PV-powered UPS

Batteries and InverterWell, that’s sorta what it is. Here are the pieces I started with:

  • Four 280W PV panels left over from another project
  • A Bed and Breakfast that needs some reliable power
  • A lot of knowledge of electronics in general and PV solar in particular

I have a diesel generator here so the reliable power issue is much more along the lines of not wanting the Internet connection and my computer from going down. Additionally, I have a very irritating gas hot water heater that needs electricity to operate and losing power for one second causes it to turn itself off. Easily corrected — you just need to get out of the shower, go outside and press one button. Yup, irritating.

Solar PanelsDoing the arithmetic, on a sunny day I can expect about 280 * 6 * 4 watt hours of power or 6720 Wh. That’s 280 watts per panel times four panels times six hours of full sun equivalent. Huh? It is pretty much full sun for four hours and then less for the morning and late afternoon hours. My guestimated based on experience in Nicaragua and in my house in Panajachel Guatemala.

Some of that power will be used directly to run equipment connected to the inverter with the “leftovers” being used to recharge the batteries. Assuming a continuous consumption that gives me 6720 / 24 or 280 watts available continually. If we assume the panels will produce at least 280 watts for 8 hours a day (we are likely talking more like 10) then we only have to store 6720 – 2240 or 4480 watt hours in the batteries. At 24 volts, that amounts to 186 ampere hours.

After looking at battery type alternatives I decided that AGM lead-acid was still the best choice. When these batteries die, some Lithium-based battery will be the better choice but, not yet. As  you really don’t want to discharge lead-acid units below 50% as it substantially decreases their life, that means I need 4480 Wh * 2 of battery capacity. I picked a 24 volt system so that means I need 4480 * 2 / 24 of capacity or 375 Ah of battery capacity.

Now, battery capacity depends on discharge rate. That is, the slower you discharge a battery the higher its capacity. If I am consuming 280 watts, we are talking about around 15 amperes of consumption. (280 / 24 equals 11.7 but let’s add a bit for inefficiencies). At that discharge rate the batteries I have chosen will have about 380 Ah of capacity. So, all is well. That is, if we really use 280 watts continually, the batteries will only end up discharged to about 50%. It may sound too close to the limits. But, there are more things to consider:

  • While that is the design capacity, it appears consumption will generally be less. In particular, the computer and Internet connectivity equipment consumes 103W with the screen on, 80W with it off. As I usually don’t work all night on the computer, that consumption is pretty low.
  • The refrigerator consumes about 150W when running but it is not on that much.The stove can consume a few hundred watts but only when the over is operating. That is not very much of the time.
  • Finally, the inverter is programmed to sense a low battery and start charging it. This doesn’t reduce my electric bill but it does protect the batteries if, for example, we baked all night or there was a lot of cloud cover.

That’s the current status. The system has been in operation for a few hours. In a few days I will have way more information about how it work.


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