The Frustrating Thing about Solar Sizing Calculators that Nobody Talks About

[eric@psmnv]

I've had money burning a hole in my pocket for years as I try to figure out the best system for our off-grid cabin. Various web sites have system sizing calculators, and they all take a similar approach: How many sun-hours do you get per day? What is your daily power requirement? How may days of autonomy do you want? From these answers, they calculate the required wattage of your array and the storage capacity of your battery bank. That's great, but what nobody ever tells you is that the amount of sun you get per day is not enough to charge your bank. If you buy the system based on the calculations, then you will run a daily energy deficit and end up under-charging your bank or running a generator for hours to supplement. Is there an honest system calculator out there that will provide unvarnished, realistic answers about what you really need, taking into account battery bank recharge time? Presumably, depending on how many sun hours you get, you may need more panels just to provide enough energy to recharge your bank in the hours available. And a lot depends on the the type of batteries, as different chemistries recharge at different rates. And what about the risk of overcharging? This is complicated stuff for people like me without the experience or the math skills. A good, all-encompassing calculator would be an awesome find.

[organic farmer]

Something else to consider. Proper maintenance of a battery bank requires an occasional 'equalize' charge.

My charge-controller has a pre-set whereby it can attempt to perform an equalize charge. But its programming prohibits it from ever actually doing an equalize charge. You see, when I command it to do an equalize charge, its first step is the charge the battery up to 100% SOC and hold it there for 24 hours, then only after that step can it increase the voltage to the level required for an equalize charge. So for this to all happen, I would first need about 48 hours of continuous sunlight.

[Ampster]

You also should make a philosophical decision about battery chemistry as an initial assumption for the other system components. There is a significant difference in the charging efficiency between Pb (Lead Acid) batteries and Lithium batteries. That in turn will affect your choice and size of components needed for charging.

[Bala]

If you are honest with the calculators they will likely get you close. Most people understate consumption and overstate sun hrs.

My system recharges to 100% about 350 days a year, so about 15 days where I need generator.

The variables of weather, your consumption, and ageing components make a perfect calculator impossible.

Your best option is education. A good reliable system that is fit for purpose will generally be much more expensive than people anticipate.

[eric@psmnv]

If you are honest with the calculators they will likely get you close. Most people understate consumption and overstate sun hrs.

My system recharges to 100% about 350 days a year, so about 15 days where I need generator.

The variables of weather, your consumption, and ageing components make a perfect calculator impossible.

Your best option is education. A good reliable system that is fit for purpose will generally be much more expensive than people anticipate.

I'm not looking for perfect, just one that factors in battery bank recharge time. I am very honest with the calculators. I use a Kill-a-Watt to measure the draw of my whole house, and I use the low estimates of solar sun time. (Some sites say 3.9 hrs. in winter, others say 4.1 hrs.)

How many hours do you get? Can you describe your system?

[Mike90250]

I'm in northern california. My 5000w of panels are angled for winter, to get the most of the short sun window. Long summer days have no issue
on a cloudy & rainy day. I can harvest 400wh - not much at all. Summer days I'm easily in float by 11am, and lots of excess power to pump water with.
Winter - sun hits the panels about 9am, but no real harvest till about 10am. Then by 2 pm. the western ridgeline puts the panels in shade, and sunset is at 4:30pm

[Bala]

My system is 16 years old. It started with 14 160w panels and a 24v 1800ah fla battery bank. It now has 12 panels as 2 caught fire and 3 years ago I put in a new battery bank of 700ah sealed lead acid. I get approx 60A of charge when the panels are cool and it drops to 40A when they are hot. I budget on 1kwh a day each for the fridge, freezer and general. I have a 10kva lister diesel generator and am not afraid to use it.I live in a wet tropics area with average 5 meters rainfall with good sun position all year. My best production is winter rather than summer. I would normally start making power from 8am and go through until 5pm. True sun hrs would be much less than that.

The ability to recharge in a day is a factored into the calculation from the sun hrs and your use, that is where the panel required comes from. But it will still be a general estimate.

My caravan has a drop in 200Ah lithium battery panelsand a DCDC 40A / MPPT charger, it is a lot more efficient at charging than my lead acid system on the tow ute and my home system.

What sort of daily KWH usage are your working on?

[GeorgeF]

Im just a KISS person. If you daily make it to the end of bulk, ok. Once a week start your genny

[Ampster]

Im just a KISS person. If you daily make it to the end of bulk, ok. Once a week start your genny

I am a KISS person also. One of the things I like about LFP batteries is that I don't even need to worry about making it to the end of Bulk or starting a generator to keep them from sulfating. Just charge from the available sunshine and forget.

[eric@psmnv]

I am a KISS person also. One of the things I like about LFP batteries is that I don't even need to worry about making it to the end of Bulk or starting a generator to keep them from sulfating. Just charge from the available sunshine and forget.

No concerns about under-charging with LFP?

[eric@psmnv]

My system is 16 years old. It started with 14 160w panels and a 24v 1800ah fla battery bank. It now has 12 panels as 2 caught fire and 3 years ago I put in a new battery bank of 700ah sealed lead acid. I get approx 60A of charge when the panels are cool and it drops to 40A when they are hot. I budget on 1kwh a day each for the fridge, freezer and general. I have a 10kva lister diesel generator and am not afraid to use it.I live in a wet tropics area with average 5 meters rainfall with good sun position all year. My best production is winter rather than summer. I would normally start making power from 8am and go through until 5pm. True sun hrs would be much less than that.

The ability to recharge in a day is a factored into the calculation from the sun hrs and your use, that is where the panel required comes from. But it will still be a general estimate.

My caravan has a drop in 200Ah lithium battery panelsand a DCDC 40A / MPPT charger, it is a lot more efficient at charging than my lead acid system on the tow ute and my home system.

What sort of daily KWH usage are your working on?

I need about 3.6 kWh.

[Bala]

I need about 3.6 kWh.

You said it was confusing stuff when you don't have experience or the math skills. If you want a system to work you do need to understand it so do follow up research on what is posted on forums, don't just believe what is written. Every situation is different. You have done well to ask before spending.

So now you have a consumption figure, how and when will it be used. I don't run high wattage appliances, I use lpg for cooking, no electric kettle, coffee machine, no heating or cooling. I only need 1kwh to get through the night and most days are sunny.

How often will you use the cabin, for how long, and do you get conditions below freezing.

When and how you use power and climate makes a difference to system size and battery type.

How much maintenance you want to do. I put in sealed batteries even though they are not recommended as the best return on investment. But now I look at by batteries every now and then rather than weekly.

Then there is budget, I don't really have one as long as it works and I don't have to fuss over it, what is your budget like.

[Ampster]

No concerns about under-charging with LFP?

Not the same concerns as Pb batteries. The Lithium cells do not care.

[eric@psmnv]

So now you have a consumption figure, how and when will it be used. I don't run high wattage appliances, I use lpg for cooking, no electric kettle, coffee machine, no heating or cooling. I only need 1kwh to get through the night and most days are sunny.

How often will you use the cabin, for how long, and do you get conditions below freezing.

When and how you use power and climate makes a difference to system size and battery type.

How much maintenance you want to do. I put in sealed batteries even though they are not recommended as the best return on investment. But now I look at by batteries every now and then rather than weekly.

Then there is budget, I don't really have one as long as it works and I don't have to fuss over it, what is your budget like.

We live in the cabin full-time as our primary residence. We only use electric for lights, TVs, computers, etc. For heating and cooking we use wood or propane. I determined our usage by putting a Kill-A-Watt on the output of the inverter that feeds the house and letting it run for a week. (We have a small solar setup that came with the cabin, but it is inadequate and we often end up running the genny 4-5 hours per day.) It would be awesome to run our well pump off the solar if possible.

We're in the Texoma area with about 4 solar sun-hours per day. We get about 20% more precip than the national average, but we don't have that many cold days and we get about 2" of snow per year, on average, and it only hangs around for a few days.

In terms of budget, I would like to keep it under $15K if possible.

[Bala]

Is that 3.6Kwh you quoted a day or a week?

Will you be doing all this yourself or getting a contractor in?

The well pump, anything is possible but cost of a system to run it depends on the pump specs and runtime. So first find the pump specs.

For batteries in lithium you can have drop in or make a pack. My solar systems are not a hobby so in my caravan I have a drop in that works well. There are in Australia available some really neat large kits that are basically mount and turn key but I prefer DIY component as I can always just change parts if needed. A problem with lithium is charge and discharge in cold weather. My lithium drop in can not be charged when it is below 0 Celcius, but some people manage that with heating when needed. Drop in batteries can range a lot in price and likely quality.

FLA is available in a wide range of price and quality, forgiving of treatment and generally easy to get.

Sealed lead acid. I now have for the house BAE brand sealed batteries. Sealed batteries have not been promoted on this site for solar but I have looked at the specs and I am happy that they will do the job. They were expensive but are doing what I want.

I know Im asking more questions than Im giving answers but all I can do is let you know what you need to know to end up with a system that works. Where I live the coldest night of the year will be12 celcius and then it will still be over 25 celcius the next day. Refrigeration is my biggest power use.