Formula for Calculating Loads on PV'S & Bat
tcvb
Solar Expert Posts: 44 ✭
Can anyone give me the formulas for calculating my loads i will need to put my system together:
Load for PV'S and Batteries
Thank's
Load for PV'S and Batteries
Thank's
Comments
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Re: Formula for Calculating Loads on PV'S & Bat
This starts with: what do you want to run, and for how long?
And are you planning on a 120 VAC system or a straight 12 VDC system?
Gotta know what you want to get out of it before you can figure out what you need to put in. -
Re: Formula for Calculating Loads on PV'S & Bat
For simple AC plug-in loads... The Kill-a-Watt meter is the way to go. For DC loads, a cumaltive DC Amp*Hour / Watt*Hour meter is very nice to have.
For "simple" DC and AC loads--The basic equations are:- Volts=Current * Resistance (Amps * Ohms)
- Power in Watts = V*I = V^2 / R = I^2 * R
- Work = Power * Time = Watts*Hours (for "our needs")
- Work = 1,000 Watt*Hours = 1 kilo Watt*Hours = 1 kWH
- Amp*Hours => is Watts without the voltage
- Work = Amp*Hours * Volts
- 100 kWhrs per month / 30 = 3.3 kWHrs per day = 3,300 WH per day
- 3,300 WH per day / 24 hours = 138 Watts average
A typical off grid system is (very) roughly 52% efficient at gathering energy based on Name plate rating... So to calculate panel size (by the way, I toss out the low three months of sun--typically winter--as most people are OK with using a generator during bad weather). using 3,300 WH per day (largish off-grid system):- 3,300 WH per day * 1/2 hours of sun per day * 1/0.52 derating = 3,173 worth of solar panels for deep winter
- 3,300 WH per day * 1/5 hours of sun per day * 1/0.52 derating = 1,269 worth of solar panels for most of summer
- 3,300 WH per day * 1/0.85 inv eff * 1/12 volt bank * 3 day * 1/0.50 max disch = 1,941 AH @ 12 volt bank
- 3,300 WH per day * 1/0.85 inv eff * 1/24 volt bank * 3 day * 1/0.50 max disch = 971 AH @ 24 volt bank
- 3,300 WH per day * 1/0.85 inv eff * 1/48 volt bank * 3 day * 1/0.50 max disch = 485 AH @ 48 volt bank
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Formula for Calculating Loads on PV'S & Bat
Hi Bill,
Been working on battery sizing for school. Need about 14kW per day. Need to do 3 days of autonomy.
Keep coming up with 16 Trojan T-105 6 volt in 2-48 volt strings. The example shows twice as many batteries as I get.
What am I missing on the rated usage. Is that considering no power from the panels and a grid failure?
Thanks
Dan -
Re: Formula for Calculating Loads on PV'S & Bat
Dan, we actually use 6x daily average load--3 days of no-sun and 50% maximum discharge = 6x daily load.
Is that the missing factor?
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Formula for Calculating Loads on PV'S & Bat
Thanks Bill,
Guess that's why you got a good sine wave gen-set.
For some reason the Honda sounds way better, anymore nowadays than two tons of dead batteries.
Dan -
Re: Formula for Calculating Loads on PV'S & Bat
Dan,
When I do posts like above--I keep revisiting my own system and wondering about swapping in an XW system... But--I keep coming back to the fact that we may lose power for 1-2 hours every year or two (and maybe a couple few minute outages during the year).
My "big one" is a once in a hundred year earthquake... I keep my loads low--and a small 1,600 watt inverter/genset running a fridge/freezer and a few lights for 10 days on less than 20 gallons of gasoline. And a siphon hose for our cars can give me more--I just cannot justify the costs of a battery bank.
And that stored fuel can be used for cooking or in the car for evacuating the area if needed. And the genset is very portable in an emergency--can't say that for a solar PV off-grid system.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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