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diy solar

Matt's simple off grid rule of thumb to power a AC load via battery bank using solar power.

Mattb4

Solar Wizard
Joined
Jul 15, 2022
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Location
NW AR
Strictly off grid: Given a specific load in watt-hours per day. Size your inverter to handle double the running watts plus surge current. Size your battery capacity to handle the loads watt-hours per day and double it for Lithium based batteries and triple it for Lead acid. Size your solar system to provide sufficient wattage to recharge the battery in a day + supply all the loads daily watt-hours, then multiply by 2.

Example: 1200wh daily load of a freezer that pulls 5 amps when running compressor.
Inverter size: Load is 5a x 120vac = 600 watts running > Inverter minimum would be 1200 watts.
Battery size: 1200wh X 2 for lithium would be a 2400wh battery, For lead acid X 3 would be 3600wh
Solar panels: Daily watt-hours of load and battery(1200wh + 1200wh to recharge battery = 2400wh) divide by average hours of sun for your location, lets say 5 hours, to get watts worth of panels X 2 (2400/5 *2) = 960 watts worth of panels

Now toss in a small gas generator when the weather refuses to co-operate.

If someone has a better and more accurate way to figure this please comment.
 
No question the daily average for watt-hr produced is going to be location dependent. Other factors that can effect it are panel orientation and mounting distances from controls/chargers.
 
My designed system for my house.
Daily loads x 2 for inverter
x 3 for solar
x 7 for storage
 
Design system for minimum cost/kwh over a ten year period.

This is generally achieved by minimising battery size and incorporating regular generator use.

It is significantly different to design a system that uses <5kwh/day to one that uses >15kwh/day.

For the latter, i’ve found that sizing your batteries at 150% of maximum overnight use yields the lowest cost.

If you can get PV cheap (it’s virtually free second hand in Australia) - put up as much as you can. Otherwise run the numbers on ten year cost of PV vs diesel cost for equivalent power from generator.
 
No question the daily average for watt-hr produced is going to be location dependent. Other factors that can effect it are panel orientation and mounting distances from controls/chargers.
For people who are totally off grid your and Tim's calcs are good. But if you have a hybrid inverter and utility power already, it is upfront and daily less expensive to size your system closer to your actual use. I mean adding 10 extra panels and a second inverter just to cover high usage days or cloudy days, to me is a waste of money. I'll pay the man for 10% of my usage, rather than invest an extra 3 or 4 grand upfront. of course I live in California off the coast where we rarely have cloudy days anymore.
 
No question the ability to use AC in addition to solar changes the calculation. Being totally off grid means you need to size for worse case scenario since you do not have the utility backup. A gas generator can serve in this capacity but you must find and pay for fuel.

It is hard to size an off grid solar system big enough to cover loads without over sizing it to cover for lack of sun. An oversized system means a lot of time and money for more panels and battery storage. It than means potential power not being utilized when the sun is available.
 
No question the ability to use AC in addition to solar changes the calculation. Being totally off grid means you need to size for worse case scenario since you do not have the utility backup. A gas generator can serve in this capacity but you must find and pay for fuel.

It is hard to size an off grid solar system big enough to cover loads without over sizing it to cover for lack of sun. An oversized system means a lot of time and money for more panels and battery storage. It than means potential power not being utilized when the sun is available.
battery storage is the biggest expense. my pge bill says I used 10kwh a day last year. I have two 5k batteries and they cover my nighttime usage most days. I've had this system up for a couple months, so we'll see what winter brings
 
Strictly off grid:

Now toss in a small gas generator when the weather refuses to co-operate.
I have an old Honda EX650 (650W) camping generator. It has 2 DC terminals, meant for charging batteries. Would connecting this generator to my inverter be prudent? (I just realized the voltage is wrong)
 
12v dc outlets on small generators are usually ~4a. They're small enough to not wreck a lead battery by overcharging it, so they can save money by having NO controls on the circuit.

It is next to useless, unfortunately. It is still a ~50w power source, but given the lack of adjustability or control it would generally be more prudent to power a more customizable DC charger/power supply from the AC side of the generator, than to use the 12v outlet in addition to some other AC device. It is definitely NEVER cost effective to run the generator for ONLY that outlet unless it's in case of emergency (using a pull start generator to charge a dead start battery on the vehicle you rode in on, when that's all you've got..). Even then i have found that using it to charge a discharged battery it will constantly trip its manual reset thermal breaker and require you to sit there babysitting it. So it is even hard to use in the case of discharged batteries!
 
12v dc outlets on small generators are usually ~4a. They're small enough to not wreck a lead battery by overcharging it, so they can save money by having NO controls on the circuit.

It is next to useless, unfortunately. It is still a ~50w power source, but given the lack of adjustability or control it would generally be more prudent to power a more customizable DC charger/power supply from the AC side of the generator, than to use the 12v outlet in addition to some other AC device. It is definitely NEVER cost effective to run the generator for ONLY that outlet unless it's in case of emergency (using a pull start generator to charge a dead start battery on the vehicle you rode in on, when that's all you've got..). Even then i have found that using it to charge a discharged battery it will constantly trip its manual reset thermal breaker and require you to sit there babysitting it. So it is even hard to use in the case of discharged batteries!
So, use the generator as a source of emergency AC power. It's probably bad waveform power.
 
Most small non inverter generators have a fairly good AC waveform with low HD. Where they lack is governor control for changing loads. Dump a big load on a generator and it takes a bit for the mechanical governor to open allowing more fuel and thus bring voltage and frequency back into spec. Turn off a big load and the governor has to back off which also causes fluctuations.*

Bigger generators have more inertia going for them and electronic controlled governors that can anticipate load changes.

* Honda standard generators have rather sophisticated voltage control that compensate for load changes. You will see labels like IAVR (intellegent automatic voltage regulation), DAVR (digital automatic voltage regulation)
 
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