Powering a pellet stove

Re: Powering a pellet stove

Welcome to the forum Eskkar.

Lots of questions (where will the system be installed, is this for backup power or true off grid, will you have a backup generator, anymore loads too, etc.).

But--To just give you an idea of what we are looking at with a nominal design... First I would suggest, if you can, run your loads on a Kill-a-Watt type meter. Basically, actual power/current readings and how many Watt*Hours per day to run your stove/loads.

In general, you would want to find a stove with the lowest amount of power/energy usage--Off Grid power is very expensive, and conservation of your electrical loads is almost always a better investment vs building a larger solar power system.

Let's make some guesses... Say you will run the stove about 12 hours per day:

• 1.1 Amps * 115 VAC * 12 hours per day = 1,518 Watt*Hours per day

That is actually a fair amount of energy usage--Equivalent to a good sized energy star rated refrigerator/freezer (one of the larger single loads a person find in an off grid home/cabin).

So, to size a battery bank for such a load (nothing else). 1-3 days of "no sun" and 50% maximum discharge, choose 2 days and 50% as a good nominal system:

• 1,518 Watt*Hours * 1/0.85 inverter eff * 2 days of storage * 1/0.50 max discharge * 12 volt battery = 595 AH @ 12 volt battery bank

That is about the largest battery bank I would suggest for a 12 volt battery bank. Next step would be a 24 volt bank.

2x 6 volt @ 220 AH "Golf Cart" batteries in series, with three parallel strings for a 12 volt @ 660 AH battery bank would be "interesting" and about the cheapest bank you can build. I am not in love with 3 parallel strings--But at the same time many (most?) folks kill their first battery bank and should not spend too much money on their first.

Next, sizing the solar array... Two things to check. The charging current from the array, and the amount of sun you get and need the array to supply. First based on charging current (5% to 13% rate of charge is a good place to start--If fully off grid and using batteries daily, 10% or more rate of charge is recommended):

• 595 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 560 Watt array minimum
• 595 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,120 Watt array nominal
• 595 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,457 Watt array "cost effective" maximum

Sizing the array based on the amount of sun you get using PV Watts, assuming Phoenix Az, fixed array tilted to Latitude (33.5 degrees from flat):

Month Solar Radiation (kWh/m 2/day)
1 5.09
2 6.06
3 6.61
4 7.54
5 7.53
6 7.28
7 7.13
8 7.17
9 7.15
10 6.75
11 5.59
12 4.88
Year 6.57

Looks like a good amount of winter sun--Pick 4 hours per day minimum (larger array means smaller genset/less fuel needs in in winter):

• 1,518 Watt*Hours per day * 1/0.52 end to end system eff * 1/4.0 hours per day = 730 Watt array

An array from 730 to 1,457 Watts would work fine, with a full time off grid home having a 1,120 Watt minimum array for a 10% rate of charge.

Depending on your needs and what and amount of solar panels you purchase, you could get away with a PWM controller (~$150) on the smaller array, but for a system this size, a MPPT controller is probably a better choice (~$300-$600).

For this size load, this AC inverter may be a good choice (about the smallest you could get away with on a 12 volt system):

MorningStar 300 Watt 12 volt TSW inverter

Your thoughts?

-Bill