Question (from Jason)

I have been living in a 336 sq foot house for over a year. I dont use electricity or hooked up to city water. I use well water. But my question is how many 45 watt solar panels do i need to buy in order to run a 30 amp house. Thank you very much for your help.

Answer (provided by Sherman in Maryland)

I’d recommend reading a bit about solar energy and how to size alternative energy systems. Here are some good online resources:

That’s just off the top of my head. There are thousands of sources of good basic info on solar energy systems. Google is a great way to get answers to general questions.

I’m not sure what is meant by, “…a house wired with 30 amp electrical.” The main breaker is just 30 amps? Is it 240 volt or 120 volt service? The former is twice the potential load or power draw (VoltsxAmps=Watts).

Of course, knowing the absolute maximum power that can be used before the main trips is of little practical use. A house could have the typical 240 volt/200 amp service but the owner might only use 100 watts to run a couple LED lights and a laptop computer. The “30 amp” house owner might use say 240V x 25A = 6,000 watts (6KW).

What is needed when designing an alternative energy system is the average amount of power used per day or per week. Load calculations are required. The idea is to list everything in the house, the wattage it uses, and how many hours per day or per week it is used.

Then, knowing how many watts/day are required, one can determine _approximately_ how many PV panels are needed — whether they are 45 watt or 450 watt doesn’t matter, just the total average output per day.

I say approximately because obviously there’s no way to determine precisely how much sun the panels will get on any given day, or even any week. But there are “insolation” charts that show approximately how much sun any given location can expect to see on average, over time.

Batteries will of course be needed (unless it is a grid-tied system, even then batteries are good for a UPS) — the greater the load; the lower the total wattage of the panels; and/or the longer the expected duration of cloud cover, the more battery capacity is needed.

It’s good to use 12Vdc when possible simply because inverters aren’t 100% efficient (but many are over 90%) so it saves energy — particularly if the inverter(s) would not otherwise be operating. Inverters are most efficient in certain range. I’m looking at the manual for my Trace DR2424 inverters. There is a chart labeled “Power vs. Efficiency”. It shows that the DR2424 is over 90% efficient (up to 95%) from about 75 watts up to 1600 watts (max. sustainable output is 2,400 watts each). So, what that means is, if a person only wants to run a LED light say (or some other small load) if it can be run on 12Vdc that’s preferable because at 25 watts the DR2424 is only about 70% efficient (and its efficiency rating continues to drop as the load decreases).

Of course, efficiency is important but it becomes more important at higher loads. IOW, 70% efficiency at 25 watts output means the total power used is about 36 watts — 11 watts are wasted. That’s not desirable but it’s a relatively small number. Imagine if the inverter were only 70% efficient at higher loads. Say at 1,600 watts — that would be 2,286 watts used to power a 1,600 watt load! As it is, the DR2424 is 90% efficient at a 1,600 watt output, so ‘only’ 1,778 watts are used. Only 178 watts wasted instead of 686!

Batteries themselves are only about 75% efficient, which makes inverters seem pretty good in comparison.

While 12Vdc is preferable when practical to use, it isn’t always possible. Some devices must have 120Vac. Well pumps typically need 240 Vac — that’s why I purchased two (2) DR2424 inverters — they can be “stacked” to put out both 120 and 240Vac.

In addition, sometimes there is simply no AC vs. DC choice — or if there is, the DC appliance/device is inferior and/or much more expensive. There is a much greater selection of AC powered items.

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