Help wanted: Designing my off-grid system

Re: Help wanted: Designing my off-grid system

Bmet has it about right about losses...:

• 0.77 solar panel+controller losses * 0.80 flooded battery losses * 0.85 AC inverter losses = 0.52 end to end losses...

Hot solar panels derate by around 0.82, charge controllers by 0.95 (0.77 overall). The battery may do as well as 0.90 (and AGM/Sealed batteries can be 0.90 to 0.98 ). AC inverter, depending on size and how much you use--Inverters have around 6-20 watts "standby losses"--so if your average loads are small, inverter efficiency goes down.

Anyway, back to your needs. Say you want 5kWH per day (3.3 kWH per day would be nice if you can cut some power usage). Batteries are recommended for 1-3 days autonomy--Use 2 days for here. And 50% maximum discharge (for longer battery life):

• 5,000 WH * 1/24 volt battery bank * 1/0.85 inverter eff * 2 days no sun * 1/0.50 max discharge = 980 AH @ 24 volt bank.

Note that this is a fairly large battery bank--You might want to think about a 48 volt system. Depending on your array choices, you can get away with one MPPT charge controller at 48 volts vs upwards of two required for 4 volt bank (roughly 60-80 amp maximum per solar MPPT charge controller).

Anyway, now two ways to calculate array size... One is based on the size of the battery bank. Looking for 5% to 13% rate of charge:

• 980 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,845 watt array minimum
• 980 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 3,691 watt array "nominal"
• 980 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 4,798 watt maximum "cost effective" array

Note that with AGM batteries, you can get down to 5% rate of charge--if not too deeply discharged.

For Flooded Cell, better towards 10% or more. And Crown Forklift batteries, definitely towards 10% to 13% as forklift batteries, when they get old, have higher self discharge (towards 1%+ per day near end of life).

And, we need to look at how big of array you need to power your 5kWH per day loads. Using PV Watts for Belize Intl Airport, with fixed array tilted to ~17 degrees:

Month   Solar Radiation (kWh/m2/day)
1      4.40 
2      5.01 
3      5.62 
4      5.69 
5      5.28 
6      4.93 
7      5.20 
8      5.18 
9      5.12 
10      4.83 
11      4.57 
12      4.41 
Year      5.02

Because of location near equator and weather patterns, you appear to have very little seasonal variations. I will pick your minimum "average" Hours of noontime sun per day as 4.40 hours (January):

• 5,000 WH * 1/0.52 system eff * 1/4.40 hours of sun = 2,185 Watt Array Minimum

So between this calculation and the battery based array calculations, around 3,691 to 4,798 Watt array would be a good recommendation for a flooded cell/forklift based battery system.

You could go down towards 2,185 watt array (based on hours of sun)--But this is an average number. You should not plan on using 100% of your "average" sun per day--Probably closer to 66-75% for "needed" loads (account for a few days of cloudy weather)... If you have a genset and fuel handy, you could cut down on the array size a bit (at the expense of added fuel costs).

Power usage is a highly personal choice, and pretty difficult to know if this is your first installation/home/office off-grid setup.

Your 150 Watts of LED lighting for 8 hours sounds on the high side (that would give me a suntan in my home)... If these include security lights, I would suggest installing motion sensor activated lights. Actually a bit better for security (lights turning on at night attract attention instead of lighting things you don't want stolen) and can save a bunch of power.

Laptops, you can choose small guys that use ~15 watts, medium that take 30 watts, or high power engineering workstations that use upwards of 60 watts or more. Use a Kill-a-Watt meter, or similar, to figure out your needs (plus network, printer, modem, external monitors, etc.).

Fans, any mechanical devices can really use power too... Measure those loads too. Anything that runs 10+ hours per day, even if using small wattage levels, still add up over time (i.e., your lighting+laptops can use more energy "WH/kWH" than a microwave oven or even your refrigerator).

Anyway, hope this helps.

-Bill