I have a small computer server rack that I run 24/7 for business. I've also had two serious (4+ hour long) power outages in the last year, and am interested in reducing my energy bill as well. Originally, I was just going to buy an APC brand UPS with external battery pack which I calculated would give me about 8 hours of UPS runtime, and cost $500.
But I'm also a solar geek, and was wondering if I could put this money towards a solar/battery/grid powered system instead.
I'm looking for comments on my plan :
* 50W x 24 hours = 1200WH/day for a small server
* Pure sine wave
* Solar power first
* Battery power second
* Grid power third
* 1200WH needed / .70 derating (round trip Solar panel -> battery -> inverter) = 1700WH
* Average insolation where I live is about 5 hours, solar panel(s) will be due south with azimuth tilt
* 17000WH/5H = 340 watts of solar needed
* Samlex TN1500
* 6V Costco brand Golf Car batteries (estimate 200 AH each). Qty 4. Gross watt hours = 200*6*4 = 4800, Usable watt hours (33%) = 1600WH
* Solar Panel: generic 200 watt panels, qty 2, e.g. 400W
* The panels, batteries, and inverter can all be mounted close to one another, so wiring runs are minimal.
Equipment would cost about $1500. I was going to have to spend $500 on a UPS anyway, so I can rationalize away $500 and say it's an even $1000.
This would save me on average 1.2KWH/day, or about $10/month in electricity (SDGE tier III =$0.29/KWh). So it would pay for itself in a little over 8 years. Of course, batteries don't last that long, so perhaps it would never pay for itself. But it's providing me additional benefit (long-lasting UPS) so I don't care if it's cost positive or negative, really.
* From reading the user manual, the Samlex TN-1500 looks perfect for what I need: it claims to prioritize Solar and Battery first, switching to grid power as a last resort.
* However, the TN-1500 12volt model has a max solar voltage of 25V, which means I'd have to get 12V panels which are rather expensive. Would it make sense to wire this up as a 24V system instead and get the 24-volt samlex? Could I then use a typical generic grid-tie 250watt solar panel which are 30VOC?
* It's not clear from me whether the samlex has a "charge controller" or not, and if so is it PWM or MPTT or what...?
* How is my proposed battery bank size: too small? too big? I don't really care if the grid power is used when we have a stretch of cloudy days, however it would seem that constantly switching between grid AC and inverter AC might be hard on the system. Since two batteries cost about the same as a 200W solar panel, would it make more sense to have 2 batteries + 600W solar? 4 batteries + 400W solar, or 6 batteries + 200W solar? Not sure how to calculate the sweet spot.
Feedback appreciated, thanks!