With all the hubbub around the falling cost of solar power, I thought I’d look into it. Since there are hidden cost structures for those places that will install systems and rent it to you or no upfront cost but it gets incorporated into you electric bill or whatever other sleight-of-finances they think up, I’m doing this investigation on parts purchase alone.
And this is for the system I would want – full battery so it will work when the grid is down (yes, there are solar power setups that stop working when the grid is down, check that before signing up), no connection to the grid (avoids billing and meter issues, plus simpler and safer), and enough to power the fridge and freezer so we don’t lose any food (not trying to power the whole house).
Load Requirements
We have 3 different fridge/freezer appliances, and I added up the draw on each and the result was 600 watts.
Assuming each might run for about 8 hours a day: 600 watts * 8 hours = 4800 Wh, or 4.8kWh per day.
That’s the draw requirements, now on to solar panel sizing.
Panel Sizing
I need 4800 Wh per day. We don’t have the sunniest location, so I’m being generous by saying I expect 6 hours of sun a day. That’s to keep cost down, not for a decent buffer of solar power. This is going to be a minimally-sized system.
4800 Wh / 6 hours of sun = 800 watts. So I need solar panels that will output 800 watts.
That’s the panel requirements, now on to battery sizing.
Battery Sizing
There’s a balance between battery voltage and inverter availability and there are a few different ways to go here.
In general, 12-volt batteries are cheapest and easiest to obtain. And 12-volt power inverters are cheapest and easiest to obtain.
But 12-volt systems aren’t good for higher power output. They’re fine for a camping fridge or charging phones, but not for houses. You could do it, but you’d have some thick cables to handle the current.
In general, 48-volt inverters are good for houses. But 48-volt batteries are not cheap nor plentiful. You could make a 48-volt battery out of four 12-volt batteries, but then you have to worry about keeping them all balanced. If one battery gets low or goes bad, then it’ll take the others down too.
For my plans, I’m choosing the middle ground – a 24-volt system. It can handle higher loads with moderately-sized cables, and for battery balancing you need to manage pairs of batteries, not quads of batteries.
So I have 4800 kWh per day, and a 24-volt battery system (note: pick solar panels that can make a 24-volt output), so I need to size my batteries. 4800 kWh / 24 volts = 200 amp-hours per day of usage.
If I were to get 200 amp-hours worth of batteries, I could start with a full charge and go one day without sunlight before my fridge would stop working.
But the thing about batteries is that they don’t like being run all the way down. In general, you want double the battery capacity of your expected daily load. And if you want extra days of power (to survive longer with more cloudy days), you would keep adding capacity.
In my case, I’m going for typical use and not having to ride out a multi-day weather event. So 200 Ah * 2 = 400 Ah. I need to buy 400 Ah-worth of batteries.
Other Items
And I also need a power inverter good for at least 600 watts continuous.
One more thing: I need a charge controller. That goes between the solar panels and the batteries, to make sure batteries are managed well and not overcharged. Bad things happen if the batteries are full, the fridge doesn’t need to run so there’s no load, and it’s sunny so the panels keep pouring electrical charge into the batteries.
Prices
Here’s what I got for prices for those components:
- Inverter: $220
- Charge controller: $175
- Solar panels: $600
- Batteries: $1440
- Total: $2435
That was solar system cost, now on to payback rate.
Conclusion
Around here, electricity is about 10 cents/kWh. And this particular load is 4.8 kWh, so that is $0.48 per day that I’m trying to avoid with this system. That works out to $175 per year.
With a system cost of $2435 / $175 per year, that gives me 13.9 years to start coming out ahead with my system.
In general, the panels and inverter and stuff should last that long, except for the batteries. Which, of course, are the most expensive part of the system. The AGM lead-acid batteries I picked for their cheaper entry price will last about 7 years.
So before the 13.9 year payback period is up, I would have to spend $1440 again for another set of batteries, which bumps the payback period up, and then before that period is up, the second set of batteries would expire, requiring a third set, and then once more before we actually get to a point where the system has paid for itself before the batteries expire.
So really, the total cost is $2435 + ($1440 * 3) = $6755. And that’s a 38.6 year payback period. And then the batteries expire shortly after that anyway and you don’t get to turn much of a profit.
If I do ever setup solar power for my house, I will have to do it without a big battery bank.
They gathered it morning by morning, everyone as much as he would eat; but when the sun became hot, it would melt.
Exodus 16:21
This is Alpha, the first-born, when he was 2YO.
This is Beta, the second-born, when he was about 2YO.
This is Gamma, the third-born, when he was about 18MO.
