30KW inverter battery requirement

ged46

Looking at creating a inverter based back up generator and wondering if any can tell me how many Simpliphi PHI-3.8-48-60-M 3.8kWh 48 Volt Lithium Ferro Phosphate Batteries it would take to run a 30kw inverter for 5 days before a recharge is required.

I planned on using the grid to recharge my backup batteries.

BB.

Welcome to the forum ged46.

Just to answer your question as asked... And some SWAGs (scientific wild a** guesses):

• 30 kWatt rated inverter--Assume using 25% of inverter capacity "on average" over the 24 hour day for 5 days:
• 30,000 Watts * 0.25 average loading * 24 hours per day * 5 days = 900,000 WH = 900 kWh

This battery:

https://www.solar-electric.com/simpliphi-phi-3-8-48-60-m-lithium-ferro-phosphate-battery-with-metal-case.html

3.8 kWH of capacity--Derate to 80% of capacity (aging, avoid "battery shutdown by BMS at 0% and 100% SoC):

• 900 kWH * 1/0.85 AC inverter efficiency * 1/3.8 kWH per battery = 279 batteries
• 279 batteries * $2,690 per battery (retail pricing, no shipping charges incl.) = $750,510 for battery bank (excluding wiring, building, climate control, etc.)

And if the loads are not "random" home loads--But larger fixed capacity loads (such as a computer server farm), the 25% average power usage may be 50-75% of 30 kW inverter capacity, meaning that battery bank will be 2-3x larger.

The general "sever farm" or inverter+generator+battery bank system is not setup for 5 days of battery operation... But instead the genset is sized/optimized to charge the battery bank (and run the loads) for X hours per day. And let the batteries only discharge during small load times (such as a village--Genset in morning for cooking, and evening for cooking and lights, battery bank for low loads mid-day and overnight when genset is less efficient)--Basically the "Prius" hybrid model for gensets--Fuel for high power needs, battery for low power needs and regenerative braking.

The fuel is your "stored" energy (much cheaper to store fuel than have a battery bank of stored energy)... For example, just using random SWAG numbers.

http://www.centralmainediesel.com/order/Kubota-25-kW-Diesel-Generator.asp?page=KB2500

Say only using 25% of 25 kWatt diesel genset power (on average)--Roughly 0.9 gph at 6.25 kW average load:

• 0.9 gph * 24 hours * 5 days = 108 gallons of fuel
• 108 gallons * $4.00 per gallon = $432 worth of "stored" fuel

3/4 of a million dollars of batteries to "store" the equivalent of $432 worth of Diesel fuel... To store:

• 900 kWH of electricity * $0.20 per kWH (California price for "off peak power") = $180 "worth" of utility power

Can do more "math" modeling. Your actual power/fuel costs. Capital (up front) vs long term costs (genset, building, inverter, batteries and long term fuel replacement, maintenance, battery replacement 10+ years down the road, cost of electricity, etc.).

How much could you save adding solar to the system? If you have room for a large array and are committed to cycling the batteries anyway (i.e., batteries during peak power costs, recharging during off peak from utility)--Solar panels themselves are not that costly (compared to the price of everything else).

http://www.solarelectricityhandbook.com/solar-irradiance.html

Seattle
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 42° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.70	2.81	3.54	4.17	4.57	4.78
Jul	Aug	Sep	Oct	Nov	Dec
5.23	5.21	4.71	3.15	1.97	1.53

• 30,000 Watts * 0.25 average loading * 24 hours per day = 180,000 WH per day = 180 kWH per day
• 180,000 WH per day * 1/0.61 Li Ion off grid AC system eff * 1/2.8 hours of sun per day (average Feb) = 105,386 Watt array "Feb" break even
• Roughly $0.50 to $1.00 per Watt for solar => $53,000 to $105,000 worth of solar panels
• Smaller battery bank, put savings into solar power system, save on electrical bills, use diesel for true "daily" backup power, solar panels for most of your base power needs especially during summer to reduce summer utility bills/Air Con costs, etc..

I obviously do not know your actual power needs (tell us more about your application?) and your situation. For example, in California, there are all sorts of pollution rules (air, ground, water) that are making backup gensets a problematic answer to backup/off grid power.

What we can do here is help you get a handle on the basic system requirements. For anything this large--You will probably need to hire a Power Engineer / Engineering Firm to do the actual design and permits. Something that we as website cannot do.

Your thoughts?

-Bill