Heyrtgunner, free is a hard price to beat!
I'll be working with my power provider to see exactly what they will be willing to help me with. Fortunately, they're pro solar, and are happy to buy any extra electricity I produce. In fact, they have a big solar array on their headquarters property which the community paid into. Every month, my power bill gets a tiny credit from the power it produces.
I'm not sure if my electronic meter is two-way ready, but they will swap it out if need be. At some point I will have to get together with them and figure out details. I do know they have a list of contractors who do PV installations. I've got tons of reading and preparation to do yet before I'm ready to begin installation though. Hopefully, hybrid systems will be more common by then.
I've been working on energy usage calculations to figure out what kind of battery I will need to run my house. I've got some assumptions that I've had to make in order to figure it all out, so I'll start with those.
- So energy usage is measured (for billing purposes) in kWh, which apparently accumulate over time. I'm assuming that can be expressed as kWh/month.
- I'm also a little confused as to what is energy vs. power. If I understand correctly, Watts are a measurement of power, and Watt-hours are a measurement of energy. Is kWh/month energy or power? Does it even matter?
- I'm also assuming that 1Wh/hour is the same as 1 Watt. Since the hours would cancel each other.
- Lastly, I'm assuming that DC Watts are close enough to AC Watts (orVolt-Amps?) in our application to figure if I use 1 Watt of power at 240VAC, that it's the same as 1 Watt of power at 48VDC.
Assuming all that is true, or close enough, I can translate my power bill into something I can use to design a house battery. So far as I can see, my peak power usage at any one point has been 5,600W. This is from looking back at the data collected from the electronic meter that got installed last year. I figure I'll round that up to 6,000W just to be safe. Drawing 6,000W from a 48V battery would mean drawing 125A.
6,000W / 48V = 125A
Then my maximum Amp draw will be 125A. This is the first critical bit of data I will need for the battery. And also the Voltage of the battery, which I just decided would be 48V, because all the cool people are doing it that way. And I want to be cool too!
Next is the average power usage. This one is a little tricky, because my power bill fluctuates wildly throughout the year. Ultimately, I decided just to average all the monthly totals for the last year together, because I don't know a better way to figure it. Then I needed to reduce it down to an hourly figure, as I'm calculating for a battery that will run for some number of hours. Here's what I came up with:
Average power => 3,008.333kWh/month x 1000Wh/kWh => 3,008,333Wh/month / 30days/month => 100,278Wh/day / 24h/day => 4,178Wh/h => 4,178W
Still with me so far? I hope so. Knowing the average Watts that will be used per hour, I can find the average Amps per hour too. Everything is calculated based on nominal voltage by the way. Different chemistries are going to have different nominal voltages, even when connected in series, but they all seem to be able to be set up to around 48V so I'm sticking with that.
4,178W / 48V = 87A
Next I've got to pick how long the battery should last, powering the house unassisted. It seems to be the general consensus that 3 days is the ideal. But I'm starting small, and plan to expand later with 3+ days being a goal to work toward. For now, I'm using a nice safe number like, say, 3 hours. (Long enough for a typical power outage in my area.) The minimum capacity I'll need can be calculated by the average Amps and how many hours I want to draw the Amps for.
87A x 3h = 261Ah
The minimum energy I'll need is capacity times Voltage:
261Ah x 48V = 12,535Wh
So now I have all the properties I need to design a battery that can power my house. Here's a summary of these properties:
- Nominal Voltage: 48V
- Maximum Current: 125A
- Maximum Power: 6,000W
- Average Energy Consumption: 4,178Wh
- Average Current: 87A
- Continuous Unassisted Runtime: 3 hours
- Minimum Capacity Needed: 261Ah
- Minimum Energy Storage Needed: 12,535Wh
I'll stop here for now. I'm pretty jazzed that I can finally express my power requirements in a way that y'all can (hopefully) understand. But please do let me know if you catch any mistakes I may have made.
I actually had a power outage yesterday during a morning windstorm. There were 4 substations out, along with about 12,000 families. The power company was able to get the vast majority of us connected back up within a couple of hours. In the meantime, I turned off the furnace thermostat, water heater, and a few other breakers in the hope that my local transformer wouldn't blow up when the power got restored. (This happens a lot where I live. Lights turn on, followed by several KABOOMs!)
Anyway, I was reminded that even using a backup battery for power outages, lots of things can be done to keep the battery from discharging as fast. The biggest would be shutting off the furnace and water heater. Those are the biggest power users in my house. In fact, during the coldest part of winter, I used around 5,500kWh of power. The mildest month I used only 1,000kWh. I figure the difference was my furnace. That's 4,500kWh difference!
There is something to be gained by doing without power for a few hours. It cleared the place out! My wife and son both fled in search of TV and Wi-Fi. I on the other hand, sat on the couch with a blanket and a good book, and had a nice quiet house for a while, all to myself.