable average. This is based on reasonably clear conditions, but in NZ we rarely get a day without some clouds. My set up is 6 x 170 w panels flat mounted into a 24v system. Even on an overcast day I'm still getting 1.5 x panel size. Example. With 1000w of panel i get about 5kw/hr of charge per day. I run a 200l compressor fridge, microwave, jug, coffee machine and small washing machine, I have 200 a/hr of lithium at 24v and have never needed to plug into shore supply or use a generator. The lowest I have got after 4 rainy days was 38% soc. I hope this helps. Jon

 

[Dzl]

I'm in New Zealand and travel in latitude ranges from 34s to 42s. I work on getting about 5x panel size in Kw/hr per day over summer and 3 x panel size in winter. This has proven to be a pretty reliable average. This is based on reasonably clear conditions, but in NZ we rarely get a day without some clouds. My set up is 6 x 170 w panels flat mounted into a 24v system. Even on an overcast day I'm still getting 1.5 x panel size. Example. With 1000w of panel i get about 5kw/hr of charge per day. I run a 200l compressor fridge, microwave, jug, coffee machine and small washing machine, I have 200 a/hr of lithium at 24v and have never needed to plug into shore supply or use a generator. The lowest I have got after 4 rainy days was 38% soc. I hope this helps. Jon

It helps very much, thanks Jon!

If I understand correctly, in the conditions you operate in, you estimate your 1000w array produces 5000 watt hours on average in the summer, 3000 watt hrs in the winter, and 1500 watt hours on an overcast day. That's pretty good, especially in the summer. I live/travel in a similar latitude range (35-45 North).

 

[Wellydex]

That's correct. One other point. A lot of people I meet on the road are surprised how much power I'm pulling compared to them as a percentage of panel capacity. As can be seen from picture on my avatar, I have a very clean roof with no AC or tv antennae to shade the panels. I think this makes a big difference. I also clean them often to get rid of bird sh1t etc. I also have them connected with 2 banks of 3 in series. I am convinced the higher voltage gives me better harvest in the first and last few hours of the day.