Question on battery sizing

Re: Question on battery sizing

Welcome to the forum Bad Wolf,

Designing a cost effective solar power system really depends on learning about the loads you want to supply.

Usually, I suggest getting a Kill-a-Watt type meter (for plug-in 120 VAC 15 amp maximum loads), perhaps a whole house type monitoring system (like T.E.D.) and/or a Current Clamp DMM (to estimate power needs).

Take the above information, and then work on 1) conservation and 2) what is your minimum power needs for the off grid situation.

In general, for short term outages (say 2 weeks or less), a small genset + fuel supply is probably a better "investment". For longer term outages (2-4+ weeks), solar can be helpful--But is not really "cost effective" unless you are pulling power 9+ months from the system.

Off Grid solar power systems need new batteries every ~5-8 years, and new electronics (charge controllers, inverters, etc.) every ~10+ years (can last longer, but have cash on hand for repairs/replacements if needed).

A smaller genset only draws fuel when running and it does not require "that much" in the way of maintenance/costs (and replacement is still not that expensive).

To give you a rough idea--An off grid system running ~9+ months of the year, over 20 years amortization+maintenance costs, will run you (very roughly) ~$1-$2+ per kWH--Where utility power runs you around $0.10 to $0.20 per kWH and a genset (fuel costs) will be on the order of $1.00 per hour...

To give you an idea of what "classes of off grid power systems" you are looking at:

1kWH (1,000 WH) per day: Run lights, small fan, laptop computer, charge cell phone, shallow well pump/pressure pump for cabin/home.
3.3 kWH per day: Add a very efficient full size refrigerator, deep well pump, washing machine.
10 kWH per day: Almost normal electrical life (lots of conservation, some electric cooking, perhaps a bit of A/C).
30 kWH per day: Standard North American Home daily power usage (national average). Some electric heat/hot water/cooking.
100 kWH per day: Living in Texas, full A/C, Electric Heat/Cooking/Hot Water, etc...

If you install the system yourself--Something like $5,000-$10,000 per kWH per day up front capital costs. And for a "true" off grid system, most people will still need backup genset for ~3 months of the year (bad weather).

Again, these are very rough numbers for US based systems to give you some back of the envelope starting point for sizing your system... And there is always the "scaling factors" (larger systems can be more cost effective--but need some professional design/installation help, etc.).

I hope these numbers do not cause you "grief"--The are just very rough starting numbers to help you understand how conservation/new Energy Star appliances and simply turning off stuff not needed during outages can help keep costs "reasonable".

Sometimes, you can compromise... Use a "small off grid solar system" (~1 kWH per day) for 24x7 power (lights, radio, laptop, cell phone charger, water pumping from cistern) and use a genset for morning/evening cooking/clothes washing/pumping to cistern.

I have some 40 lb propane tanks and a propane stove for cooking and a multi-fuel camp stove that will run on unleaded gasoline (or diesel, etc.). Sort of look at a power emergency like camping--But I live in a moderate climate and our "big one" will probably be an earth quake (which, in California, has relatively localized effects--Rather than wiping out a hundred square miles of homes/business/infrastructure).

It is easy to buy larger systems/equipment--But there are costs too... Large battery banks that still need maintenance/replacement... A "small" Honda eu2000i (1,600 watt peak) may run 4-9+ hours on a gallon of fuel. An 8 kW genset may run on ~1/2 to 1 gallon of fuel per hour. All depends on your power needs.

I keep looking at Off Grid/Hybrid (both off grid and GT support when utility is running) power systems... But I don't need much power (no A/C, natural gas for heating/cooking/hot water)--So a Honda eu2000i and 20 gallons of fuel will last me 10+ days pretty easily (more if I siphon fuel from my car/pickup, use fuel stabilizer and recycle fuel to vehicles ~once per year). It just never made any sense for me to install an off grid solar power system. Beyond that, lack of city infrastructure (water, sewage, etc.) may become more of an issue and may need to evac/move. Plus a home knocked off its foundation with a 1/2 ton of batteries and a large solar array is not exactly portable.

Energy usage is a highly personal set of choices--What works well for me, may not work well for you.

-Bill