Energy consumption & generation calculation

[SunDevil]

Originally Posted by Sander

I'm not sure to what extent the choice of batteries and panels affects these numbers. I assume some brands are more efficient than others.

Would it be a good idea to have more battery capacity / AH than I need or is it best to try to balance the consumption, generation and storage?

What happens if my panels generate more power than I can store?
Or if my batteries are underutilized?
Maybe not so important....?

The RV Battery Charging Puzzle « HandyBob's Blog
I haven't closed this link since the last time I suggested it.

There are different types of batteries that matter more than brand. If they are cheap, they probably cut corners somewhere. As for solar panels, look at the type, make sure the numbers are good for what you are trying to do. Efficiency is still about the same, the big players in the market kick out the smaller inefficient panels or if the costs are too high, the market decides pretty quickly.

It is best to have more. Look at the depth of discharge cycle numbers for your battery type. If you are using deep cycle, and will be draining them at night, then charging during the day you might be buying a lot of batteries. If you look into LiFePO4 batteries, you get 10x more cycles, but at 3x-4x the price (I think). If you don't drain the amphours down very much and recharge them to full, you can see many years of service out of a battery. I think underutilizing is good for most batteries, but we have a Battery Minder device on one of our batteries at work and it has been going strong for years and years, but it doesn't have to power much, and is 125Ah.

Usually charge controllers will handle excess power either by disconnecting the panels or connecting a variable high amp load. Or you can probably setup some to do something useful like heat or cool when there is any additional power.

[rebel heart]

Calculating solar is nearly impossible. You will have cloudy days with near zero output and big bright sunny days with ten hours at 100%. Rig as much solar as you reasonably can, and have a backup source (engine, generator, genset).

For us a lot depends on the weather, and whether or not we want to sit around watching movies on the laptop for a few hours. It's really hard to nail that stuff down in advance and the last thing you want to be is the electricity police. We have some rules (no charging AC appliances after the sun goes down), but being able to handle the wild swings in available sun and watts required is the tricky part.

[colemj]

Originally Posted by rebel heart

Calculating solar is nearly impossible. You will have cloudy days with near zero output and big bright sunny days with ten hours at 100%. Rig as much solar as you reasonably can, and have a backup source (engine, generator, genset).

For us a lot depends on the weather, and whether or not we want to sit around watching movies on the laptop for a few hours. It's really hard to nail that stuff down in advance and the last thing you want to be is the electricity police. We have some rules (no charging AC appliances after the sun goes down), but being able to handle the wild swings in available sun and watts required is the tricky part.

Yep, this is the truth. All that calculation and stuff is ballpark at best. Put on as much solar as you can fit, think about wind if you can't do much solar, get a backup source for the off days, and forget about it.

We always find that when it is sunny and the solar is pulled back into float, we don't have any need for the power. However, when it is cloudy, we always seem to have an unplanned acute requirement for heavy power loads. That need never seems to line up with good solar.

In fact, I think the boat knows when it is cloudy and does it on purpose...

Mark

[noelex 77]

Originally Posted by Sander

What happens if my panels generate more power than I can store?
Or if my batteries are underutilized?

With both of the above you, and your batteries, will be happy.

It's not really about storage, however, if all, or most of your power will come from solar.
The batteries are just a means of evening out good /bad days and night/day.
The important numbers are that you generate more power than you use on average. A larger battery bank just means the averaging period can be longer.

[denverd0n]

Originally Posted by Sander

Would it be a good idea to have more battery capacity / AH than I need or is it best to try to balance the consumption, generation and storage?

More than you need doesn't hurt, but it costs money. Do you have more money than you need? Might as well spend it!

Seriously, the whole idea behind coming up with consumption calculations, and then basing battery bank size, and solar arrays on that is so that you don't spend money on more than you really need. It's a trade-off. A little more than you need is probably a good thing. A lot more than you need is just a waste of money.

[westwinds]

Originally Posted by Captain Bill

I would say that you are being overly optimistic on how much power you'll get from the solar panels. I have 300 rated watts and averaged about 55 ah per day over the winter in the Bahamas and about the same this summer in Maine. The reason we did so well in Maine is because we had less clouds than in the Bahamas last winter. Using your formula we should have had about 1275 watt hours available but in actuality we had about 660. Our very best day was in the Bahamas in May when we got 88 ah or 1056 wh. That was a very sunny day with a very high sun angle and few clouds and the wind blowing the right direction so that we had few shadows from the rigging on the panels.

I like these numbers. A solar panel needs to put out 17 volts for a charge controller to work so divide 300 watts by 17 and get 17.6 amps. Figure 5 hours full output in 24 hours as a rule of thumb and get 88 amp hours. Considering cloudy days or high latitude and 55 amp-hours per day seems about right to me. If we go with the power consumption calculations of the OP, then he would need four 300 watt panels.

[Dockhead]

Originally Posted by Sander

Thank you for all the valuable comments and ideas.
I am going to have to fine tune the consumption figures.
I'm not sure to what extent the choice of batteries and panels affects these numbers. I assume some brands are more efficient than others.
Would it be a good idea to have more battery capacity / AH than I need or is it best to try to balance the consumption, generation and storage?
What happens if my panels generate more power than I can store?
Or if my batteries are underutilized?
Maybe not so important....?

The more battery capacity you have, the better, for your electrical life. It means you can soak up and store more power when power is abundant, such as when you are motoring, or hooked up to shore power, or have a great sunny day. More battery capacity also means you can charge more efficiently, since charge acceptance goes down rapidly over the charge cycle.

More battery capacity also means you can more easily smooth out periods of unusually high consumption, as well as soak up unusually high production.

The downside of course is cost and weight. Weight is a big minus on a cat.

I just replaced a whole bank of batteries with Trojan lead-acid ones. But I note that LiFePo technology seems to be right on the cusp of being ready for prime time. Cost is coming down, and there is new battery management equipment. These batts have several times the usable power per kg of mass than lead-acid batteries do, and would really be just the thing for a cat. I'm pretty sure my next battery bank won't be lead-acid. I drool a little thinking about having 500 or 600 amp/hours (at 24v) of lithium batteries -- it would transform electrical life on board. Basically you wouldn't need to think about power at all unless you sit on the hook for more than a couple of days.

[Dockhead]

Originally Posted by denverd0n

More than you need doesn't hurt, but it costs money. Do you have more money than you need? Might as well spend it!

Seriously, the whole idea behind coming up with consumption calculations, and then basing battery bank size, and solar arrays on that is so that you don't spend money on more than you really need. It's a trade-off. A little more than you need is probably a good thing. A lot more than you need is just a waste of money.

I respectfully disagree, where battery capacity is concerned. More is really a lot better, at least until you get into thousands of amp/hours. There are many reasons, just a few:

1. The shallower the discharge, the longer lead acid batteries will last.
2. Especially with solar -- you will always have bad (cloudy) days -- more battery capacity makes it easier to get through these days.
3. Larger battery bank is charged more efficiently, so you capture more power when power is abundant -- acceptance rate goes down steeply during the charging cycle, and twice the battery capacity means twice the acceptance rate at any given point on the curve.
4. Less Peukert effect when you are running heavy loads.

[noelex 77]

The ideal battery capacity depends on a number of factors, but time away from shore power is one of the greatest factors.
As this varies with different styles of crusing there is, understandably, a big variation in recommended battery capicity.

If we restrict our discussion to boats without generators who produce most power from solar.

For example if we take a boat that connects up to shore power every 3 days. Each extra 100AHrs in lead acid battery capicity give s an extra 16.7 Ahrs that can be spent ( assuming 50% discharge =50AHrs/3 ).
A boat in this situation may strongly recommend a large battery bank. If we take the difference between a small battery bank of say 300AHrs compared to a large battery bank of 700AHrs. The extra 400AHrs means the boat with the larger battery bank has almost an extra 67AHrs that can be used each day.

However for boats that spend much longer away from shore power the advantage of a large battery bank rapidly diminishes. If you spend a week away from shore power the larger bank only provides an extra 28AHrs.For a boat like myself that spends 2 years away from shore power, no battery bank, no matter how big can store up any worthwhile amount of shore power.

You do need to provide a battery bank large enough to even out good and bad days as well as low and high consumption days, but in many climates a relatively small battery bank does this adequately.
No battery bank will even out summer and winter.

Batteries have a finite life even if their discharge is kept shallow. If you have access to shore power frequently a larger battery bank makes sense, but if you are planning on living away from shore power a maximium investment in solar panels and energy reduction is generally more satisfactory and cost effective. Only when these have been maxed out does it make sense to go or a large battery bank. Even then the end result is frequently more expensive, per year, with only a small gain in usability.

(Note for boats running generators, or large amounts of motoring the advice is very different, here a large battery bank can be very beneficial, even for those boats that spend long periods of time away fom shore power.)

[denverd0n]

Originally Posted by Dockhead

I respectfully disagree, where battery capacity is concerned. More is really a lot better, at least until you get into thousands of amp/hours.

I think we're really saying the same thing. It's a trade-off. No point putting 1,200 KWH worth of batteries on a Catalina 22. Probably need more than a couple of AA rechargeables, though. Somewhere in between is the number where you have enough to serve your needs, without wasting money on excess.

The whole point of going through all of the consumption calculations is so that you can understand where the right "in between" point is for you.

And, yes, of course you need to take into account the fact that deeply discharged batteries don't last as long as more shallowly discharged ones. That, too, is part of the trade-off.